Desarrollo de nuevos morteros de restauración de cal con ...
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Transcript of Desarrollo de nuevos morteros de restauración de cal con ...
A mi madre y a mi padre
“La ciencia, muchacho, está hecha de errores,
pero de errores útiles de cometer, pues poco a
poco, conducen a la verdad”
Julio Verne
Agradecimientos
Agradezco a todas las personas que de una u otra forma, han colaborado y me han
ayudado en esta etapa de formación durante estos cuatro años.
Le doy las gracias a mis directores de tesis, Dr. José Ignacio Álvarez Galindo y Dr. Íñigo
Navarro Blasco, por motivarme a realizar esta tesis doctoral, así como por dedicarme parte de
su tiempo y su paciencia para la elaboración de este trabajo, así como ser la guía para terminar
con éxito pese las circunstancias adversas que vive nuestra sociedad.
Agradezco en especial al Dr. Íñigo Navarro Blasco, porque gracias a él comencé esta
aventura, además de que fue la persona que me encaminó a la elaboración de este trabajo y fue
quien me ayudó a realizar la travesía de México a España.
Agradezco también el Dr. José María Fernández, por su asesoramiento a lo largo de este
trabajo, por lo que esta Tesis también es resultado de su esfuerzo.
A Cristina Luzuriaga y a Marta Yárnoz, por haberme apoyado a lo largo de mi doctorado
con sus consejos y su ayuda en la experimentación. Gracias por ayudarme, ser muy serviciales
conmigo y facilitarme lo necesario para el desarrollo de este trabajo.
A todos los miembros del Departamento de Química, que siempre me han ayudado con
alegría y buena disposición. Quisiera dar las gracias particularmente a mis compañeros y
amigos Max Petitjean, Burcu Taşcɪ, María Pérez, Joan Puig, Beatriz de Diego, Leire Goñi,
Mikel Domeño y David Lucio, por compartir conmigo el tiempo en el laboratorio, y compartir
gratas experiencias a lo largo de este periodo
Agradezco a la Asociación de Amigos de la Universidad de Navarra por el soporte
económico recibido durante estos cuatro años de formación.
A todos los amigos que he conocido durante todo este tiempo que, aunque no tuvieron
relación con mi formación Doctoral, me han ayudado a sentirme en casa, al abrirme las puertas
de su hogar y brindarme su amistad, compartiendo momentos inolvidables conmigo. En
especial agradezco a Alberto, Sandy, María, Andrea, Óscar, Javier, a mis actuales compañeros
de piso: Jhony, Shirley, Laura y Araceli con quien he pasado estos días difíciles de cuarentena,
pero gracias a su compañía los han hecho amenos y todos mis amigos de Logroño quienes me
han animado a seguir adelante en momentos difíciles.
Agradecimientos
Agradezco a mi familia a mis padres María Guadalupe y Fidel por apoyarme siempre,
confiar en mí y ser compresivos conmigo durante todo este tiempo: sin ustedes no sería lo que
soy ahora. A mis hermanos Jessica, Marco y Lizbeth por hacerme saber que cuento con todos
ellos aun estando lejos.
Resumen
Resumen
El objetivo principal de este trabajo ha sido optimizar morteros de cal aérea mediante la
incorporación combinada de diferentes aditivos para obtener tres gamas de morteros cal aérea
para la restauración de obras arquitectónicas del Patrimonio Cultural. Entre los aditivos
combinados están: materiales puzolánicos (nanosílice, microsílice o metacaolín),
superplastificantes (investigando diversos tipos como éteres de policarboxilato,
lignosulfonatos, condensados de naftaleno–formaldehído y de sulfonato de melamina–
formaldehído), hidrofugante (oleato sódico), fotocatalizador en la masa (TiO2), aditivo
incrementador de la adherencia (copolímero de etileno–vinil–acetato) y modificador de la
reología (almidón).
En la primera gama se desarrollaron morteros de cal de inyección (grouts) combinando
puzolana, superplastificante e hidrofugante. Se estudiaron cinco superplastificantes
poliméricos: lignosulfonato, éter de policarboxilato, sulfonato de naftaleno y condensado de
melamina–formaldehído sulfonato. Se añadió oleato de sodio para reducir la absorción de agua
y se usaron como minerales puzolánicos microsílice y metacaolín para la mejora de la
resistencia y el tiempo de fraguado. Se estudió la compatibilidad entre las diferentes mezclas y
el mecanismo de acción de los diferentes polímeros mediante medidas de potencial zeta e
isotermas de adsorción. Se prepararon e investigaron diversas mezclas de grouts evaluando su
inyectabilidad, fluidez, estabilidad, resistencia a la compresión, hidrofobicidad y durabilidad.
La mezcla multicomponente compuesta de cal, metacaolín, oleato de sodio y PCE (éste al 1%
en peso), resultó ser la composición más efectiva, mejorando la resistencia mecánica, la
inyectabilidad y la hidrofobicidad.
Posteriormente se estudió una gama de morteros de cal con capacidades fotocatalíticas,
de descontaminación del entorno atmosférico y de autolimpieza. Se empleó la nanosílice, como
aditivo puzolánico para mejorar la resistencia de los morteros, y se añadió nanotitania (TiO2)
para proporcionar a las mezclas propiedades fotocatalíticas. Se estudió el efecto de cinco
aditivos dispersantes (superplastificantes) diferentes para mejorar la actividad fotocatalítica,
asumiendo su función dispersante de las partículas de nanotitania, reduciendo la velocidad de
recombinación hueco positivo–electrón. Se incluyó también oleato de sodio, como en la
primera gama, como agente repelente de agua con el fin de aumentar la durabilidad de los
morteros. Dado que la hidrofilicidad fotoinducida, responsable del efecto de autolimpieza,
podría verse afectada por el hidrofugante, se investigó la compatibilidad entre este aditivo y el
TiO2. Los resultados mostraron que la actividad fotocatalítica mejoró debido a la acción de los
superplastificantes (un aumento promedio del 33% de la degradación del NO), significativo
Resumen
para la actividad descontaminante de estos morteros. Además, estos morteros también
mostraron una liberación muy reducida de compuestos intermedios tóxicos, principalmente
NO2: el factor de selectividad (NOx/NO) alcanzó valores de hasta el 87%. La capacidad de
autolimpieza de los morteros, estudiada a través de la degradación del colorante rodamina B,
se incrementó al utilizar a los superplastificantes. En relación con la capacidad de autolimpieza,
y a pesar de la presencia de oleato, las mezclas con superplastificante conservaron e incluso
elevaron la hidrofilicidad fotoinducida de los morteros de cal, alcanzando una buena
humectabilidad de la superficie de los morteros (ángulos de contacto de aprox. 10º),
demostrando compatibilidad de los aditivos y permitiendo obtener una nueva gama de morteros
de cal con capacidades descontaminante y autolimpiante.
Por último, se usaron diferentes aditivos para el desarrollo de una gama de morteros de
cal de revoque con mejor adherencia y durabilidad, así como con reducida fisuración. Para ello,
se ensayaron combinaciones de un mejorador de la adherencia (copolímero de etileno–acetato
de vinilo, EVA), un hidrofugante, un incrementador de la viscosidad (un derivado del almidón)
y una adición puzolánica de nanosílice o metacaolín. Las mezclas resultantes se aplicaron en
forma de monocapa de 15 mm de espesor sobre cuatro sustratos diferentes (arenisca, caliza,
granito y ladrillo) para evaluar su desempeño. Se estudió la influencia de la combinación de los
aditivos sobre la fluidez, el tiempo de fraguado, la adherencia, la formación de microfisuras, la
resistencia a la compresión, la estructura porosa y la durabilidad (resistencia a heladas y al
ataque por sulfatos). Se observó que el EVA mejoró la adherencia cuando se usa en
combinación con oleato, metacaolín y almidón. Esta combinación condujo además a una
mínima fisuración. Además, se observó que la formación de fisuras y la adherencia dependieron
de la porosidad de los sustratos y de la presencia de poros capilares de pequeño tamaño (0.01 a
1 micra). Las mezclas con nanosílice mostraron elevadas resistencias a compresión, debido al
efecto de relleno del aditivo y a la formación de C–S–H, y mejoraron claramente la durabilidad
frente a los ciclos de hielo–deshielo y al ataque por sulfatos.
Índice
i
I. Introducción
1. Mortero de cal............................................................................................................. 3
1.1. Introducción ...................................................................................................... 3
1.2. Componentes .................................................................................................... 4
1.2.1. Cal .......................................................................................................... 4
1.2.2. Árido ....................................................................................................... 7
1.2.3. Agua ....................................................................................................... 8
1.2.4. Aditivos .................................................................................................. 8
2. Empleo del mortero de cal en restauración del Patrimonio Edificado ....................... 9
2.1. Estudios actuales y justificación ....................................................................... 9
2.2. Los aditivos como solución a los problemas de los morteros de cal .............. 12
3. Aditivos en morteros de cal ...................................................................................... 14
3.1. Panorámica general ........................................................................................ 14
3.2. Tipos de aditivos empleados en morteros de cal ............................................ 16
3.2.1. Superplastificantes (reductores de agua) .............................................. 16
3.2.2. Agentes puzolánicos ............................................................................. 22
3.2.3. Hidrofugantes ....................................................................................... 24
3.2.4. Aditivos fotocatalíticos ........................................................................ 26
3.2.5. Incrementadores de la viscosidad ......................................................... 29
3.2.6. Mejoradores de adherencia ................................................................... 31
4. Interés del estudio sobre combinaciones de aditivos en morteros de cal ................. 33
4.1. Morteros de inyección .................................................................................... 33
4.2. Morteros autolimpiantes ................................................................................. 34
4.3. Morteros de adherencia mejorada .................................................................. 35
Referencias ................................................................................................................... 36
Índice
ii
II. Objetivos
Objetivos....................................................................................................................... 53
III. Material y métodos
1. Materiales empleados ............................................................................................... 57
1.1. Materiales generales ....................................................................................... 57
1.1.1. Cal ........................................................................................................ 57
1.1.2. Árido ..................................................................................................... 57
1.1.3. Hidrofugante ......................................................................................... 58
1.1.4. Aditivos puzolánicos ............................................................................ 58
1.1.5. Agua ..................................................................................................... 59
1.2. Aditivos específicos de la Gama 1: morteros de inyección de cal con
elevada resistencia, durabilidad y buena fluidez ........................................... 59
1.2.1. Aditivos superplastificantes ................................................................. 59
1.3. Aditivos específicos de la Gama 2 de morteros de cal con capacidad
autolimpiante ................................................................................................. 61
1.3.1. Aditivos superplastificantes ................................................................. 61
1.3.2. Aditivo fotocatalítico: TiO2 .................................................................. 63
1.4. Aditivos específicos de la Gama 3 de morteros de cal de reología
controlada y adherencia mejorada ................................................................. 63
1.4.1. Modificador de reología ....................................................................... 63
1.5. Potenciador de la adherencia .......................................................................... 64
2. Caracterización de los materiales ............................................................................. 65
2.1. Difracción de Rayos X (XRD) ....................................................................... 65
2.2. Isotermas de adsorción gas–sólido ................................................................. 65
2.3. Determinación de potencial zeta .................................................................... 66
2.4. Determinación de tamaño de partícula ........................................................... 66
2.5. Espectroscopía IR ........................................................................................... 66
2.6. Determinación de adsorción mediante carbono orgánico total (TOC) ........... 66
Índice
iii
3. Preparación y estudio de las mezclas ....................................................................... 68
3.1. Dosificación .................................................................................................... 68
3.2. Mezcla y amasado .......................................................................................... 68
3.3. Elaboración de las probetas ............................................................................ 69
3.4. Ensayos del mortero fresco ............................................................................ 69
3.4.1. Determinación de la consistencia (mesa de sacudidas) ........................ 69
3.4.2. Determinación de la densidad y el contenido de aire ocluido .............. 70
3.4.3. Determinación del periodo de trabajabilidad ....................................... 70
3.4.4. Estudio del proceso de hidratación ....................................................... 71
3.4.5. Determinación de la capacidad de retención de agua .......................... 71
3.4.6. Evolución del extendido sobre diferentes superficies .......................... 71
3.4.7. Inyectabilidad ....................................................................................... 72
3.5. Ensayos del mortero endurecido .................................................................... 73
3.5.1. Determinación de la resistencia a compresión ..................................... 73
3.5.2. Estudio de la estructura porosa ............................................................. 74
3.5.3. Estudio químico y mineralógico .......................................................... 74
3.5.4. Análisis térmico .................................................................................... 74
3.5.5. Estudio del ángulo de contacto ............................................................. 74
3.5.6. Estudio de actividad fotocatalítica ....................................................... 75
3.5.7. Estudio de la durabilidad ...................................................................... 76
3.5.8. Estudio biocida ..................................................................................... 77
4. Metodología de Estudio ............................................................................................ 78
IV. Resultados y discusión
Capítulo I: Desarrollo de morteros de cal de inyección (grouts) .................................. 85
Parte A. Polymer–based superplasticizers to prepare lime–metakaolin grouts:
mechanical performance and durability assessment .............................. 87
Índice
iv
Parte B. Combination of polymeric superplasticizers, water repellents and
pozzolanic agents to improve air lime–based grouts for historic
masonry repair ..................................................................................... 123
Capítulo II: Desarrollo de morteros de cal con actividad fotocatalítica mejorada y
autolimpiables ......................................................................................... 161
Improvement of the depolluting and self–cleaning abilities of air lime mortars
with dispersing admixtures .................................................................................. 163
Capítulo III: Diseño y obtención de morteros de revoco con fisuración reducida y
adherencia mejorada ............................................................................. 201
Improving lime–based rendering mortars with admixtures ................................ 203
V. Discusión general
1. Aditivos puzolánicos .............................................................................................. 253
1.1. Microsílice .................................................................................................... 253
1.2. Metacaolín .................................................................................................... 255
1.3. Nanosílice ..................................................................................................... 257
2. Superplastificantes .................................................................................................. 262
2.1. Lignosulfonato .............................................................................................. 262
2.2. Éteres de policarboxilato .............................................................................. 263
2.3. Sulfonato de naftaleno .................................................................................. 266
2.4. Condensado de melamina–formaldehído sulfonato (SMF) .......................... 268
3. Aditivo hidrofugante: oleato de sodio .................................................................... 270
4. Fotocatalizador: TiO2 ............................................................................................. 272
4.1. Estudio biocida ............................................................................................. 272
4.2. Abatimiento de NO y autolimpieza .............................................................. 273
5. Modificador de la viscosidad: almidón de patata modificado ................................ 274
6. Modificador de la adherencia: copolímero de etileno–acetato de vinilo (EVA) .... 276
Índice
v
7. Resumen de resultados y recomendaciones de combinaciones de aditivos ........... 277
7.1. Morteros de inyección .................................................................................. 277
7.2. Morteros autolimpiantes ............................................................................... 277
7.3. Morteros de adherencia mejorada ................................................................ 278
Referencias ................................................................................................................. 279
VI. Conclusiones
Conclusiones............................................................................................................... 287
Introducción
Introducción
3
1. Mortero de cal
1.1. Introducción
Desde la antigüedad la cal ha sido uno de los principales conglomerantes que el
hombre ha utilizado debido a su fácil obtención a partir de rocas carbonatadas abundantes
en la corteza terrestre. A través de la historia, sus aplicaciones han sido múltiples,
empleándose para revestimientos o en forma de morteros para rellenos, solados, levante,
etc., así como en materiales hidráulicos y resistentes a la acción del agua del mar
añadiendo puzolanas como aditivos [1,2].
Además, se sabe que a los morteros de cal se les agregaban aditivos para su mejora
desde la época egipcia, en la cual se emplearon aditivos orgánicos tales como la sangre
animal, huevos, caseína, etc. En la época griega y romana se empezaron a utilizar
adiciones inorgánicas como tejo, ladrillo triturado o polvo volcánico, todos ellos con
actividad puzolánica más o menos intensa. Los romanos fueron los primeros en darse
cuenta de que las mezclas con puzolana poseían carácter hidráulico y eran capaces de
endurecer bajo el agua, alcanzando además elevadas resistencias mecánicas y muy buenas
durabilidades [3,4].
El empleo de la cal como ligante se atribuye al extensivo uso en el período
neolítico y posteriormente por parte de las culturas griega y romana, siendo estos últimos
los que perfeccionaron la técnica de fabricación y aplicación [5]. Los morteros de cal
romanos estaban formados por cal y arena, generalmente en dosificación 1:3 o 2:5 en
función de la calidad del árido, y en ocasiones incluían materiales hidráulicos como
cenizas volcánicas, tejas trituradas o puzolanas [3,6,7].
Posteriormente, la fabricación y utilización de los morteros de cal fue una práctica
razonablemente común hasta la primera guerra mundial, en combinación con los
cementos naturales. A partir de ese momento, la gran evolución de los cementos Portland
(con mayor velocidad de endurecimiento y resistencia mecánica más elevada) hizo que
los morteros de cemento desplazaran a los tradicionales morteros de cal en prácticamente
todas sus aplicaciones [8].
Sin embargo, hoy en día el mortero de cal constituye la alternativa más deseable
para acometer la restauración del Patrimonio Edificado. Su aplicación y uso es muy
diverso y abarca desde la reparación de daños estructurales hasta meramente decorativos
Introducción
4
[9–11]. Todo ello es factible gracias a la obtención de morteros con diversas
características y propiedades por variación de sus componentes o la proporción de los
constituyentes [6,12].
1.2. Componentes
1.2.1. Cal
La cal es una sustancia alcalina constituida por óxido de calcio, que al contacto
con el agua genera una reacción de hidratación fuertemente exotérmica. La normativa
enuncia el concepto de cal para construcción, definida como “Conglomerante cuyos
principales constituyentes, dados por el análisis químico, son los óxidos e hidróxidos de
calcio (CaO, Ca(OH)2), con cantidades menores de magnesio (MgO, Mg(OH)2), silicio
(SiO2), aluminio (Al2O3) y hierro (Fe2O3)” [13]. La cal forma el mortero o argamasa
cuando se mezcla con arena y agua.
Comúnmente en el ámbito de la construcción se utiliza la cal apagada, o hidróxido
de calcio, que no produce una reacción exotérmica al hidratarse. La cal apagada (a la que
comúnmente se extiende la denominación de cal) se produce mediante la hidratación
controlada de la cal viva proveniente de la calcinación de roca caliza o dolomítica. Esta
hidratación puede hacerse mediante proceso industrialmente controlado o por
embalsamiento en agua durante tiempos prolongados, en procesos artesanales [14].
Pueden distinguirse en función de la composición química y del mecanismo de
fraguado, cales aéreas o hidráulicas. En el caso de cales aéreas, se componen
principalmente de óxido o hidróxido de calcio. Debido a que este tipo de cal carece de
componentes hidráulicos, el material en contacto con el agua no endurece. El agua se
inmoviliza y se genera adherencia, aportando humedad al mortero evitando, hasta cierto
punto, un secado demasiado rápido.
El fraguado es un proceso en el que el material tras ser mezclado con agua deja de
comportarse como una suspensión líquida [15,16]. En el caso de la cal, durante el
fraguado se experimenta un proceso de carbonatación y posterior liberación de agua.
Dependiendo del tipo de cal, los procesos de fraguado y endurecimiento se desarrollarán
a través de diferentes mecanismos. Las cales aéreas endurecen como consecuencia de un
doble proceso, evaporación de parte del exceso de agua de amasado y carbonatación del
hidróxido de calcio por contacto con el dióxido de carbono atmosférico. La evaporación
Introducción
5
inicial del agua es la que proporciona el endurecimiento inicial, y la carbonatación es un
proceso irreversible que modifica la microestructura del material a largo plazo [16].
Debido a la presencia de agua, el CO2 y la portlandita se disuelven previamente,
dando como productos ácido carbónico (Ec. 1) y una pasta de cal respectivamente.
𝐶𝑎(𝑂𝐻)2 + 2𝐻2𝑂 + 𝐶𝑂2 → 𝐻2𝐶𝑂3+ + 𝐶𝑎(𝑂𝐻)2 + 𝐻2𝑂 𝑬𝒄. 𝟏
El ácido carbónico generado reaccionará con la portlandita en disolución,
transformándose en carbonato cálcico y agua (Ec. 2).
𝐻2𝐶𝑂3 + 𝐶𝑎(𝑂𝐻)2 → 𝐶𝑎𝐶𝑂3 + 𝐻2𝑂 𝑬𝒄. 𝟐
Esta reacción forma un producto intermedio (bicarbonato cálcico) que se
descompone por evaporación de agua, lo que forma finalmente cristales de carbonato
cálcico (Ec. 3).
𝐶𝑂2(𝑔) + 𝐶𝑎(𝑂𝐻)2(𝑠) → 𝐶𝑎(𝐻𝐶𝑂3)2(𝑠) → 𝐶𝑎𝐶𝑂3(𝑠) + 𝐻2𝑂(𝑙) + 𝐶𝑂2(𝑔) 𝑬𝒄. 𝟑
El proceso global de carbonatación de la cal aérea se describe en la Fig. 1 [17–
19].
La velocidad e intensidad del proceso de carbonatación se ve afectado por diversos
factores como son: la temperatura, la humedad relativa, la existencia de agua en el
mortero, la estructura porosa y espesor del material, el tiempo de reacción, la
permeabilidad del medio, la composición de la cal, la adición de un ligante o la presencia
de aditivos orgánicos; todo ello tendrá repercusión directa sobre la microestructura del
material [20].
La resistencia a compresión de cales aéreas depende directamente de su proceso
de carbonatación. Llegará a su máximo valor, dependiendo de su propiedades, en un
periodo de aproximado de 1 a 3 años [14,15,21–23].
Figura 1. Proceso de carbonatación de la cal
Introducción
6
La cal hidráulica se obtiene a partir de rocas que contienen mezclas de margas y
arcillas ricas en sílice, aluminio y hierro. Durante la calcinación de la cal se forman, en
función de la carga mineral, de la temperatura del horno y del tiempo de residencia,
diferentes silicatos de calcio, particularmente C2S y gehlenita. La disponibilidad de
hidróxido de calcio y de estos compuestos permite que este tipo de cal endurezca en
contacto con el aire y con el agua, y más rápido que la cal aérea, lo que permite acelerar
el ritmo de su aplicación en obra. Su capacidad de endurecimiento en ausencia de aire
permite su empleo en obra hidráulica y grandes macizos de albañilería.
En cales hidráulicas, el proceso de carbonatación es análogo al acontecido para
las cales aéreas; sin embargo, adicionalmente se lleva a cabo un proceso paralelo de
hidratación de silicatos y aluminatos presentes, principalmente bajo las formas químicas:
silicato dicálcico (C2S), y en mucha menor medida de silicato tricálcico (C3S) y aluminato
tricálcico (C3A), que darán lugar a los correspondientes productos hidratados
responsables de la resistencia mecánica del material, y también originará portlandita que
sufrirá posteriormente la reacción de carbonatación (Ec. 4–6).
2(3𝐶𝑎𝑂 ∙ 𝑆𝑖𝑂2) + 6𝐻2𝑂 → 3𝐶𝑎𝑂 ∙ 2𝑆𝑖𝑂2 ∙ 3𝐻2𝑂 + 3𝐶𝑎(𝑂𝐻)2 𝑬𝒄. 𝟒
3(𝐶𝑎𝑂) · 𝐴𝑙2𝑂3 + 6𝐻2𝑂 → 3𝐶𝑎𝑂 ∙ 𝐴𝑙2𝑂3 ∙ 6𝐻2𝑂 𝑬𝒄. 𝟓
2(2𝐶𝑎𝑂 ∙ 𝑆𝑖𝑂2) + 4𝐻2𝑂 → 3𝐶𝑎𝑂 ∙ 2𝑆𝑖𝑂2 ∙ 3𝐻2𝑂 + 𝐶𝑎(𝑂𝐻)2 𝑬𝒄. 𝟔
En morteros de cal deben tenerse en cuenta las siguientes características y
comportamientos observables:
a) Modificaciones de volumen: como consecuencia directa de dos fenómenos
principales: retracción y expansión. La retracción es debida a la disminución
de volumen experimentada por el mortero de cal, durante y después del
fraguado, tras la exposición al aire, térmica (cales hidráulicas), plástica e
hidráulica o de secado (cales aéreas).
La expansión, por el contrario, es un fenómeno que puede suceder en cualquier
tipo de cal, que puede contener cierta cantidad de cal viva y sufrir un proceso
de hidratación tras su aplicación, dando como resultado un mercado incremento
de volumen. Este hecho origina roturas y despegues de material, conocidos
como caliches. Este fenómeno puede desencadenar un notable inconveniente y
debe ser debidamente controlado.
Introducción
7
b) Plasticidad, esta propiedad proporciona a las pastas y los morteros de cal la
capacidad para moldearse con facilidad. La cal puede absorber las
deformaciones originadas por esfuerzos mecánicos [16,24].
c) Retención de agua, dicho atributo depende del tamaño de grano del hidróxido
de calcio, así como de su estructura. La alta capacidad retenedora de agua de
los diferentes tipos de cal es consecuencia directa de su reducido tamaño [25].
d) Demanda de agua, se considera la cantidad de agua necesaria ya sea para apagar
la cal viva o para llevar acabo la etapa de amasado.
1.2.2. Árido
Por lo general, los áridos que forman parte de los morteros de cal son materiales
granulares inorgánicos de tamaño variable. Se considera oportuno dispongan de un
carácter inerte ya que por sí mismos no deben actuar químicamente en la mezcla. Sin
embargo, la vinculación con la cal ejercerá una influencia determinante en las propiedades
físicas del mortero. En los últimos años se ha reconocido ampliamente la importancia de
seleccionar el tipo de áridos y su efecto en el cambio de propiedades del hormigón y
mortero de cal. En el contexto de la rehabilitación de revestimientos de muros, este factor
cobra especial importancia, ya que se pretende obtener morteros con características
específicas y prestaciones compatibles con los existentes en mampostería antigua. Los
áridos, al ser parte integrante de los morteros de cal, y en algunos casos definidos como
el "esqueleto" de los sistemas de revestimiento, influyen directamente en sus propiedades,
tanto en estado fresco como endurecido. El comportamiento de los morteros depende en
gran medida de su microestructura, que a su vez está condicionada por varios aspectos,
entre los que destacan: las características de los componentes utilizados (a saber, tipo de
ligante y naturaleza mineralógica y tamaño del agregado); la formulación (proporción con
la que se mezclan los componentes y cantidades de agua de mezcla); la cura; los
procedimientos de aplicación y el tipo de soporte. Los áridos, que constituyen alrededor
del 75 al 85% del volumen de mortero, asumen un papel fundamental en el
comportamiento físico, químico y mecánico de los morteros, así como en el acabado y
aspecto final de los morteros, principalmente en el caso de morteros de cal [14,26–28].
Introducción
8
1.2.3. Agua
El papel desempeñado por el agua de amasado en un mortero de cal es variable.
En el caso de las cales hidráulicas, el agua es necesaria para que se lleven a cabo la
reacción de hidratación durante el endurecimiento, denominándose entonces agua de
curado. En el caso de las cales aéreas, el papel ejercido se puede calificar de intermedia:
el agua no participa directamente en las reacciones químicas del mortero, pero es
necesaria como medio para que se produzca la carbonatación [18].
Cualquier tipo de agua hallada en la naturaleza , siempre y cuando no contenga
abundantes sales o impurezas, es apta para su empleo en la preparación de un mortero. El
agua utilizada para el amasado o el curado en obra no debe contener ningún residuo o
constituyente perjudicial en cantidades tales que afecten a las propiedades del mortero,
según las prescripciones para cementos y hormigones [29]. En definitiva, toda agua de
consumo público es válida para su empleo, y en general, es preferible el agua de ríos que
la procedente de pozos y pantanos, debido a su mayor contenido en materia orgánica,
fangos, limos, arcillas y finos en suspensión, y que por su pequeño tamaño disminuyen
considerablemente la adherencia de la pasta y el árido.
Se conoce que las aguas que presentan un contenido en sales naturales en torno al
5% producen pérdidas de resistencia de hasta un 30%, y que las aguas de origen mineral
carbonatadas que contienen pequeñas cantidades de sulfatos y cloruros pueden inferir
caídas de resistencia de hasta un 80%. Además, un alto contenido en sales conlleva
necesariamente a la aparición de eflorescencias.
1.2.4. Aditivos
Estos compuestos son materiales añadidos antes o durante la mezcla del mortero
o pasta, en una proporción inferior al 5% en masa del contenido de ligante. Ejercen su
función principal con objeto de mejorar las propiedades del mortero, en estado fresco o
endurecido, con determinadas modificaciones bien definidas y con carácter permanente
[30,31].
Introducción
9
2. Empleo del mortero de cal en restauración del Patrimonio
Edificado
2.1. Estudios actuales y justificación
En el momento de elaborar el presente trabajo, los investigadores dedicados a la
restauración del Patrimonio Edificado recomiendan, de forma apremiante, la utilización
de morteros de cal en procesos de restauración de obras monumentales de interés
histórico–artístico [23,32–34], dado que dichos materiales de reparación exhiben
características, composición y propiedades similares a los materiales originales de la obra
arquitectónica en donde son aplicados [35].
Durante las últimas décadas del siglo XX, el uso a gran escala del cemento
Portland postergó, cuando no evitó, los estudios sobre los morteros de cal a un segundo
plano, lo que explica relativamente su limitada aplicación en obra de intervención. El
cemento Portland constituyó, tras su extensión como material conglomerante, el foco
neurálgico para la comunidad científica dedicada a los materiales de construcción
aglutinantes [14] y, además, se convirtió en el material de referencia para ser usado en la
práctica totalidad de los procesos constructivos, incluyendo las actuaciones de
restauración de obras monumentales [14,36].
Sin embargo, los problemas asociados a la utilización del cemento Portland
explican y avalan el creciente uso de los morteros de cal. Son destacables varios
argumentos positivos a favor de los morteros de cal, que minimizan los inconvenientes
de los morteros de cemento, e incluso justifican el empleo y aplicación de aquellos
[37,38]:
• Los morteros de cal presentan mayor compatibilidad con los métodos de
edificación y los materiales antiguos desde los puntos de vista químico, estructural
y mecánico.
• La cantidad de sales solubles aportada por el mortero de cal es notablemente
inferior a la proporcionada por el cemento Portland. Esto evita el importante daño
en el sistema conjunto piedra/mortero originado por los ciclos de recristalización
(disolución y precipitación) y/o hidratación.
• Manifiestan una mayor flexibilidad bajo determinadas condiciones mecánicas,
aspecto esencial en previsión de los esperados movimientos de las fábricas de
Introducción
10
mampostería: los morteros de cal poseen una compatibilidad tecnológica con los
materiales antiguos muy superior en comparación con los análogos de cemento.
• Los morteros de cal proporcionan una aventajada estabilidad estructural del
edificio a largo plazo, ya que en el caso de que se originen fracturas en el mortero,
éstas pueden subsanarse mediante un proceso de autosellado (self–healing en el
término inglés), vinculado a los ciclos de disolución/reprecipitación de la calcita.
Junto a estas premisas iniciales, en favor del uso de morteros de cal, cabe esgrimir
criterios históricos, que sostienen el principio de mínima intervención sobre obra
patrimonial, en aras de conservación preventiva o, en su defecto, en caso de necesidad, la
utilización de materiales y técnicas de construcción análogos a los empleados en la
edificación original, atendiendo a la salvaguarda de los valores intrínsecos y extrínsecos
propios de la obra integrante del Patrimonio Histórico [22,37].
Existen, además, revestimientos interiores y exteriores (enlucidos y revocos), en
las que estos morteros proporcionan ventajosa aplicación, ya que permiten i) una buena
plasticidad, ii) una débil retracción,, iii) una gran elasticidad que favorece su adaptación
a las deformaciones del soporte sin provocar agrietamientos, iv) permeabilidad apreciable
al vapor de agua que favorece el intercambio gaseoso, v) escaso contenido en sales
solubles por lo que disminuye el riesgo de aparición de eflorescencias, vi) buen
aislamiento térmico y acústico, vii) buen aspecto estético y homogéneo, y viii) facilidad
de coloración con garantía de sellado y estucado [39,40].
La cal es, además, un material tradicional, barato y localmente disponible en la
mayoría de las zonas. En determinadas aplicaciones, como hormigones de cal y cáñamo,
puede llegar a tener un impacto de carbono negativo. Es decir, en un análisis completo de
su ciclo de vida, incluyendo su extracción, producción y aplicación, no libera CO2 a la
atmósfera, sino que actúa como absorbente del mismo. Entre otras razones, el menor coste
energético en transporte y producción (por la menor energía de calcinación requerida, en
comparación con el cemento), y su capacidad de carbonatación, explican estos resultados.
Además, es un material saludable, con notable capacidad biocida por su pH alcalino
[41,42].
En definitiva, resulta, por ello, de enorme interés abordar el estudio de las
propiedades de los morteros de cal y su grado de afectación por diversos materiales,
métodos y técnicas [30,43]. A modo de ejemplo, varios estudios han mostrado la
Introducción
11
implicación de las propiedades de morteros de cal aérea cálcica, dolomítica e hidráulica,
como factores críticos para la resistencia mecánica del material [14,22,30,44,45]. En este
mismo sentido, la adición de agentes puzolánicos procuran un resultado muy positivo
para dichos morteros de cal [44,46–48].
En su detrimento, la literatura señala un largo periodo de endurecimiento para que
el mortero de cal alcance su resistencia mecánica definitiva, de forma que los valores
máximos se alcanzan tras uno o dos años [14,15,21–23]. La lenta carbonatación de estos
morteros, responsable principal de su lenta evolución mecánica, se ha relacionado con la
alteración en la distribución de tamaño de poro producida durante el propio proceso de
carbonatación y con el secado del material. Por un lado, la transformación de portlandita
en calcita conduce al bloqueo de los poros de mayor tamaño, de forma que la reacción de
carbonatación se dificulta mediante la restricción del acceso de agua necesaria la
disolución de la portlandita y la transformación del CO2. Por otro, el secado y evaporación
del agua de amasado, en las primeras etapas, y del agua remanente en el interior de los
poros, en los siguientes estadios, limita el contenido de agua requerido, según se ha
comentado, para la carbonatación. En conclusión, el proceso de carbonatación en los
morteros de cal se considera autolimitante [49,50].
Añadido a ello, también es sencillo enumerar las desventajas que se relacionan
con el mortero de cal. Entre ellas destacan la relativa baja resistencia mecánica, alta
sensibilidad a los procesos de deterioro debidos a su baja cohesión interna y alta porosidad
−factores que aportan una elevada capacidad de retención de agua−, su pequeña
resistencia a ciclos hielo–deshielo y el alto grado de afectación de la cristalización de
sales [38]. Además del menoscabo de estos factores físicos sobre las características
estructurales, se deben considerar los factores químicos (reacciones químicas directas
entre los morteros y reactantes agresivos, procesos de disolución en agua, con
contaminantes atmosféricos: CO2, SO2, NO2) o los biológicos (crecimiento de
microorganismos que conllevan destrucción química por los productos de su
metabolismo, variación del pH en el sustrato o colonización biológica desestabilizando la
integridad del mortero), como aspectos relevantes en el de deterioro de los morteros de
cal [21,51,52].
La incorporación de uno o varios aditivos químicos o puzolánicos adecuadamente
seleccionados a un mortero conlleva una mejora muy considerable en una o varias de sus
propiedades y, por lo tanto, del comportamiento del material. Por esta razón, los aditivos
Introducción
12
han adquirido en los últimos años una importancia enorme en la industria de la
construcción y son muchos los estudios que se han desarrollado en torno a este tema para
morteros de cemento y hormigones [53–55].
2.2. Los aditivos como solución a los problemas de los morteros de cal
A pesar de la abundancia de investigación en el campo de los materiales de
construcción sobre la inclusión de aditivos de diversa naturaleza y puzolanas sobre
agentes conglomerantes basados en cemento, los estudios de incorporación de aditivos y
empleo de puzolanas a morteros de cal son limitados, especialmente los primeros. Dado
su diferente carácter químico, los resultados obtenidos en los ensayos con matrices de
cemento no son directamente extrapolables a los sistemas de cal, en los que el
comportamiento de aditivos requiere un estudio independiente para conocer su función
desarrollada, así como su eficacia y actividad, incluyendo el mecanismo de acción
específico en la matriz de cal.
Asimismo, la mejora de las propiedades de los morteros de cal podría ser de
enorme interés, enfocada a aplicaciones modernas (revocos y enlucidos) o encaminada
hacia obras de restauración llevadas a cabo con materiales compatibles pero mejorados,
con la ayuda de productos y técnicas de fabricación modernos. En este sentido puede
indicarse que muchos de los aditivos que se proponen son compuestos químicos de
naturaleza orgánica, como oleatos, estearatos, derivados de goma guar o almidón; otros
son derivados químicos sintéticos, pero, en todo caso, se añaden en proporciones muy
bajas, en torno al 0.5% respecto al peso de cal, porcentaje muy inferior todavía respecto
al peso del mortero total en seco. Por ello, los sistemas que se proponen son respetuosos
con las técnicas y materiales clásicos, aportando las mejoras lógicas del avance científico
y técnico, y preservan además las características de la cal relativas a su sostenibilidad
medioambiental.
Todo ello se apoya en investigaciones previas que han estudiado la incorporación
individual de superplastificantes a morteros de cal aérea o hidráulica, el uso de
retenedores de agua, aireantes, hidrofugantes o el empleo de puzolanas como el
metacaolín o nanosílice como aceleradores del fraguado [23,30,38,47,56–61], o la adición
de aditivos fotocatalíticos, basados en TiO2 [62,63]. Los resultados son muy
prometedores, ya que se han observado ventajas claras en la incorporación de un único
Introducción
13
aditivo: mejora en resistencias mecánicas, en tiempos de fraguado o en durabilidad han
sido resaltadas recientemente [50,60,64].
Sin embargo, en la mayor parte de los trabajos, toda la información disponible se
circunscribe al efecto de un único aditivo, sin contemplar el posible efecto conjunto o
incluso sinérgico de las combinaciones más interesantes de dos o más aditivos y/o
puzolanas. Sólo de forma reciente se han publicado algunos trabajos que plantean la
combinación binaria de una puzolana con otros aditivos, principalmente
superplastificantes, con muy buenos y esperanzadores resultados [48,50,58,65].
Introducción
14
3. Aditivos en morteros de cal
3.1. Panorámica general
El uso de aditivos con objeto de mejorar las propiedades del mortero de cal no es
una idea moderna. Los primeros datos correspondientes a su empleo se remontan a la
cultura egipcia, como se ha comentado, en la que se aplicaron una variedad grande de
aditivos orgánicos disponibles, de origen animal o vegetal, como la sangre animal,
huevos, caseína, etc. En la época griega y romana se dispusieron de adiciones inorgánicas
como teja y ladrillo triturado o polvo volcánico (la conocida Tierra de Santorini), todos
ellos con probada actividad puzolánica más o menos intensa. Así, los romanos alcanzaron
morteros de elevada resistencia mecánica y muy alta durabilidad a largo plazo [66,67].
El empleo de aditivos se ha generalizado, y a lo largo de los años han ido
apareciendo nuevos compuestos y productos capaces de mejorar enormemente el
comportamiento de los morteros. Gracias al notable avance tecnológico, se permite llevar
a cabo la modificación de materiales naturales o la síntesis de nuevos aditivos con el
objeto de conseguir productos que cumplan unas expectativas tan concretas como se
desee. Sin embargo, este gran desarrollo industrial, se ha centrado en la última década en
los aditivos para productos con base cemento (hormigones, morteros, etc.).
Existen numerosas referencias acerca de distintos grupos de aditivos de aplicación
íntegra a hormigones y morteros de cemento: aditivos hidrofugantes, reductores de agua
y superplastificantes, retenedores de agua, aireantes, aceleradores de fraguado y
retardadores [68–71]. La descripción detallada de los mecanismos de acción de cada uno
de los aditivos y el conocimiento pleno de los procesos que tienen lugar en las mezclas,
han permitido sentar las bases para el desarrollo de nuevos y mejores productos
comerciales.
Como estrategia convergente, la adición mineral conjunta ha abierto una nueva
perspectiva de investigación. Las adiciones minerales plantean la incorporación de un
material añadido a la mezcla, en porcentaje generalmente más alto que los aditivos, que
asimismo implica una modificación de la mezcla, sea física, química o fisicoquímica.
Estas adiciones persiguen rebajar la cantidad de cemento incorporado, tanto en el proceso
de clinkerización como a posteriori, abaratando costes, e incluso permitiendo el reciclado
y la reutilización de residuos. La incorporación de uno o varios aditivos o puzolanas
adecuadamente seleccionados a un mortero conlleva una mejora muy considerable en una
Introducción
15
o varias de sus propiedades y, por lo tanto, del comportamiento del material. Por esta
razón, los aditivos han adquirido en los últimos años una importancia enorme en la
industria de la construcción y son muchos los estudios que se han desarrollado en torno a
este tema para morteros de cemento y hormigones [68–70].
Entre los aditivos más frecuentemente estudiados se encuentran los materiales de
relleno o reciclados y las adiciones minerales, que engloban los materiales con mayor
potencial, aquellos de naturaleza puzolánica como el humo de sílice, cenizas de cáscara
de arroz, cenizas volantes, escorias metalúrgicas, tobas volcánicas, arcillas calcinadas,
nanosílice, microsílice, etc. [72]. La incorporación de dichos agentes minerales con
carácter puzolánico, denominados materiales cementicios suplementarios
(Supplementary Cementitious materials, SCMs) representa una eficaz estrategia para
solventar, parcial o totalmente, algunos de los principales inconvenientes que presentan
hoy en día los morteros de cal. Las puzolanas, bien documentadas en la química de los
materiales cementantes ordinarios, llevan a cabo una intensa reacción con el hidróxido de
calcio, formando silicatos de calcio que permiten su posterior hidratación resultando en
una matriz de evidente carácter hidráulico, dotando al sistema un notable incremento de
su resistencia mecánica y aminorando el tiempo de fraguado [34,37,38,50]. A modo de
ejemplo, en el caso específico de morteros de inyección o de relleno, la presencia de
aditivos se hace indispensable para adquirir adecuada reología del mortero fresco que
posibilite su proyección, ya que dichos morteros deben fluir oportunamente durante su
aplicación, garantizando a posteriori, además de su estabilidad de volumen, un oportuno
fraguado y posterior durabilidad [53,56].
Por último, cabe destacar los aditivos fotocatalizadores, compuestos entre los que
sobresale de manera clara el TiO2, son generalmente semiconductores basados en óxidos
de los elementos de transición, que mediante la acción de luz (en el caso del TiO2 en el
espectro ultravioleta), permiten la descomposición/oxidación química de contaminantes
y depósitos de materia orgánica facilitando su eliminación [63]. Además, estos aditivos
muestran eficacia biocida, evitando la colonización biológica sobre los morteros, tanto de
algas, como por ejemplo de líquenes o cianobacterias [73].
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16
3.2. Tipos de aditivos empleados en morteros de cal
En este apartado se describen los diferentes tipos de aditivos empleados en los
morteros de cal, su uso, aplicación y así como las propiedades que modifican, utilizando
como base la clasificación realizada por Izaguirre [74]. Dicha clasificación considera los
grupos de aditivos más frecuentemente descritos en la bibliografía o que han tenido mayor
importancia. La mayoría constituyen una categoría propia y, como se detallará después,
han sido utilizados en esta tesis doctoral, en que se discutirán los siguientes tipos de
aditivos:
• Superplastificantes (reductores de agua)
• Agentes puzolánicos
• Hidrofugantes
• Aditivos fotocatalíticos
• Incrementadores de la viscosidad
• Mejoradores de adherencia
3.2.1. Superplastificantes (reductores de agua)
Estos aditivos permiten reducir la necesidad de agua de amasado (si se quiere
mantener similar consistencia) o aumentar la fluidez del material (si se quiere mantener
la cantidad de agua de amasado). Estos aditivos pueden ser útiles para morteros de cal ya
que su acción reductora de agua puede evitar el exceso de agua de amasado, lo que puede
ser beneficioso para el proceso de carbonatación y el desarrollo de la resistencia mecánica
final [26]. Existen algunas evidencias de que estos aditivos pueden disminuir en los
morteros de cal su largo tiempo de fraguado e incrementar su resistencia mecánica
[75,76]. Además, un menor contenido de agua puede representar una menor contracción
por secado y una menor porosidad asociada con la absorción de agua (poros capilares), lo
que puede reducir la susceptibilidad a la degradación de los morteros de cal. En estado
fresco, la mayor fluidez que se puede conseguir con el uso de aditivos reductores de agua
puede mejorar la aplicación de estos morteros, en particular de inyección o grouts, y su
adherencia al soporte.
Introducción
17
Figura 2. Esquema del mecanismo de acción de los superplastificantes
Los superplastificantes consisten en agentes tensioactivos o tensioactivos
aniónicos que se adsorben en las partículas de la cal, aportando frecuentemente carga
superficial negativa. Esta carga electrostática conduce a la repulsión entre las partículas
(repulsión electrostática) y, por tanto, a su dispersión. Dado que las fuerzas de atracción
entre las partículas se reducen, es más fácil superarlas para que el material fluya
(reducción del límite elástico), razón por la cual estos aditivos son responsables de un
aumento de fluidez, cuando el agua de amasado se mantiene igual [77]. La diferencia
entre un plastificante y un superplastificante es la reducción de agua: mientras que los
primeros conducen a una reducción de agua entre un 5 y un 10%, los segundos permiten
reducciones de agua de hasta un 40% [77].
También se conoce que estos aditivos pueden ejercer su papel a través de
impedimento estérico entre las partículas, incluso con la formación de multicapas, con lo
que el efecto se amplía [69,78]. La Fig. 2 muestra un esquema de la posible interacción
de los superplastificantes con las partículas del mortero de cal.
Los superplastificantes más utilizados en la actualidad son los basados en
lignosulfonato (LS), policondensados de sulfonato de naftaleno (PNS), sulfonato de
melamina formaldehído (SMF) y éteres de policarboxilato (PCE) [50,75,79,80].
• Lignosulfonatos (LS)
Estos plastificantes son un polímero natural que se deriva del procesamiento de la
madera. La lignina de la pulpa de madera se elimina mediante una reacción de sulfito y
luego se procesa antes de usarse para aditivos Los lignosulfonatos empleados como
reductores de agua son principalmente cálcicos y sódicos. La molécula básica (Fig. 3). es
un fenilpropano sustituido, que contiene grupos hidroxil, carboxil, metoxi y ácido
sulfónico. El polímero final no se dispone linealmente, sino que forma esferas situando
sus cargas en la superficie exterior.
Introducción
18
Figura 3. Lignosulfonato
Los lignosulfonatos más solubles son los sódicos, lo que es de utilidad ya que evita
la sedimentación a bajas temperaturas. En sistemas conglomerantes con cal, el LS
aumenta considerablemente la fluidez de las muestras además de que puede formar
complejos de Ca2+ que producen moléculas de LS "libres" en la suspensión lo que provoca
que estas moléculas generen un fuerte efecto estérico que evita la floculación, explicando
de esta manera el efecto plastificante de este aditivo. Al mismo tiempo, la complejación
de Ca2+ dificulta la carbonatación de los morteros de cal. En el estudio realizado por Pérez
et al en 2016, este aditivo −a una dosis de 1% con respecto a la masa de cal− alcanzó un
valor de trabajabilidad de aproximadamente 1200 minutos, provocando un retraso en el
tiempo de fraguado y menores resistencias mecánicas, por lo que este plastificante debe
ser usado en porcentajes menores a este. También a través de un ensayo de potencial zeta
se observó que este aditivo es adsorbido por la nanosílice y completa una adsorción en
dos capas en el sistema de cal. La primera capa se formó debido a la adsorción del
plastificante sobre las partículas de la cal, lo cual generó una disminución continua del
potencial zeta por la carga negativa del LS; posteriormente, debido a la formación de esa
monocapa, y al exceso de cationes Ca2+ que apantallaron a esas moléculas adsorbidas de
LS, se observó una sobrecarga y un aumento pronunciado hacia valores más positivos.
Por último el LS se volvió adsorber en forma de una segunda monocapa, reflejado en la
disminución continua del potencial zeta a partir de unos 7 mL de titulante añadidos, como
se muestra en la Fig. 4 [75].
Introducción
19
Figura 4. Figura tomada del trabajo de Pérez et al. donde se muestra la curva de
potencial zeta de pastas de cal tituladas con una solución acuosa de LS al 1%[75]
• Policondensados de sulfonato de naftaleno–formaldehído (PNS)
El sulfonato de naftaleno formaldehído (Fig. 5) se sintetiza mediante reacciones
químicas sucesivas a partir del naftaleno. Pueden obtenerse pesos moleculares muy
diversos, siendo los de mayor peso los que se consideran más efectivos. Este
superplastificante puede ser útil para mejorar los morteros de cal, ya que promueve un
aumento de la resistencia mecánica incluso en edades tempranas, y mantiene la estructura
porosa similar a la de un mortero de cal pura. Al no alterar las propiedades de un mortero
de cal, la compatibilidad con materiales antiguos puede mantenerse potencialmente,
evitando daños prematuros y la consiguiente necesidad de reparación [79].
En el trabajo de Pérez et al. [75] se comprobó que este superplastificante limitaba
la formación de fases CSH, advirtiéndose una reducción en el rango de poros entre 0.1 y
0.01 m, que son los poros atribuidos a la estructura gelificada de CSH. Además, debido
a su arquitectura molecular y a su elevada carga aniónica (2,44 meq de carga aniónica / g
de polímero), tiene una adsorción plana sobre partículas de cal o de cemento, a diferencia
del LS, que se adsorbe de manera perpendicular. También al realizar el ensayo de
potencial zeta se observó un comportamiento de adsorción en multicapa, que debido a la
fuerte carga aniónica y a la adsorción plana generó el perfil indicado en la Fig. 6 (patrón
en dientes de sierra).
Figura 5. Sulfonato de melamina formaldehído (PNS)
Introducción
20
Figura 6. Figura tomada del trabajo de Pérez et al. donde se muestra la curva de
potencial zeta de pastas de cal tituladas con una solución acuosa de PNS al 1% [75]
• Sulfonato de melamina formaldehído (SMF)
El sulfonato de melamina formaldehído (Fig. 7) se obtiene a partir de la melamina,
mediante técnicas de resinificación. Dependiendo del proceso de polimerización, se
pueden obtener diferentes pesos moleculares, siendo 30.000 el orden considerado más
efectivo. Este superplastificante puede emplearse de manera aislada o en combinación
con PNS. Al ser utilizado de forma individual, produce un efecto mínimo de introducción
de aire o retraso en el fraguado. Este compuesto reduce la resistencia a la flexión de 4.2
a 2.9 MPa y a la compresión de 16.5 a 14.3 MPa [81], sin embargo, produce morteros con
valores suficientemente altos para competir con las prestaciones de un mortero
convencional. La adherencia superficial en el estudio donde se ha investigado ha sido
aceptable, con una interfaz de rotura cohesiva y valores aceptables para un mortero de
revoque. La capilaridad de los morteros de cal en los que se ensayó este plastificante
permite predecir una adecuada permeabilidad al vapor de agua, válida para la aplicación
de esos composites [81].
Figura 7. Sulfonato de melamina formaldehído (SMF)
Introducción
21
• Éteres de policarboxilato (PCE)
Los éteres de policarboxilato constituyen la tercera y más nueva generación de
superplastificantes, siendo más eficaces que los basados en SMF o PNS, debido a su
estructura molecular. Las moléculas de PCE tienen forma de peine, con un esqueleto
lineal principal con grupos carboxílicos y cadenas largas de grupos éter unidos (Fig. 8)
[58,76]. En este caso, los grupos carboxílicos ionizados son los responsables de la carga
negativa de estas moléculas y, por tanto, de la repulsión electrostática cuando se unen a
las partes superficiales cargadas positivamente de las partículas aglutinantes. Sin
embargo, las cadenas laterales, generalmente largas e hidrofóbicas, del polímero son
responsables de fuerzas repulsivas adicionales (obstáculo estérico), siendo este último el
mecanismo de dispersión dominante en este tipo de superplastificantes y la razón de su
mayor efectividad [58]. En los trabajos de Fernández et al. y de Silva et al. [50,79] se
comprobó que en las muestras que contienen PCE se redujo sustancialmente la demanda
de agua de amasado, se redujo el tiempo de fraguado y hubo un notable aumento de la
resistencia mecánica. Se advirtió también una disminución de la porosidad debido a una
fuerte reducción de los poros en el rango de 1 a 10 µm de diámetro y cambios drásticos
en la microestructura del mortero. El empleo de PCE a una dosis del 1% incrementó la
resistencia mecánica hasta un 161%. La pérdida de fluidez a lo largo del tiempo fue
moderada y también se observó una pequeña acción de incorporación de aire [79].
En el trabajo de Fernández et al. se comprobó a través del estudio de potencial
zeta y de microscopia óptica el papel que tiene este tipo de superplastificantes para
dispersar a las moléculas de cal (Fig. 9)[50].
Figura 8. Éter de policarboxilato
Introducción
22
Figura 9. Tomada del trabajo de Fernández et al. [50]
La Fig. 9 muestra fotografías de microscopía óptica de suspensiones de cal: a)
Suspensión de cal pura: las flechas blancas muestran grandes aglomerados, representados
como áreas oscuras, que van desde 50 a 100 μm. También se puede observar una gran
población de partículas de portlandita de 10 μm de tamaño (áreas oscuras). b) Suspensión
de cal pura a mayor aumento. Además de las partículas de 10 µm, se detectó una cantidad
significativa de partículas pequeñas de 0.3 µm y las áreas que abundan en estas partículas
pequeñas se indican mediante círculos blancos. c) Suspensión de cal con NS, que presenta
una estructura más densa, con grandes aglomerados de partículas. Las partículas más
pequeñas de 0.3 μm de tamaño casi han desaparecido. d) Microfotografía de suspensión
de cal–PCE, que muestra pequeñas partículas y ausencia de grandes aglomerados como
resultado de la fuerte acción dispersante del PCE.
3.2.2. Agentes puzolánicos
La adición de aditivos puzolánicos a los morteros de cal aérea es una práctica
común en la construcción, especialmente en el sector de la restauración, ya que esto
mejora las propiedades de los morteros de cal aérea tanto en estado fresco como
endurecido (por ejemplo, resistencia mecánica, permeabilidad al agua y durabilidad) [82–
Introducción
23
84]. Los materiales puzolánicos reaccionan a temperatura ambiente normal con el
hidróxido de calcio disuelto (Ca(OH)2) para formar compuestos de aluminato de calcio y
silicato de calcio que desarrollan fuerza. Se ha observado que las puzolanas conducen a
la formación de fases hidratadas como C–S–H, C–A–S–H y C–A–C–H [44,50,85]. La
literatura ha mostrado interés en la incorporación a matrices de cal aérea de materiales
con actividad puzolánica, como el metacaolín [84,86–88], con el fin de superar algunos
de los inconvenientes de este tipo de aglutinantes, especialmente los relacionados con sus
bajas resistencias mecánicas [26]. En este trabajo de investigación se han utilizado la
sílice de tamaño nano y micro–métrico que se han estudiado ampliamente en sistemas de
cemento [24,83], y también recientemente en ligantes de cal aéreos [54,58,75].
El metacaolín generalmente se procesa mediante la calcinación de arcilla de caolín
de alta pureza a temperaturas que oscilan entre 650 y 800 °C. Contiene sílice y alúmina
en forma activa que reaccionan con el hidróxido de calcio produciendo fases de silicato
de calcio hidratado (CSH), y también C2ASH8 y C4AH13 como, respectivamente, fases de
silicoaluminato de calcio hidratado y aluminato de calcio hidratado. El efecto de relleno
de MK y la producción de nuevas fases hidratadas proporcionan la mejora de varias
propiedades de los morteros y pastas a base de cal aérea, como su tiempo de fraguado o
resistencia a la compresión, y también reducen la microfisuración [33,53,87].
La microsílice (MS), generalmente compuesta por dióxido de silicio amorfo en
forma de polvo fino, es un producto del silicio y ferrosilicio, y se produce en las industrias
de fundición. Su componente químico es principalmente una gran cantidad de sílice activa
(SiO2), que básicamente tienen reacción puzolánica. después de mezclarse con la cal,
haciendo su mezcla compatible con materiales de construcción antigua, con un gran
potencial de aplicación [83].
La adición de nanosílice a un material aglutinante a base de cal se advirtió que
modificaba drásticamente la distribución de la porosidad debido al comportamiento
probado como nano–filler, que provocó una disminución de los poros en el rango de 20 a
100 nm. Entre las partículas de cal se intercalaron partículas de nano–sílice dando lugar
a una población enriquecida de poros gel (<10 nm), incluyendo el rango de microporos.
Estos dos hechos dieron como resultado una mejora en la resistencia mecánica en los
morteros de cal donde fue empleada y pueden tener propósitos prácticos relevantes para
mejorar la resistencia a la compresión de los morteros de cal aéreos [50,58].
Introducción
24
3.2.3. Hidrofugantes
El objetivo principal de estos aditivos es minimizar la absorción de agua por
capilaridad del mortero endurecido. Con estos aditivos el mortero no es totalmente
impermeable, sino que reduce su capacidad de absorción de agua a baja presión. Como
consecuencia de esta acción hidrofugante, se controlan las eflorescencias, se mantiene la
superficie más limpia y seca y se mejora la durabilidad del material frente a ciclos de
hielo–deshielo y de humectación–secado [60].
Los aditivos hidrofugantes actúan como agentes aireantes, por lo que su uso puede
conllevar un aumento en el porcentaje de aire ocluido, y, por ende, se acompaña de una
mejora en la trabajabilidad, una disminución en la densidad y explica la mayor
durabilidad frente a los ciclos de hielo–deshielo. Cuando los aditivos son partículas muy
finas, su incorporación puede aumentar la cantidad de agua requerida para obtener un
material trabajable, pudiendo disminuir las resistencias mecánicas y aumentar la
permeabilidad por un exceso de agua [60,89].
Se han propuesto tres mecanismos de acción que explican el impedimento en la
entrada de agua por capilaridad: i) las finas partículas de hidrofugante ocluyen los huecos
presentes en el material; ii) son capaces de colmatar los poros y la superficie del material
formando una fina película hidrofóbica; ya que el hidrofugante posee una estructura con
una parte polar y otra apolar, y iii) una acción combinada de ambos mecanismos. La Fig.
10 ilustra el mecanismo de formación de una película hidrofóbica, aunque en función de
la naturaleza del producto, la vía por la que se forma esta película es sustancialmente
diferente:
• Los ácidos grasos reaccionan con los productos de hidratación del cemento,
generando una capa protectora.
• Las emulsiones de cera sufren coalescencia al ponerse en contacto con el medio
alcalino del mortero, formando la película hidrofóbica.
• Los materiales hidrófobos finamente divididos tienen la capacidad de crear la
película gracias a su elevada superficie específica.
Introducción
25
Figura 10. Mecanismo de acción de los aditivos hidrófugos de masa
En definitiva, la formación de la película hidrofóbica depende de la naturaleza del
hidrofugante y permite su vinculación a cada uno de los tres mecanismos de formación
de película hidrofóbica expuestos, relacionados en la Fig. 11 e incluyendo productos más
comunes.
La influencia de los hidrofugantes en los morteros de cal se ha estudiado
someramente mediante compuestos orgánicos duales, con un resto polar (generalmente
un grupo carboxílico) y una cola hidrófoba, como el estearato de calcio y el oleato de
sodio [60,90,91]. Se destaca la importancia de la hidro–repelencia para minimizar la
absorción de agua por capilaridad donde la permeabilidad no se ve afectada. Además,
queda patente la prevención de la disolución de sales que da lugar las eflorescencias y
previene los severos daños mecánicos en la mampostería provocados por los ciclos de
congelación–descongelación. La hidrofobicidad impartida por los aditivos hidrófugos
mejora la resistencia a largo plazo de las lechadas [60].
Figura 11. Productos químicos hidrofugantes
Introducción
26
a) b) c) d)
Figura 12. a) Suciedad causada por deposición de material carbonáceo (Fuerte de San
Bartolomé, siglo XVIII, Pamplona, España; b) microalgas and c,d) líquenes y depósitos
de suciedad (Iglesia de San Miguel, siglo XII, Cizur, España)
3.2.4. Aditivos fotocatalíticos
Estos aditivos, usualmente semiconductores basados en óxidos de los elementos
de transición, mediante la acción de luz (en el caso del TiO2 en el espectro ultravioleta,
UV), permiten la descomposición/oxidación química de contaminantes y depósitos de
materia orgánica facilitando su eliminación [92,93]. Además, estos aditivos muestran
eficacia biocida, evitando la colonización biológica sobre los morteros, tanto de algas,
como por ejemplo de líquenes o cianobacterias [73].
La deposición de partículas atmosféricas, aerosoles o incluso la formación
irreversible de incrustaciones negruzcas (depósitos de partículas de carbón, muchas veces
sulfatadas), y, en general, depósitos de compuestos hidrocarbonados, así como la
aparición de colonización biológica, causan daños estéticos en los materiales
constructivos de las obras del Patrimonio Edificado (Fig. 12), y son una vía de inicio de
alteraciones severas, a veces irreversibles, en estos materiales (piedra y mortero,
fundamentalmente). Además, la aparición de estos depósitos obliga a elevados costes de
mantenimiento, eliminándolos mediante procesos de ablación por láser o por chorro de
arena, que pueden generar daños irreparables en la obra patrimonial [94,95].
En el caso de depósitos biológicos sobre el Patrimonio Edificado, debe tenerse en
cuenta que algas y cianobacterias están presentes en las superficies expuestas al exterior
y permiten la colonización sobre diversos sustratos, en función de su composición
química y de su estructura porosa, que afecta a la retención de agua sobre dichas
superficies. Posteriormente, tras la invasión de algas y cianobacterias surge el crecimiento
de líquenes y musgos, llegando a acumularse notables cantidades de materia biológica.
El biodeterioro en los materiales constructivos se debe a la producción de metabolitos
Introducción
27
(ácidos orgánicos, pigmentos y metabolitos ) que los dañan, incluso comprometiendo su
durabilidad [96]. Ciertos tratamientos disponibles en el mercado para prevenir la
colonización biológica no aseguran la protección a largo plazo y obligan a renovar la
aplicación cada cierto tiempo [73].
Por tanto, el uso de fotocatalizadores tiene un campo de aplicación especialmente
relevante, con objeto de reducir la suciedad y el depósito de contaminantes y de
microorganismos y sus consiguientes efectos perjudiciales en los morteros de cal. Por
acción de la luz, la reacción fotoquímica que tiene lugar en la superficie del
fotocatalizador permite la descomposición química y eliminación de los contaminantes,
además de destruir los enlaces formados entre los microorganismos y los sustratos
(piedras y morteros) [62,92,97].
En el caso del TiO2, la activación química se alcanza por luz ultravioleta (<387
nm), cuya incidencia genera en la estructura del semiconductor pares hueco positivo –
electrón (h+ e–) (Fig. 13). En presencia de agua (humedad), los huecos son atrapados por
los iones OH– o moléculas de H2O presente sobre las superficies y los electrones reducen
el oxígeno adsorbido dando lugar a la aparición de radicales fuertemente oxidantes como
el hidroxilo (OH–•), el hidroperóxido (HO2•) y el ion superóxido (O2−•), responsables de
la oxidación de las especies químicas adyacentes. Es necesario fijar o adsorber el
fotocatalizador para permitir su acción [92,98].
La elección de TiO2 como fotocatalizador se basa en su baja toxicidad, elevada
compatibilidad con materiales de construcción y gran actividad fotocatalítica en
comparación con otros óxidos metálicos [99–101].
Figura 13. Actividad fotocatalítica del TiO2 bajo radiación UV
Introducción
28
Las partículas contaminantes como los óxidos de nitrógeno (NOx) que se
encuentran presentes en la atmósfera, reaccionan con estos agentes oxidantes a través de
una cascada de reacciones (Ec. 7–13):
TiO2 + h → e− + h+ Ec. 7
H2O + h+ → H+ + OH• Ec. 8
O2 + e– → O2•− Ec. 9
NO + O2•− → NO3− Ec. 10
NO + OH• → HNO2 Ec. 11
HNO2 + OH• → NO2 + H2O Ec. 12
NO2 + OH• → NO3− + H+ Ec. 13
Cuando la luz UV irradia al TiO2, se forman los huecos par–electrón (Ec. 7).
Posteriormente las moléculas de oxígeno y agua presentes en el medio formarán radicales
oxidantes tras reaccionar con los electrones y los huecos positivos existentes en la
estructura interna del fotocatalizador (Ec. 8–9). Dichos radicales pasarán a oxidar las
especies de NO y NO2 presentes en el medio, que reaccionarán hasta producir iones nitrito
y nitrato (Ec 10–13) [102–104].
Investigaciones anteriores han reportado que la generación de radicales hidroxilo
mejora la actividad y eficiencia fotocatalítica [39,44]. Asimismo, se ha demostrado que
la anatasa es más activa como fotocatalizador que el rutilo debido al tipo de radicales OH·
que genera cada poliformo. La anatasa genera radicales móviles, mientras que el rutilo
solo es capaz de producir radicales a partir de las sustancias adsorbidas [105].
Cabe mencionar que existe la posibilidad de que, en ausencia de agentes aceptores
de electrones, el electrón excitado se recombine volviendo a su banda original. Esto afecta
al rendimiento cuántico de la reacción, así como a su eficiencia [106].
Los aglutinantes, como morteros, yesos o lechadas, se han explorado como
materiales capaces de alojar fotocatalizadores añadidos a granel. El uso de mortero de cal
presenta varias ventajas potenciales en cuanto al desarrollo sostenible de estos materiales:
se ha informado que su producción produce una menor huella ambiental, debido a un
Introducción
29
menor consumo de energía y emisiones de CO2 en comparación con el cemento [107–
109].
Además, en la literatura, las mezclas binarias de Cal–TiO2 debido al aumento de
la concentración de dióxido de carbono, resultante de la fotocatálisis (fotooxidación) de
los contaminantes orgánicos han exhibido una aceleración considerable en la tasa de
carbonatación, tanto en el laboratorio como en ambientes exteriores. La adición de TiO2
a los morteros de cal asegura que cuando se logra la fotocatálisis, se logra una mayor
concentración de CO2 en forma de gas en la superficie del material [62].
Si bien la fotocatálisis es un fenómeno de superficie, el CO2 producido se disuelve
en vapores de agua y se absorbe en la masa de mortero por capilaridad hasta
profundidades superiores a 2 mm. Además, la adición de TiO2 en toda la masa asegura el
potencial de descomposición de contaminantes orgánicos incluso después de la
degradación de la superficie inicial de los morteros [62].
Por tanto, está claro que la adición de TiO2 en los morteros de reparación a base
de cal es beneficiosa, ya que puede asegurar una mayor durabilidad frente a
contaminantes orgánicos, mejores características estéticas, mayor profundidad de
carbonatación y una velocidad de carbonatación acelerada [32,110].
3.2.5. Incrementadores de la viscosidad
La mayoría de estos aditivos son polímeros orgánicos hidrófilos solubles en agua,
siendo los más utilizados los basados en éteres de celulosa, a saber,
hidroxipropilmetilcelulosa (HPMC) e hidroxietilmetilcelulosa (HEMC), aunque el uso de
almidones y gomas ha aumentado en los últimos años [77]. Este tipo de aditivos actúan
fijando moléculas de agua en su estructura, reduciendo así la cantidad de agua libre en la
mezcla y provocando un aumento de la viscosidad. Además, las cadenas de polímeros
pueden sufrir un proceso de entrelazado y pueden adsorberse en partículas de aglutinante
vecinas, manteniéndolas físicamente juntas, lo que aumenta aún más la viscosidad Los
aditivos modificadores de la viscosidad se utilizan frecuentemente en morteros de
cemento para mejorar sus propiedades en estado fresco, especialmente en productos
premezclados, con consecuencias para sus propiedades de endurecimiento. Sin embargo,
el conocimiento sobre la influencia de estos aditivos sobre los materiales a base de cal es
aún incipiente. Cuando son usados en morteros de cal estos aditivos conducen a un
espesamiento de la suspensión de cal en estado fresco.
Introducción
30
Además, algunos de estos aditivos tienen la capacidad de retener agua dentro del
mortero, lo que puede ser útil para morteros de restauración ya que las mamposterías
antiguas están hechas con materiales altamente porosos que pueden absorber el agua del
mortero, deshidratarlo y dificultar la carbonatación [30]. Sin embargo, también se sabe
que tienen una acción retardadora del fraguado, y que muestran un fuerte comportamiento
dependiente de la dosis: pueden tener efectos espesantes o dispersantes según la dosis
utilizada [111–113].
Algunos estudios, como el de Seabra et al. [61], llegaron a la conclusión de que la
presencia de HPMC provocó inicialmente un efecto espesante seguido, luego de un
tiempo de agitación, por un efecto fluidificante debido a la alineación de las cadenas de
polímero en la dirección del flujo y la acción de incorporación de aire del aditivo.
Izaguirre et al. [30] compararon el efecto de dos de estos aditivos: uno a base de éter de
celulosa (HPMC) y otro a base de goma guar. En estado fresco, se necesitaron mayores
cantidades de agua para obtener la consistencia requerida en los morteros con estos
aditivos. Los autores concluyeron que la goma guar se comportó como espesante para
dosis de hasta 0.3% (del peso total de los morteros secos), es decir, condujo a una
disminución de los valores de flujo. Sin embargo, actuó como plastificante por encima de
ese valor [61,114].
Estos aditivos también influyen en las propiedades del estado endurecido. Por
ejemplo, la mejora de la trabajabilidad se puede atribuir a la acción de incorporación de
aire de estas sustancias que reduce la fricción entre las partículas. A su vez, estos vacíos
de aire pueden reducir la resistencia mecánica, lo que en los morteros de cal ya débiles
puede ser un problema; pero también pueden cortar la red capilar y, por tanto, reducir la
absorción de agua y mejorar la resistencia de los morteros a los ciclos de hielo– deshielo
y la cristalización de sales [30]. Con base en los resultados obtenidos, los autores
concluyeron que los éteres de celulosa no eran tan adecuados como los éteres de quitosano
o la goma guar para ser utilizados como aditivo modificador de viscosidad en morteros
de cal.
Esta acción potenciadora de la viscosidad podría ser de gran utilidad para mejorar
los morteros de cal aérea a efectos de revoque. Además, al usar estos aditivos, el
contenido de aire aumenta en los morteros, cambiando la distribución del tamaño de los
poros y dando lugar a algunos aspectos positivos, como una disminución de la absorción
de agua y una mejora de la durabilidad mediante ciclos de hielo–deshielo. Sin embargo,
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31
se deben tener en cuenta algunos efectos no deseables relacionados con su acción de
retención de agua, como el retraso en el tiempo de fraguado, ya que pueden afectar
negativamente su desempeño en los morteros de cal [25,30].
Su principal función en los morteros de cal es evitar la desecación rápida, mejorar
la hidratación y evitar fisuras por retracción. Estos aditivos como la goma de guar
presentan mayor cantidad de grupos ionizados a pH alcalino, debido a su gran densidad
de carga, reducen su capacidad de adsorción sobre partículas de Ca(OH)2. Los de esta
manera los grupos ionizados permiten aumentar la capacidad para la unión del calcio,
dando lugar a un incremento de la viscosidad a través de un fenómeno de reticulación
[115,116].
En la revisión de la literatura se señala al almidón de patata como posiblemente el
modificador de viscosidad más útil para los morteros de cal: además de poder mejorar sus
propiedades en estado fresco, fue capaz de incrementar las bajas resistencias mecánicas
propias de estos morteros y mantener sus propiedades físicas, minimizando así los
problemas de compatibilidad [61,114,117,118].
3.2.6. Mejoradores de adherencia
La adherencia del mortero de cal sobre un sustrato depende de la humedad y de la
porosidad abierta en la interfaz sustrato/mortero [28]. Algunos de los principales
problemas pueden ser: agrietamiento por tracción a través del espesor del mortero y
cizallamiento en la interfaz entre los dos materiales. El agrietamiento por secado de los
morteros de revestimiento depende en gran medida de las condiciones ambientales y del
soporte donde se aplica (rugosidad, porosidad, etc.). Si la absorción de agua del sustrato
es demasiado alta, el mortero secará rápidamente, lo que perjudica especialmente a los
aglutinantes hidráulicos ya que dificulta las reacciones hidráulicas. Como soluciones para
evitar este efecto, se puede humidificar del sustrato antes de aplicar el mortero o bien
emplear aditivos en el mortero que ayuden a controlar el secado y mejorar la adherencia.
Algunos aditivos ya han sido utilizados en el cemento Portland y el hormigón, y
pueden ser opción para los morteros de cal, como la metilcelulosa y el copolímero de
etileno–acetato de vinilo (EVA), para mejorar las adherencias [78,119]. La metilcelulosa
mejora la dispersión y estabiliza los productos de hidratación y contribuye a la resistencia
a la flexión reduciendo el daño por secado en un mortero [119]; y los EVA se pueden
formular como polvos redispersables aumentando la resistencia a la flexión porque los
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32
grupos activos en sus moléculas también pueden reaccionar con los cationes de los
productos de hidratación del cemento además de mejorar la adhesión entre los agregados
y la matriz del material cementoso, reduciendo el módulo de elasticidad del hormigón y
mejora su capacidad para absorber tensiones en condiciones de temperatura variable
[120–122].
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33
4. Interés del estudio sobre combinaciones de aditivos en
morteros de cal
Como se ha mencionado anteriormente, la mayor parte de la información
disponible se enfoca al estudio del efecto de un único aditivo, sin contemplar el posible
efecto conjunto o incluso sinérgico de las combinaciones más interesantes de dos o más
aditivos y/o puzolanas. Queda por desarrollar el estudio del comportamiento,
interacciones y el mecanismo de acción de diversas combinaciones de aditivos y/o
adiciones minerales puzolánicas que, para morteros de restauración del Patrimonio
Edificado revisten especial relevancia, y que desempeñarán funciones específicas en
restauración.
En particular, en esta memoria de Tesis Doctoral, se presta atención especial al
desarrollo mediante combinaciones adecuadas de aditivos de morteros de inyección,
morteros autolimpiantes y morteros de adherencia mejorada. Esta clasificación puede
ofrecer a los responsables de la intervención en edificaciones del Patrimonio tres gamas
de morteros mejorados de base cal mediante la inclusión de aditivos (incluyendo
puzolanas), con propiedades específicas de interés para acometer, con garantía de éxito,
diversos procesos de restauración de obras del Patrimonio Arquitectónico. Estas tres
gamas de morteros serán el objeto de estudio de este trabajo.
4.1. Morteros de inyección
La bibliografía ha detallado la posibilidad de incrementar en la preparación de
morteros de cal, las resistencias, acortar tiempos de fraguado y permitir el
endurecimiento, aun cuando el acceso del CO2 esté dificultado, mediante la inclusión de
puzolanas a las mezclas. El metacaolín ha sido uno de los aditivos más clásicamente
estudiados, aunque también la nanosílice ha sido objeto de algunas investigaciones.
Existen algunos trabajos que han comprobado la compatibilidad de diversas
combinaciones entre nanosílice y metacaolín con superplastificantes de tipo éteres de
policarboxilato [50], pero no se han realizado estudios de compatibilidad con otros
superplastificantes del mismo tipo y diferente peso molecular ni con otros habituales en
la química de los conglomerantes (lignosulfonatos, condensados de naftaleno, sulfonato
de melanina, ácido poliacrílico, todos ellos reductores de la cantidad de agua de amasado
y mejoradores de la trabajabilidad y muy ventajosos para su uso en morteros de inyección
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34
y de relleno) ni de ellos con agentes hidrofugantes, reductores de la absorción de agua
por capilaridad (entre los que cabe mencionar a los oleatos y estearatos). El empleo de
estos hidrofugantes o repelentes de agua es crucial con objeto de minimizar las vías de
entrada de agua a los morteros, y estas combinaciones entre cal – puzolana –
superplastificante – hidrofugante pueden dar lugar a una gama de morteros de enorme
utilidad en la restauración del Patrimonio, muy particularmente para morteros de
inyección o de relleno [53,57,60].
4.2. Morteros autolimpiantes
Otra de las combinaciones que aún no se ha estudiado y que es de gran importancia
es la que surge al obtener morteros de restauración con capacidad autolimpiante mediante
la inclusión de aditivos fotocatalizadores. Estos morteros podrán ser usados en
monocapas, revocos o rejuntados. La presencia de estos fotocatalizadores, muchas veces
nanoestructurados, obliga a una adecuada dispersión de estos por lo que la combinación
de aditivos fotocatalíticos con aditivos dispersantes o superplastificantes resulta
imprescindible. Por ejemplo, aditivos fotocatalíticos de tamaño nanométrico tienden a
aglomerarse, reduciendo los puntos de contacto activos del fotocatalizador y aumentando
la velocidad de recombinación hueco–electrón, por lo que es necesario un estudio en
profundidad de las interacciones de diversos fotocatalizadores con distintos aditivos
superplastificantes.
Además, debe tenerse en cuenta que, en presencia de aditivos fotocatalíticos como
el TiO2, la acción de autolimpieza se ha relacionado con una superhidrofilicidad
fotoinducida, es decir, la formación de una película acuosa en la superficie del material
tratado con fotocatalizador [123], que permite el paso de la radiación electromagnética
mientras disuelve algunos de los compuestos responsables de la suciedad que es
arrastrado.
Dado que el agua es un factor preeminente de deterioro, recientemente se ha
trabajado en recubrimientos en piedra con materiales hidrofóbicos [94,124]. También los
materiales autolimpiables se fundamentan en algunos casos en la superhidrofobicidad. En
este sentido, para recubrimientos en piedras del Patrimonio, se han avanzado algunas
combinaciones con olígomeros de dióxido de silicio, obteniéndose composites de SiO2
hidrofóbica con TiO2, ofreciendo resistencia a la penetración de agua y propiedades
autolimpiantes (self–cleaning) [125].
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35
En los morteros objeto de preparación y estudio, además del adecuado reparto del
fotocatalizador, las combinaciones de interés a estudiar deberían incluir también aditivos
hidrofugantes, de naturaleza hidrófoba y repelentes de agua, de manera que, en morteros
de cal, donde esta combinación no ha sido estudiada, se consiguiera un efecto
autolimpiante con una alta resistencia a la penetración de agua. Por supuesto, estas
combinaciones deben contemplar, además, la inclusión de las puzolanas mencionadas en
el primer bloque, para casos de interés en los que las resistencias mecánicas elevadas y/o
los cortos tiempos de fraguado sean críticos. Se debería, a su vez, estudiar con detalle la
compatibilidad entre la hidrofilicidad fotoinducida (y, por tanto, el mantenimiento del
efecto self–cleaning) y el aditivo hidrofugante.
Los compuestos fotocatalíticamente activos se plantea que sean incorporados
mediante adición en masa, durante la preparación del mortero en seco. La incorporación
en masa es muy frecuente, tanto en los productos comerciales basados en TiO2 como en
las referencias bibliográficas, ya que permite asegurar la eficacia a largo plazo, evitar
problemas de deterioro superficial del agente activo por abrasión, por ejemplo, y abarata,
en obra nueva, el procedimiento de incorporación del fotocatalizador [97].
4.3. Morteros de adherencia mejorada
Finalmente, otras combinaciones potencialmente interesantes de aditivos para
morteros de restauración con base cal que no se han estudiado hasta la fecha son las
mezclas de morteros incluyendo aditivos para la mejora de adherencia del mortero (tipo
emulsiones de estireno–butadieno o látex) aplicado junto con los otros modificadores de
la consistencia, como pueden ser modificadores de la reología (incrementadores de la
viscosidad, como el hidroxipropilguarán o el almidón) y con hidrofugantes. Estas mezclas
son extremadamente importantes para tareas de restauración que incluyan aplicaciones
en revocos, enlucidos o morteros de revestimiento, ya que la disposición en paramentos
verticales exige una alta adherencia sobre el sustrato además de una reología de la mezcla
en fresco adecuada, para evitar problemas como el escurrimiento del mortero, la
segregación de algunos de sus componentes o la fisuración. Algunos morteros de cal han
mostrado ciertas características no adecuadas en lo relativo a estas propiedades, por lo
que el estudio de combinaciones de estos aditivos (en mortero con de cal aérea pura o con
puzolanas) resulta de gran interés [60,61,114].
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36
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Objetivos
Objetivos
53
El objetivo principal de este trabajo es obtener morteros de cal aérea con
propiedades mejoradas mediante la incorporación combinada de diferentes aditivos como
agentes puzolánicos (nanosílice, microsílice o metacaolín), superplastificantes (éteres de
policarboxilato, lignosulfonato, condensado de naftaleno–formaldehído y sulfonato de
melamina), hidrofugante (oleato sódico), fotocatalizador (TiO2), aditivo incrementador
de la adherencia (copolímero de etileno–acetato de vinilo, EVA) y modificador de la
reología (almidón). Diferentes combinaciones múltiples de estos aditivos se aplicarán a
matrices de cal aérea para obtener tres gamas de morteros, especialmente enfocados a la
restauración de edificaciones del Patrimonio Cultural.
La primera gama la configuran morteros de inyección de elevada resistencia y
durabilidad; la segunda gama a desarrollar la componen morteros de cal con aditivos
fotocatalíticos de capacidad autolimpiante; y la tercera es una gama de morteros de cal
con adherencia mejorada sobre sustratos, para su aplicación en paramentos verticales
como revocos, enlucidos o monocapas. La investigación sobre las tres gamas de morteros
incluye el estudio de la combinación y diferentes dosis de aditivos, el comportamiento en
estado fresco y endurecido de estos materiales y su aplicación.
Objetivos
Por tanto, los objetivos específicos del trabajo son:
1. Optimizar y obtener morteros de inyección de cal aérea, con propiedades de
elevada resistencia y durabilidad, mejorando su fluidez e inyectabilidad, mediante
la incorporación combinada de puzolanas (metacaolín y microsílice),
superplastificantes (éteres de policarboxilato (PCE), lignosulfonatos (LS),
condensados de naftaleno–formaldehído (PNS) y sulfonato de melamina (SMF))
y un aditivo hidrofugante (oleato sódico).
2. Estudiar morteros de inyección de cal aérea mediante medidas de tiempo de
fraguado, fluidez, inyectabilidad, resistencias mecánicas a compresión a
diferentes tiempos de curado, capacidad de hidro–repelencia, distribución de
tamaño de poro y microestructura. Analizar el mecanismo de acción y la
compatibilidad entre los aditivos utilizados mediante isotermas de adsorción y
medidas de potencial zeta.
3. Preparar una gama de morteros de cal aérea con capacidad autolimpiante,
mediante el uso de un aditivo fotocatalizador nanoestructurado (TiO2), mejorando
Objetivos
54
la eficacia fotocatalítica y de autolimpieza reduciendo la aglomeración del TiO2 y
ralentizando la recombinación hueco positivo–electrón mediante la incorporación
de, agentes dispersantes (superplastificantes). Mejorar las prestaciones mecánicas
y de durabilidad de estos morteros a través de una adición puzolánica (nanosílice)
y un agente hidrofugante (oleato sódico) que reduzca la penetración de agua.
4. Analizar en morteros de cal aérea con capacidad autolimpiante la mejora en la
actividad fotocatalítica en función del dispersante empleado y la compatibilidad
entre aditivos, en particular el mantenimiento de la hidrofilicidad fotoinducida.
Evaluar la capacidad fotocatalítica estudiando la degradación de óxidos de
nitrógeno y la de autolimpieza mediante la degradación de colorantes.
5. Desarrollar morteros de cal aérea con adherencia mejorada sobre sustratos, para
su aplicación en paramentos verticales como revocos, enlucidos o monocapas,
mediante el estudio de compatibilidad y la combinación de aditivos
incrementadores de la adherencia (copolímero de etileno–acetato de vinilo, EVA),
modificadores de la reología (almidón), hidrofugante (oleato sódico) y adiciones
puzolánicas (metacaolín y nanosílice).
6. Evaluar la adherencia y el comportamiento de estos morteros sobre diversos
sustratos (caliza, arenisca, granito y ladrillo). Valorar la compatibilidad en estado
fresco entre los diversos aditivos y la durabilidad de los morteros tras someterlos
a diversas condiciones de envejecimiento acelerado: ciclos de hielo–deshielo y
ataque de sulfatos.
Material y Métodos
Material y métodos
57
1. Materiales empleados
1.1. Materiales generales
En este apartado se describen los materiales que han sido utilizados para llevar a
cabo la experimentación de las distintas gamas morteros preparadas. Posteriormente se
incluirán los diferentes materiales o aditivos específicos para cada una de ellas.
1.1.1. Cal
En las tres gamas de mortero se ha utilizado cal aérea apagada suministrada por
Cal Industrial S.A. (Navarra, España), con una clasificación CL–90 de acuerdo a la
normativa española y europea, con un tamaño de partícula medio de 10 μm y una fracción
> 50 μm inferior al 10% [1].
1.1.2. Árido
El árido de naturaleza caliza empleado fue suministrado por CTH (Navarra,
España). Proviene del procesamiento físico de dicho material con una composición global
de 52.83% (CaO), 2.28% (MgO), 1.14% (sesquióxidos de Fe2O3 y Al2O3), 0.57% (SO3),
0.49% (SiO2), 0.07% (Na2O), 0.05% (K2O) y una pérdida de ignición de 43.10%. Además
contiene un gran porcentaje de finos, representados por su granulometría mostrada en la
Fig. 1[2,3].
Figura 1. Tamaño de partícula del agregado calcáreo
Material y métodos
58
1.1.3. Hidrofugante
Se empleó oleato sódico suministrado por ADI–center S.L.U. (Barcelona, España)
bajo el nombre comercial HISA–A 2388 N®. Se contrastó la composición química y su
estructura de cadena apolar hidrocarbonada y grupo carboxílico polar en su extremo final
referida por el fabricante mediante comparación con los correspondientes productos puros
adquiridos a través de Sigma–Aldrich [4].
1.1.4. Aditivos puzolánicos
• Nanosílice (NS)
Este producto fue suministrado Ulmen Europa S.L. (Castellón, España) en forma
de suspensión al 28% con un pH de 9,68. El tamaño medio de las partículas esféricas de
nanosílice es de aproximadamente 50 nm con una superficie específica de 500 m2g−1
[5,6].
• Microsílice (MS)
Suministrada también por Ulmen Europa S.L. (Castellón, España) con un tamaño
medio de partícula en suspensión acuosa de aproximadamente 380 µm. De acuerdo con
el proveedor, las partículas de microsílice de forma esférica tienen al menos un 85% de
SiO2, con bajo contenido de carbono (Fig. 2) [7].
Figura 2. Apariencia física de la microsílice
Material y métodos
59
• Metacaolín (MK)
Se empleó el producto comercial Metaver® suministrado por Newchem AG
(Pfäffikon, Suiza), con un área de superficie específica de 20.00 m2g−1 y un tamaño de
partícula medio de 3.9 µm (Fig. 3)[8].
Figura 3. Apariencia física del metacaolín
1.1.5. Agua
Todos los morteros fueron preparados utilizando agua de consumo público de la
red de la Mancomunidad de la Comarca de Pamplona.
1.2. Aditivos específicos de la Gama 1: morteros de inyección de cal
con elevada resistencia, durabilidad y buena fluidez
1.2.1. Aditivos superplastificantes
• Sulfonato de naftaleno (PNS)
El plastificante utilizado, sal del ácido naftalen–sulfónico condensado (Fig. 2),
corresponde con el producto comercial Conplast SP430 Polvo suministrado por Fosroc
Euco S.A. (Izurtza, España) [7].
Figura 2. Estructura del sulfonato de naftaleno
Material y métodos
60
• Lignosulfonato (LS)
Se utilizó el producto comercial Lignin DS10 suministrado por Fosroc Euco S.A.
(Izurtza, España) (Fig. 3) [8].
Figura. 3. Estructura del lignosulfonato
• Éter de policarboxilato (PCE)
Se empleó el producto comercial Melfulx®, suministrado por la compañía BASF
Española S.L. (Tarragona, España) Es un policarboxilato sintetizado que consta de una
cadena principal lineal con grupos carboxilato y éter laterales en forma de estrella (Fig.
4) [5].
Figura. 4. Estructura del éter de policarboxilato
SO3-Na+
SO3-Na+
SO3-Na+
Material y métodos
61
• Condensado de melamina–formaldehído sulfonato (SMF)
Se utilizó el producto comercial Melment F10 suministrado por la empresa BASF
(Ludwigshafen, Alemania) (Fig. 5) [7].
Figura. 5. Estructura del condensado de melamina–formaldehído sulfonato
1.3. Aditivos específicos de la Gama 2 de morteros de cal con
capacidad autolimpiante
1.3.1. Aditivos superplastificantes
Cabe mencionar que se han utilizado adicionalmente al éter de policarboxilato
mencionado anteriormente y el sulfonato de naftaleno de la gama anterior, los siguientes
superplastificantes proveídos por el profesor Plank de la Universidad Técnica de Múnich
(TUM), Múnich, Alemania:
• 23APEG
Se trata de un compuesto que contiene macromonómeros de α–alil–ω–metoxi
polietilenglicol, compuestos de 23 unidades de óxido de etileno y una cantidad
equivalente de anhídrido maleico [9].
Material y métodos
62
Figura. 6. Estructura del superplastificante 23APEG
• 45PC6
Este es un polímero compuesto por un macromonómero de un éster de ω–metoxi
polietilenglicolmetacrilato con 45 unidades de óxido de etileno y de ácido metacrílico con
una relación molar 1:6 (Fig. 6) [9].
Figura. 7. Estructura del superplastificante 45PC6
• 52IPEG
Este polímero se basa en la copolimerización por radicales libres de ácido acrílico
y macromonómeros de isoprenil –hidroxipolietilenglicol. En su estructura con 52
unidades de óxido de etileno y la relación de ácido acrílico e isoprenil oxi poli
(etilenglicol) es 5,8. (Fig. 8) [9].
Material y métodos
63
Figura. 8. Estructura del superplastificante 52IPEG
1.3.2. Aditivo fotocatalítico: TiO2
Para esta gama se emplearon nanopartículas de titania suministrada por Evonik
Industries (Alemania) bajo la denominación Aeroxide® P25, la cual tiene una apariencia
física de un polvo blanco y muy fino (Fig. 9) [10].
Figura 9. Apariencia física del TiO2
1.4. Aditivos específicos de la Gama 3 de morteros de cal de reología
controlada y adherencia mejorada
1.4.1. Modificador de reología
Se utilizó un derivado soluble del almidón de patata, de marca comercial Casaplast
KO09 de la casa comercial Nova Casanova (Barcelona, España), eterificado con alto
grado de sustitución. De acuerdo con el fabricante tiene un carácter no iónico lo que le
permite una alta compatibilidad sistemas de alto contenido en iones bivalentes como el
calcio y el magnesio. Así, por tanto, se aplica como aditivo para lograr cierta viscosidad
Material y métodos
64
y/o fluidez en pastas o con función espesante para morteros de cemento y yeso, que no
aporta pegajosidad proporcionando muy buena trabajabilidad [11].
1.5. Potenciador de la adherencia
El aditivo empleado es Elotex MP 2080 proveído por Celanese (Tarragona,
España), es un polvo redispersable en agua que contiene acetato de vinilo y copolímeros
de etileno–vinil–acetato (EVA). Posee propiedades hidrofóbicas aptas para morteros de
cal o cemento. La adición de etileno–acetato de vinilo (EVA) al mortero aumenta la
resistencia a la flexión porque los grupos activos de sus moléculas también pueden
reaccionar con los productos de hidratación para mejorar la estructura física del mortero.
Especialmente recomendado para uso en exteriores donde se requiere alta hidrofobicidad
y baja absorción de agua [12].
Material y métodos
65
2. Caracterización de los materiales
La caracterización de los materiales anteriores se llevó a cabo utilizando las
siguientes técnicas:
2.1. Difracción de Rayos X (XRD)
Para poder determinar las fases cristalinas, y realizar la caracterización tanto de
materiales empleados como de las muestras resultantes, se utilizó un equipo Bruker D8
Advance Eco, de 50 kV, 60 mA y 1 kW de tensión, intensidad de corriente y potencia
máximas, respectivamente. La radiación la suministró un tubo con anticátodo de cobre.
Se estableció que la medida fuese desde 2 hasta 80º 2, con un incremento de 0.02º (2)
y una duración de 1s/step (40 KV y 30 mA). Para el análisis de los difractogramas
obtenidos se utilizó el software de la empresa Bruker llamado DIFFRACplus EVA®
utilizando la base de datos ICDD para realizar su comparación (Fig. 10). [6].
Figura 10. Difractómetro de Rayos X
2.2. Isotermas de adsorción gas–sólido
Para el ensayo de superficie específica se realizaron medidas de isotermas de
adsorción de N2 a 77K, en un equipo ASAP 2020 de Micromeritics, con el software ASAP
2020 V3.01, en el cual se determinó la superficie específica de los distintos aditivos
mediante el método BET.
Material y métodos
66
2.3. Determinación de potencial zeta
Para poder determinar la carga superficial de los diferentes aditivos, así como su
interacción con las partículas de cal se empleó el instrumento ZetaProbe de Colloidal
Dynamics (Fig.11) que determina el potencial zeta basado en una señal electroacústica y
permite la realización de medidas en sistemas muy concentrados, así como llevar a cabo
titulaciones con diversos agentes.
Figura 11. Medidor de potencial zeta
2.4. Determinación de tamaño de partícula
Para conocer la distribución de tamaño de partículas en suspensiones alcalinas
(1% peso/peso) se usó el instrumento Nanozeta Sizer de Malvern.
2.5. Espectroscopía IR
Se utilizó un espectrómetro Shimadzu IRAffinity–1S, con accesorio MKII Golden
Gate de atenuancia total reflejada y software OMNIC ESP. Las medidas se verificaron
con una resolución empleada de 4 cm–1, estableciendo como temperatura de trabajo 20ºC
y los espectros obtenidos fueron resultados de un promedio de 100 barridos en un rango
de 4000–600 cm−1.
2.6. Determinación de adsorción mediante carbono orgánico total
(TOC)
En la gama 2 para conocer la adsorción de superplastificantes sobre TiO2 se
realizó un experimento de adsorción por lotes. Se prepararon cinco muestras de referencia
con 10 mg de cada SP y 5 suspensiones con la misma cantidad de SP más 500 mg de TiO2
y se completaron hasta un volumen final de 50 mL. Las muestras se agitaron
mecánicamente durante 30 min para alcanzar el equilibrio de adsorción y posteriormente
Material y métodos
67
se centrifugaron durante 2 horas a 8000 rpm en un equipo Wobbler Heraeus Biofuge
Stratos. A continuación, se tomó el sobrenadante y se determinó el carbono orgánico total
(TOC) en un analizador de carbono orgánico total TOC–L Shimadzu. Se calculó la
cantidad adsorbida de superplastificante, así como la diferencia entre el contenido de TOC
de las muestras de referencia y el contenido de TOC del sobrenadante de las suspensiones.
Material y métodos
68
3. Preparación y estudio de las mezclas
3.1. Dosificación
En la etapa experimental y para las tres gamas de mortero se mantuvo una relación
de cal: árido 1:1 en masa, equivalente a 1:3 en volumen. Se ha utilizado esta relación
debido a que ya ha sido utilizada con éxito en trabajos previos [7,8].
Para las gama 1 se ajustó el agua al 31 % relación agua/cal y para la 2 se optó por
fijar la proporción agua al 28 % relación agua/cal, y de esta forma se pudo observar el
efecto que tuvo cada uno de los superplastificantes utilizados y se estudió el efecto sobre
la fluidez y la dispersión del aditivo fotocatalítico, respectivamente.
En la gama 3 se fijó el criterio de la consistencia (medida indirecta de la
trabajabilidad del material) para fijar la cantidad de agua necesaria para la mezcla. El
diámetro de escurrimiento en la mesa de sacudidas fue de 145 10 mm.
En cada capítulo de este trabajo se detallarán las diferentes dosificaciones de los
aditivos utilizados en cada caso.
3.2. Mezcla y amasado
Cada uno de los componentes se pesó por separado y se mezclaron en seco.
Posteriormente, se pesó el agua necesaria en cada caso, se incorporó a la mezcla anterior
y se introdujo en la amasadora.
Primero se pesaron los componentes: la cal, la arena y el agua en una balanza
METTLER PC 4000 y posteriormente en una balanza METTLER PC 440 se pesaron los
aditivos.
Acto seguido los componentes en seco se mezclaron utilizando una mezcladora
de sólidos BL–8–CA de Lleal S.A. (Fig. 12), que consta de un tambor principal y un
intensificador que giraron durante cinco minutos, asegurando de esta forma una buena
homogeneización la mezcla; posteriormente, se realizó el mezclado con agua en una
amasadora planetaria ajustada a la norma EN 196–1 [13] IBERTEST IB32–040E durante
90 segundos a velocidad lenta.
Material y métodos
69
Figura 12. Mezcladora de sólidos
3.3. Elaboración de las probetas
Para la gama 1 se realizaron probetas prismáticas de 40x40x160 mm en moldes
tripes de acero PROETI C0090, para las otras dos gamas se elaboraron probetas
cilíndricas de 40 mm de diámetro y 36 mm de altura. Se realizó el llenado en dos capas y
se empleó para compactar cada capa la compactadora IBERTEST iB32–045E–1
automática que proporcionó 60 golpes por capa con una frecuencia de un golpe por
segundo, para eliminar las burbujas de aire presentes en la mezcla, siguiendo la norma
UNE–EN 196–1 [13]. Para finalizar se enrasó con una regla y se eliminó el exceso de
masa de mortero y todas las probetas se desmoldaron a las 24 horas. Posteriormente se
dejaron curar a 20º C y 60% H.R.
3.4. Ensayos del mortero fresco
3.4.1. Determinación de la consistencia (mesa de sacudidas)
Este ensayo consistió en rellenar un molde troncocónico con el mortero fresco en
dos capas, compactando cada una de ellas con 10 golpes. Se enrasaron, se desmoldaron y
se les efectuaron 15 sacudidas con una frecuencia de un golpe por segundo y por último
se realizó la medida del diámetro en dos direcciones perpendiculares entre sí con un
calibre. Este prueba siguió los lineamientos establecidos en la norma EN 1015– 3 [14].
Para los morteros de la gama 1 y 2, el diámetro obtenido en este ensayo fue
considerado un dato en sí mismo (medida del escurrimiento o slump, que dio idea de la
fluidez de la masa), mientras que para la gama 3 este ensayo se tomó como referencia
Material y métodos
70
para determinar la cantidad de agua necesaria en cada caso para obtener el diámetro de
escurrimiento fijado: 145 10 mm.
3.4.2. Determinación de la densidad y el contenido de aire ocluido
Este ensayo se utilizó recipiente metálico con capacidad de un litro y una tapa
provista de una cámara de aire estanca y un manómetro. En primer lugar, se determinó la
densidad, por lo que se pesó el recipiente vacío, después se llenó con el mortero en fresco
y se volvió a pesar. De esta manera por diferencia de masas y conociendo el volumen del
recipiente se pudo calcular la densidad aparente de cada mezcla. Para la determinación
del contenido de aire se colocó la tapa, se cerró herméticamente el recipiente, se agregó
agua a través de la válvula de la tapa sobre la superficie del mortero considerando su
nivel. Con ayuda de presión de aire, se forzó la introducción del agua en el mortero,
desplazando de esta manera el aire contenido en los poros, lo que provocó una
disminución del nivel de agua con el cual se conoció el volumen de aire extraído del
mortero. Estos dos ensayos fueron realizados acorde con las normas EN 1015–6 [15] y
EN 1015–7 [16], respectivamente.
3.4.3. Determinación del periodo de trabajabilidad
Para esta prueba se llenó un molde cilíndrico vertiendo en él 10 capas de mortero,
sacudiendo 4 veces tras cada adición y tras su llenado se enrasó. Posteriormente el
recipiente fue colocado sobre una balanza y ésta fue tarada. Y como se muestra en la Fig.
13 se introdujo una sonda de penetración cada 15 minutos, se registró el peso y el tiempo
hasta que el peso fue superior a 1500 g, terminando de esta forma el ensayo. Este método
fue realizado de acuerdo con la norma EN 1015–9 [17].
Figura 13. Determinación del periodo de trabajabilidad del material
Material y métodos
71
3.4.4. Estudio del proceso de hidratación
Se ha utilizado un equipo TAM Air (Fig. 14) para determinar los cambios del
proceso de hidratación en las diferentes mezclas, por lo que se prepararon pastas de cal
de alrededor de 3 gramos, en unos recipientes de cristal con tapa metálica aptos para este
equipo, que se introdujeron durante al menos 24 horas a 25º C para obtener sus curvas de
calorimetría isotérmica.
Figura 14. Calorímetro isotérmico
3.4.5. Determinación de la capacidad de retención de agua
Para esta determinación se llenaron con mortero moldes cilíndricos previamente
pesados, se enrasaron y se volvieron a pesar para determinar la cantidad de mortero.
Posteriormente se colocaron dos capas de gasa fina de algodón y 8 discos de papel de
filtro previamente pesados, encima se colocó un disco de vidrio donde se colocaron 2 Kg
de pesas durante 5 minutos. Finalmente se desmontó el sistema y se pesaron las gasas y
los discos de papel para calcular el agua que éstos absorbieron y se calculó la retención
de agua de cada mezcla con los datos obtenidos. Para este ensayo se utilizó la norma
UNE 83–816–93 [18].
3.4.6. Evolución del extendido sobre diferentes superficies
Este ensayo se realizó extendiendo una capa de mortero de 15 mm de espesor
sobre la superficie de un arenisca, caliza, granito y ladrillo (Fig. 15), previamente lavados
y humedecidos y se estudió la evolución del mortero durante 1, 2, 7, y 30 días de fraguado,
con el fin de observar la aparición de fisuras, posibles descuelgues del mortero, faltas de
adherencia, etc.
Material y métodos
72
Figura 15. Sustratos utilizados para realizar la prueba de evolución del extendido
3.4.7. Inyectabilidad
Se llevó a cabo una prueba de inyectabilidad a presión constante en una columna
de metacrilato transparente con una altura de 390 mm y un diámetro interior de 21 mm,
sostenida verticalmente desde su parte inferior. La columna se llenó con material granular
(travertino) con una tamaño de partícula de 2–4 mm. Esta prueba es una adaptación de la
prueba de columna de arena (EN 1771: Determinación de la inyectabilidad mediante la
prueba de columna de arena [19]). Se utilizó una presión constante de 0.075 MPa para
inyectar el mortero durante 60 s y se registró el tiempo necesario para el llenado completo
de los cilindros (Fig. 16).
Figura 16. Sistema para ensayar la inyectabilidad del mortero
Material y métodos
73
Figura 16 (continuación). Sistema para ensayar la inyectabilidad del mortero
3.5. Ensayos del mortero endurecido
3.5.1. Determinación de la resistencia a compresión
Para determinar la resistencia a la compresión de las diferentes mezclas se utilizó
una prensa Frank/Controls 81565 con un dispositivo de rotura a compresión Proeti ETI
26.0052 y a una velocidad de rotura 5–50 Kp·s–1 con un intervalo de tiempo entre 30 y
90 segundos. Para la gama 1 se determinó este valor tras 28, 91, 182 y 365 días de curado,
para observar las posibles modificaciones con el tiempo; se rompieron 3 probetas, con el
fin de conseguir valores representativos. Para la gama dos y tres se realizó este ensayo a
28 y 91 días por triplicado.
Material y métodos
74
3.5.2. Estudio de la estructura porosa
Se utilizo un porosímetro de intrusión de mercurio (MIP) Micromeritics AutoPore
IV 9500 con un intervalo de presiones de 0.0015–207 MPa, que registró automáticamente
la presión, el diámetro de poro y el volumen de intrusión de mercurio, de esta manera se
pudo determinar la estructura porosa del material.
3.5.3. Estudio químico y mineralógico
Se estudió la composición de cada una de las muestras por medio de: difracción
de rayos X (descrito en el apartado 2.1) y espectroscopía IR (descrito en el apartado 2.5).
3.5.4. Análisis térmico
Para esta prueba se trabajó con un equipo simultáneo TGA–sDTA 851 Mettler
Toledo con muestreador automático Mettler Toledo TSO 801 RO y controlador de gases
Mettler Toledo TSO 800 GC1 conectado a un refrigerador JULABO FP 50, introduciendo
las muestras pulverizadas en crisoles de alúmina de 70 μL con tapa perforada. El
calentamiento se realizó desde 20 hasta 1000ºC, a una velocidad de 20ºC·min–1 bajo una
atmósfera de aire estático, nitrógeno como gas de purga (20 mL/min).
3.5.5. Estudio del ángulo de contacto
Se utilizó un instrumento de medición del ángulo de contacto OCA 15EC
Dataphysics (Fig. 17), con el cual se determinó el ángulo de contacto a través de 5 gotas
de 3 L de volumen de agua depositada sobre diferentes superficies de la muestra con el
fin de obtener una medida representativa, así como el tiempo para la absorción de esta
por el material. Cuando se estudió el efecto de la hidrofilicidad fotoinducida de la gama
dos se determinó el ángulo de contacto a 1, 2, 5, 8 y 30 minutos de radiación ultravioleta,
con la lampara OSRAM Vitalux de 300 W.
Material y métodos
75
Figura 17. Instrumento de medición del ángulo de contacto y algunos ejemplos de
muestras medidas
3.5.6. Estudio de actividad fotocatalítica
• Abatimiento de NO
Se llevó a cabo para los morteros de la gama 2, en un fotorreactor de forma
cilíndrica con 12 centímetros de altura y 14 centímetros de diámetro alimentado con un
flujo de NO de 500 ppb de concentración, que, tras pasar por el mismo, era analizado por
un detector quimioluminiscente (Environment AC32CM) que determina las
concentraciones de NO y NO2 en continuo (Fig. 18). Las condiciones experimentales
fueron de 50 10% H.R. y 25 2º C. Las muestras se introdujeron dentro del reactor y
se mantuvieron durante 10 minutos en oscuridad con el flujo de NOx atravesando el
reactor para conseguir una concentración estable de NO, posteriormente fueron
iluminadas por la lampara Osram Ultravitalux 300W durante 30 minutos en los cuales la
concentración de NO disminuía hasta un mínimo y finalmente se apagó la lampara y las
muestras se dejaron 10 minutos más dentro del reactor para permitir que los niveles de
concentración de NO recuperaran los valores iniciales.
Figura 18. Dispositivo de medida de NO
a)
b)
c)
d)
e)
f)
Material y métodos
76
• Test de autolimpieza: degradación de un colorante orgánico
Este estudio también fue realizado para la gama 2 y se utilizó una disolución de
rodamina B de concentración 1 mM, recubriendo con 3 capas la superficie con la ayuda
de un pincel. Se expusieron a una radiación UV–vis de la lampara Osram Ultravitalux
300W a una distancia de 20 cm. Mediante un espectrofotómetro Konica–Minolta CR–
300 se tomaron datos de la variación de color a las 5, 20, 80, 140 y 310 minutos midiendo
las tres coordenadas CIELab: L (luminosidad), a (rango entre rojo y verde) y b (rango
entre azul y amarillo) según la Commission Internationale de l’Eclairage [20], a partir de
las cuales se determinó la variación de color (C), con la ayuda de la ecuación[20]:
∆Cn=√[at
*– a0*]
2+[bt
*– b0*]
2
[aC* – a0
*]2+[bC
* – bt*]
2 Ec. 1
donde at* y bt
*son las coordenadas en el tiempo de irradiación t, mientras que aC
* y bC* se
miden en las piedras limpias antes de teñir con la rodamina B.
3.5.7. Estudio de la durabilidad
• Ciclos de hielo–deshielo
Se estudió esta durabilidad sometiendo a las muestras hasta 28 ciclos de dos
etapas: una primera durante la cual las probetas se sumergían en agua a temperatura
ambiente, y una segunda en la que se congelaban en un arcón CARAVELL 521–102 a
–10ºC.
• Resistencia al ataque de sulfatos
Las muestras se sumergieron completamente en una solución acuosa saturada de
MgSO4 a 20ºC y 95% HR durante 24 h. Después de este proceso, las muestras se secaron
en estufa a 65ºC durante 24 h y se sumergieron en agua durante 24 h a 20ºC y 95% HR.
Para concluir el ciclo, las muestras se secaron nuevamente como se describió
anteriormente. Los ciclos se repitieron continuamente hasta 28 ciclos o hasta la
destrucción total de las muestras.
Material y métodos
77
3.5.8. Estudio biocida
Para desarrollar este experimento, se utilizó una cepa ambiental de Pseudomonas
fluorescens. Se obtuvieron cultivos frescos de stocks a –80ºC almacenados en leche
desnatada al 10% y propagados en placas de medio de cultivo Luria Bertani (LB). El
crecimiento bacteriano en medio líquido se realizó en caldo LB en un horno a 37°C y con
agitación orbital (180 rpm). Para preparar el inóculo bacteriano, primero se obtuvieron
células frescas en una placa de agar LB cultivada durante 18 horas. Con estas células se
preparó una suspensión que se ajustó con solución salina estéril (NaCl al 0.9% en agua
destilada) a una densidad óptica de 0.04 m–1 a 600 nm, equivalente a 5.107 unidades
formadoras de colonias (UFC) por mL, aproximadamente. El día del experimento, los
cilindros se hidrataron durante 2 horas en LB y luego cada cilindro se inoculó en su
superficie superior con 200 L (microlitros) de la suspensión, equivalente a 1.106 CFU /
mL (es decir, un millón de UFC), aproximadamente. Después de la incubación en la
cámara durante 5 días a 37°C, la superficie superior de los cilindros se raspó de manera
homogénea con una espátula estéril y el material se resuspendió en 1 mL de solución
salina estéril. La cantidad de material desprendido de los cilindros (mg) se determinó
pesando el tubo antes y después de colocar el material del cilindro en el mismo. Después
de homogeneizar vigorosamente esta suspensión en un agitador mecánico, se determinó
el número de bacterias presentes en la suspensión mediante recuento de viables. Para ello,
se realizaron sucesivas diluciones de la suspensión en tubos que contenían suero salino
estéril y se transfirieron 50 µL a placas de agar LB que se incubaron a 37°C durante 48
horas.
Material y métodos
78
4. Metodología de Estudio
Se expone un esquema (Fig. 19) que resume la metodología de estudio básica
seguida para todas las gamas de mortero estudiadas. Los experimentos específicos
realizados, así como sus características y especificaciones para determinar el mecanismo
de acción de cada tipo de aditivo se detallan en los trabajos de investigación
correspondientes.
Figura 19. Diagrama de la metodología de estudio básico seguido
Material y métodos
79
Referencias
[1] European Committee for Standardization, UNE–EN 459–1:2016 Building lime
– Part 1: Definitions, specifications and conformity criteria, EN. (2016).
[2] J. Lanas, R. Sirera, J.I. Alvarez, Study of the mechanical behavior of masonry
repair lime–based mortars cured and exposed under different conditions, Cem.
Concr. Res. 36 (2006) 961–970. https://doi.org/10.1016/j.cemconres.
2005.12.003.
[3] J. Lanas, M. Arandigoyen, J.I. Alvarez, J.L. Perez Bernal, M. Angel Bello,
Mechanical Behavior of Masonry Repair Mortars: Aerial and Hydraulic Lime–
based Mixtures, in: Proc. 10th Int. Congr. Deterior. Conserv. Stone Stock. June
27–July 2, 2004, 2004.
[4] A. Izaguirre, J. Lanas, J.I. Álvarez, Effect of water–repellent admixtures on the
behaviour of aerial lime–based mortars, Cem. Concr. Res. 39 (2009) 1095–
1104. https://doi.org/10.1016/J.CEMCONRES.2009.07.026.
[5] J.M. Fernández, A. Duran, I. Navarro–Blasco, J. Lanas, R. Sirera, J.I. Alvarez,
Influence of nanosilica and a polycarboxylate ether superplasticizer on the
performance of lime mortars, Cem. Concr. Res. 43 (2013) 12–24.
https://doi.org/10.1016/j.cemconres.2012.10.007.
[6] I. Navarro–Blasco, M. Pérez–Nicolás, J.M. Fernández, A. Duran, R. Sirera, J.I.
Alvarez, Assessment of the interaction of polycarboxylate superplasticizers in
hydrated lime pastes modified with nanosilica or metakaolin as pozzolanic
reactives, Constr. Build. Mater. 73 (2014) 1–12. https://doi.org/10.1016/
j.conbuildmat.2014.09.052.
[7] J.F. González–Sánchez, B. Taşci, J.M. Fernández, Í. Navarro–Blasco, J.I.
Alvarez, Combination of polymeric superplasticizers, water repellents and
pozzolanic agents to improve air lime–based grouts for historic masonry repair,
Polymers (Basel). 12 4 (2020). https://doi.org/10.3390/POLYM12040887.
Material y métodos
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[8] A. Duran, J.F. González–Sánchez, J.M. Fernández, R. Sirera, Í. Navarro–
Blasco, J.I. Alvarez, Influence of two polymer–based superplasticizers (poly–
naphthalene sulfonate, PNS, and lignosulfonate, LS) on compressive and
flexural strength, freeze–thaw, and sulphate attack resistance of lime–
metakaolin grouts, Polymers (Basel). 10 8 (2018). https://doi.org/10.3390/
polym10080824.
[9] M. Pérez–Nicolás, J. Plank, D. Ruiz–Izuriaga, I. Navarro–Blasco, J.M.
Fernández, J.I. Alvarez, Photocatalytically active coatings for cement and air
lime mortars: Enhancement of the activity by incorporation of
superplasticizers, Constr. Build. Mater. 162 (2018) 628–648.
https://doi.org/10.1016/j.conbuildmat.2017.12.087.
[10] M. Pérez–Nicolás, J. Balbuena, M. Cruz–Yusta, L. Sánchez, I. Navarro–
Blasco, J.M. Fernández, J.I. Alvarez, Photocatalytic NOx abatement by
calcium aluminate cements modified with TiO2: Improved NO2 conversion,
Cem. Concr. Res. 70 (2015) 67–76. https://doi.org/10.1016/j.cemconres.
2015.01.011.
[11] A. Izaguirre, J. Lanas, J.I. Álvarez, Behaviour of a starch as a viscosity modifier
for aerial lime–based mortars, Carbohydr. Polym. 80 (2010) 222–228.
https://doi.org/10.1016/j.carbpol.2009.11.010.
[12] C. Shi, X. Zou, P. Wang, Influences of ethylene–vinyl acetate and
methylcellulose on the properties of calcium sulfoaluminate cement, Constr.
Build. Mater. 193 (2018) 474–480. https://doi.org/10.1016/j.conbuildmat.
2018.10.218.
[13] European Committee for Standardization, UNE–EN 196–1 Methods of testing
cement. Part 1: Determination of strength, EN. (2005).
[14] European Committee for Standardization, UNE–EN 1015–3:2000 Methods of
test for mortar for masonry —Part 3: Determination of consistence of fresh
mortar (by flow table), EN. (2006).
[15] European Committee for Standardization, UNE–EN 1015–6:1999 Methods of
Test for Mortar for Masonry. Part 6: Determination of Bulk Density of Fresh
Mortar, EN. (1999).
Material y métodos
81
[16] European Committee for Standardization, UNE–EN 1015–7:1999 Methods of
Test for Mortar for Masonry. Part 7: Determination of Air Content of Fresh
Mortar, EN. (1999).
[17] European Committee for Standardization, UNE–EN 1015–9:1999 Methods of
Test for Mortar for Masonry, Part 9: Determination of Workable Life and
Correction Time of Fresh Mortar, EN. (1999).
[18] European Committee for Standardization, UNE–EN 83–816–93. Test methods.
Mortars. Fresh mortars. Determination of water retentivity, EN. (1993).
[19] European Committee for Standardization, UNE–EN 1771:2005 Products and
systems for the protection and repair of concrete structures – Test methods –
Determination of injectability and splitting test, EN. (2005).
[20] L. Fornasini, L. Bergamonti, F. Bondioli, D. Bersani, L. Lazzarini, Y. Paz, P.P.
Lottici, Photocatalytic N–doped TiO2 for self–cleaning of limestones, Eur.
Phys. J. Plus. 134 539 (2019). https://doi.org/10.1140/epjp/i2019–12981–6.
Resultados y discusión
Capítulo I: Desarrollo de
morteros de cal de inyección
(grouts) Parte A. Polymer–based superplasticizers to prepare
lime–metakaolin grouts: mechanical performance
and durability assessment
Publicado en Polymers 2018, 10(8), 824
Parte B. Combination of polymeric superplasticizers,
water repellents and pozzolanic agents to improve air
lime–based grouts for historic masonry repair
Publicado en Polymers 2020, 12(4), 887
Capítulo I. Parte A
87
Polymer–based superplasticizers to prepare lime–metakaolin grouts:
mechanical performance and durability assessment
Adrián Duran 1, Jesús F. González–Sánchez 1, José M. Fernández 1, Rafael Sirera 1, Íñigo
Navarro–Blasco 1 and José I. Álvarez 1,*
1 Heritage, Materials & Environment MIMED Research Group, Departamento de Química, Facultad de
Ciencias, Universidad de Navarra, Irunlarrea, 1, 31008 Pamplona, Spain; [email protected] (A.D.);
[email protected] (J.F.G–S), [email protected] (J.M.F), [email protected] (R.S),
[email protected] (Í.N.–B.); [email protected] (J.I.A)
* Correspondence: [email protected] or [email protected] ; Tel.: +34948425600
Received: 5 July 2018 / Revised: 20 July 2018 / Accepted: 25 July 2018 / Published: 26 July
2018
Abstract
A new range of grouts prepared by air lime and metakaolin (MK) as a pozzolanic admixture
has been obtained by using as dispersing agents two polymers, namely poly–naphthalene
sulfonate (PNS) and lignosulfonate (LS), with the aim of improving the fluidity of the fresh
grouts. Fluidity and setting times of the grouts were assessed. Differences in the molecular
architecture and in the anionic charge density explained the different adsorption of the
polymers and the different performance. The higher anionic charge of PNS and its linear
shape explained its better adsorption and effectiveness. The pozzolanic reaction was
favoured in grouts with PNS, achieving the highest values of compressive strength (4.8 MPa
after 182 curing days). The addition of PNS on lime grouts slightly decreased the frost
resistance of the grouts (from 24 freeze–thaw cycles for the polymer–free samples to 19 or
20 cycles with 0.5 or 1 wt % of PNS). After the magnesium sulphate attack, grouts were
altered by decalcification of hydrated phases and by formation of hexahydrite and gypsum.
A protective role of portlandite against magnesium sulphate attack was clearly identified.
Accordingly, the polymer LS, which preserves a significant amount of Ca(OH)2, could be
an alternative for the obtaining of grouts requiring high sulphate attack resistance.
Keywords: lime–based grouts; metakaolin; polymer–based superplasticizers; freeze–thaw
cycles; magnesium sulphate attack
Capítulo I. Parte A
88
1. Introduction
Lime–based mortars play an important role in conservation and restoration procedures
thanks to their high compatibility with the raw materials employed in the artefacts comprising
the Built Heritage [1,2,3]. Grouts based in lime (either air or hydraulic lime) have, in the first
approach, an adequate chemical and mechanical compatibility with ancient supports, but may
need several additions in order to provide a suitable flowability to fill all the cracks and voids
[4,5,6]. In addition, these grouts should fulfil mechanical and durability requirements to
guarantee their safe applicability [7,8].
The addition of pozzolanic admixtures is a way of increasing both the final mechanical
strength and durability. Specifically, the utilisation of metakaolin (MK) as a pozzolanic
addition for mortar and concrete has received extensive attention in the last years. MK is
usually processed by calcination of high–purity kaolin clay at temperatures ranging between
650 and 800 °C [9]. It contains silica and alumina in an active form which react with the
calcium hydroxide (Ca(OH)2, CH) yielding hydrated calcium silicate (C–S–H) phases, and
also C2ASH8 and C4AH13 as, respectively, hydrated silicoaluminate and hydrated aluminate
phases [10,11]. The filler effect of MK and the production of new hydrated phases provide
the enhancement of several properties of air–lime based mortars and pastes, such as their
setting time or compressive strength, and also reduce microcraking [12].
To provide suitable injectability, polymeric additives (such as superplasticizers, SPs) can
be incorporated into the mixture of the fresh grout [12,13]. Superplasticizers enhance the
fluidity of the fresh grouts preventing particles from agglomeration, i.e., acting as dispersive
agents. In cement–based materials, water–soluble anionic polyelectrolytes, such as
polycarboxylate ethers, poly–naphthalene sulfonate (PNS), and lignosulfonate (LS) can be
quoted as the most widely used SPs. The chemical structure of the two latter SPs contains
hydrophilic (sulfonic groups in both, and also, methoxyl and hydroxyl groups in LS) and
hydrophobic parts (naphthalene for PNS and alkylbenzene for LS) [14]. The interaction
mechanisms of these polymers are related with the electrostatic and steric forces and also
with the adsorption onto surfaces [13,14,15,16,17]. The polymer molecules adsorbed onto
binder particles could be able to modify the surface charge (zeta potential) of the particles.
Zeta potential values exceeding the range of ±30 mV can lead to electrostatic repulsions
between particles avoiding their agglomeration. In addition, electrosteric repulsions between
these attached polymer molecules also contribute to the dispersing action [18,19,20]. PNS
has been described as a water–reducer agent more efficient than LS [21]. However, LS shows
Capítulo I. Parte A
89
a better plasticizing effect than PNS in some systems [14]. Many works dealt with the effect
of these polymers, PNS and LS, in cement systems [16,17,22,23,24,25,26], although there
are few articles regarding the performance of these superplasticizers within lime–based
mortars [14].
The composition and the relative proportions of each of the components of the grouts
affect the fresh as well as the microstructure and mechanical properties of the hardened
mortars [2]. This paper focuses on a new range of grouts prepared by air lime, MK as
pozzolanic admixture and a polymer–based SP (either PNS or LS). PNS and LS interactions
are proposed and fresh and hardened state properties are assessed. The long–term mechanical
resistance (compressive and flexural strength) was studied, as well as the durability of the
obtained grouts against freeze–thaw cycles and magnesium sulphate attack.
2. Materials and Methods
2.1. Materials
CL 90–S class slaked lime (ECOBAT Type) in powder form was used for making pastes
and grouts. Lime was provided by CALINSA (group Lhoist) (Tiebas, Spain). A limestone
aggregate with particle size lower than 2 mm was employed. Aggregate was supplied by CTH
(Huarte, Navarra, Spain) and its chemical composition was 52.83% (CaO), 2.28% (MgO),
1.14% (Fe2O3 + Al2O3), 0.57% (SO3), 0.49% (SiO2), 0.07% (Na2O), 0.05% (K2O), 43.10%
(ignition loss). The ratio lime/aggregate was 1:3 by weight. Metakaolin (MK, supplied by
METAVER, Pfäffikon, Switzerland) was used as pozzolanic admixture. The MK employed
had a specific surface area of 20 m·g−1, as measured by the BET method after N2 adsorption
isotherms (ASAP 2020, Micromeritics, Norcross, GA, USA) and an average particle size of
4.5 µm (particle size distribution determined by laser diffraction in a Malvern Mastersizer,
Malvern Instruments, Ltd., Malvern, UK) [13]. Different weight percentages of MK (0, 6,
10, and 20 wt %) with respect to the weight of lime were added.
Two polymers, poly–napthalene sulfonate (PNS) and lignosulfonate (LS) (supplied by
FOSROC EUCO S.A., Izurtza, Spain), were assessed as SPs. The characterization of the two
polymers focused on the molecular weight, the elemental composition and the anionic charge
density of the two polyelectrolytes [11]. The molecular weights, as determined by size–
exclusion chromatography (SEC), were 8620 Da for PNS and 8650 Da for LS. Elemental
composition (LECO analyser, LECO Corporation, St Joseph, MI, USA) yielded similar
values for C (ca. 50%), whereas clear differences were found for sulphur contents: 12.3% for
Capítulo I. Parte A
90
PNS, 6.2% for LS. Titration with Poly–DADMAC allowed obtaining the anionic charge
density of the polymers mainly caused by the deprotonation of sulfonate groups. The values,
expressed as meq of anionic charge/g of polymer, were 2.44 for PNS and 1.04 for LS, in good
agreement with the larger S content determined for PNS.
For testing the properties of the grouts, SPs were added in 0.5 and 1 wt % with respect
to the weight of lime. Dosages were selected according to previous values reported in the
literature [13,14,27]. To properly assess the effect of the different SPs and their dosages,
mixing water was added in a fixed 1:1 water/lime ratio by weight. This ratio of mixing water
provided an adequate workability (measured slump in the flow table test within the range 175
± 5 mm) in the control sample. Table 1 collects the grouts composition.
Table 1. Composition of the different grouts (all of them were prepared with 500 g of
air lime, 1500 g of calcitic sand, and 500 g of mixing water).
Samples Air lime
(g)
Calcitic sand
(g)
Mixing water
(g)
MK
(g)
LS
(g)
PNS
(g)
S0MK (control group) 500 1500 500 0 0 0
S0MK0.5LS 500 1500 500 0 2.5 0
S0MK0.5PNS 500 1500 500 0 0 2.5
S0MK1LS 500 1500 500 0 5 0
S0MK1PNS 500 1500 500 0 0 5
S6MK 500 1500 500 30 0 0
S6MK0.5LS 500 1500 500 30 2.5 0
S6MK0.5PNS 500 1500 500 30 0 2.5
S6MK1LS 500 1500 500 30 5 0
S6MK1PNS 500 1500 500 30 0 5
S10MK 500 1500 500 50 0 0
S10MK0.5LS 500 1500 500 50 2.5 0
S10MK0.5PNS 500 1500 500 50 0 2.5
S10MK1LS 500 1500 500 50 5 0
S10MK1PNS 500 1500 500 50 0 5
S20MK 500 1500 500 100 0 0
S20MK0.5LS 500 1500 500 100 2.5 0
S20MK0.5PNS 500 1500 500 100 0 2.5
S20MK1LS 500 1500 500 100 5 0
S20MK1PNS 500 1500 500 100 0 5
Capítulo I. Parte A
91
2.2 Experimental methods
For the preparation of the fresh grouts, lime, metakaolin, and the required amount of SPs
(all of them in a dry condition) were blended for 5 min using a solid additives mixer BL–8–
CA (Lleal, S.A., Granollers, Spain) to guarantee a proper homogeneity of the components.
Mixing water was then added and mixed for 90 s at low speed, in a Proeti ETI 26.0072 (Proeti,
Madrid, Spain) mixer. The fluidity of the fresh grouts was measured by using the mini slump
flow test according to the norm [28], in which a truncated metallic cone was filled in with
the samples and then removed. The slump measurements were recorded after 15 strokes of
the flow table, 1 per second, in line with previous works [29]. Density and air content of the
fresh grouts were also measured according to the European norms [30,31]. The setting time
of the pastes was calculated according to the workable life following the European norm EN
1015–9 [32]. All these experiments were carried out by triplicate and the depicted values are
an average value of all the recorded measurements.
Sorption experiments for both SPs (PNS and LS) were carried out following previously
referenced processes [13,14,22,33,34] in batch reactors for plain lime pastes (1 g of lime per
25 mL of water) and for lime–MK pastes (5 g of lime and pozzolanic admixture at 6, 10, and
20 wt % with respect to lime in 25 mL of water). The mixtures were stirred for 1 h and,
subsequently, centrifuged at 8000× g for 15 min. After this, the supernatant was collected
and filtered through 0.45 µm PTFE filters. The amount of both SPs adsorbed onto the
particles was determined by the difference between the concentration initially added and the
final remaining concentration of SP, as quantified by UV–VIS spectrophotometry (maxima
at λ = 296 nm for PNS and at λ = 285 nm for LS). The mathematical fitting of the adsorption
data was calculated for Langmuir, as well as Freundlich, models.
Regarding the hardened state study, prismatic specimens with dimensions of 160 × 40 ×
40 mm were prepared in a Proeti C00901966 mould. The as–prepared grouts were cured at
20 °C and 60% RH and demoulded 7 days later, and stored under those very same conditions
that had previously been established for these lime–based mortars [35,36]. Flexural strengths
were determined by triplicate in the prismatic specimens using an Ibertest STIB–200 device
(Madrid, Spain) at low loading rates of ca. 10 N·m−1. Subsequently, compressive strength
experiments were executed on the two fragments of each specimen resulting from the flexural
tests; the compressive strength experiments were conducted at a rate of loading of ca. 50
N·m−1, so that specimens broke between 30 and 90 s. All these tests were carried out
according to the European norm [37].
Capítulo I. Parte A
92
In hardened specimens different characterization methods were performed. For thermal
analysis, a simultaneous TG–sDTA 851 Mettler Toledo thermoanalyzer device
(Schwerzenbach, Switzerland) was used under the following experimental conditions:
alumina crucibles, a temperature range from 25 to 1000 °C, and a heating rate of 10 °C·min−1
and static air atmosphere. Fourier transform infrared spectroscopy—attenuated total
reflectance (FTIR–ATR) experiments were done in a Shimadzu IRAffinity–1S apparatus
(Shimadzu, Japan). The infrared spectra were registered at 100 scans over a wavelength range
of 4000–600 cm−1, with resolution of 4 cm−1. X–ray diffraction (XRD) experiments were
performed in a Bruker D8 Advance diffractometer (Bruker, Karslruhe, Germany) with a Cu
Kα1 radiation, from 2° to 80° (2θ), 1 s per step, and a step size of 0.04°. A Micromeritics
AutoPore IV 9500 apparatus (Micromeritics, Norcross, GA, USA), with a pressure range
between 0.0015 and 207 MPa, was used for mercury intrusion porosimetry (MIP)
experiments.
For the durability essays, prismatic samples (prepared and cured 28 days as described
before) were tested to assess the durability. Hardened grouts were subjected to different
processes:
(a) Frost resistance was determined by means of freezing–thawing cycles. The cycles
consisted of water immersion of the samples for 24 h and subsequently freezing at −10 °C
for 24 h. For these experiments, a CARAVELL 521–102 freezer was used.
(b) Sulphate attack resistance: the monolithic samples were completely submerged in
a MgSO4 saturated aqueous solution at 20 °C and 95% HR for 24 h. After this process, the
samples were dried in an oven at 65 °C for 24 h and submerged in water for 24 h at 20 °C
and 95%HR. To conclude the cycle, the specimens were again dried as described above. The
cycles were continuously repeated until the total destruction of the specimens.
In order to evaluate the survival of the samples after the ageing cycles, two parameters
were considered, following that previously mentioned in other papers [35,36]: (i)
compressive strength tests after 7, 14, and 28 cycles when the integrity of the samples allowed
them; and (ii) qualitative evaluation based on visual appearance after each cycle; the criterion
was the following: degree 0 (samples with no evidence of decay), degree 1 (samples showing
a slight degree of deterioration due to some short or thin cracks on surface), degree 2 (altered
samples showing some deeper cracks), degree 3 (heavily altered specimens with deep cracks
and certain swelling), degree 4 (samples with severe decay due to large and deep cracks and
also partial loss or swelling), and degree 5 (completely destroyed samples).
Capítulo I. Parte A
93
3. Results and discussion
3.1. Fresh state properties
3.1.1. Fluidity
Figure 1 shows the fluidity values as a function of the different contents of MK, PNS
and LS in the lime–based grouts (spread values as measured by the flow table test).
The addition of both polymer–based SPs in the plain lime grouts increased the fluidity
of the pastes, the incorporation of the PNS being, on average, more effective than that of the
LS. The highest dosage of PNS turned out to be the most effective and the two polymers
showed a dosage–dependant pattern. The improvement in flowability of the two polymeric
dispersive agents supports the interest of the study of these compounds for lime–based grouts.
The presence of MK for polymer–free grouts yielded lower spread values although
results did not fit to a dosage–pattern response. In grouts with dispersing polymers, the
fluidity was clearly enhanced in the presence of MK. The better efficiency was seen for PNS.
The observed results were dissimilar depending on the amount of MK. Two counteracting
factors can be taken into account to explain this behaviour: (i) the pozzolanic reaction, giving
rise to C–S–H phases and their irreversible agglomeration, resulting in a fluidity decrease;
and (ii) the lubricant effect provided by MK that allows the particles to reduce their friction
forces, thus increasing the fluidity and workability [4,38]. The increase in MK could lead to
an intensification of the pozzolanic reaction, particularly for the highest additions of MK.
Figure 1. Fluidity values (slump measured in the flow table test) of the different grouts
Capítulo I. Parte A
94
The absence of a clear trend can be explained considering that the mixing water has been
kept constant throughout the work. Samples with high percentages of MK and intensified
pozzolanic reaction would require larger mixing water content due to the fast consumption
of water. In the case of low amounts of mixing water added, a fluidity decrease would take
plce. For both superplasticizers, the addition of increasing dosages of MK at the highest SP
dosage provoked a reduction in fluidity values. This can be related to the consumption of the
polymers during the pozzolanic reaction. Similarly to cement–based materials, the polymers
can be adsorbed onto the newly formed hydrates, being then covered by the growing
hydration products. These polymer molecules would be unable to act as dispersing agents.
The formation of these organo–mineral inactive compounds has been described in the
literature [34].
Due to the concurrence of many different factors affecting the fluidity, a careful design
of the mix proportions should be considered in order to obtain the most appropriate grouts.
The experimental values of density and air content of the fresh pastes were also
determined and collected in Table 2. Although values underwent small variations, there is a
consistent slight density reduction as a function of the MK incorporation. This fact can be
explained as a consequence of the fixed water/binder ratio, which was kept constant for all
the tested grouts. The fast consumption of water on account of the pozzolanic reaction would
lead to less–dense packing systems.
Furthermore, the presence of the polymers (PNS and LS) exerted an influence on the
density and air–entrained values. LS increased the air–entrained during the mixing process
as a result of its surfactant characteristics (with both hydrophobic and hydrophilic segments
within the same molecule). Conversely, incorporation of PNS gave rise to lower levels of air
content, thus achieving a denser packing system. The excess in the air–entrained together
with the low density of the fresh paste could involve a porosity increase after the hardening
of the sample, as will be discussed below.
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Table 2. Bulk density and air content of the fresh grouts.
Samples Bulk Density of the Fresh Paste
(g·mL−1)
Air
Content
(%)
S0MK (control group) 1.89 3.2
S0MK0.5LS 1.9 3.4
S0MK0.5PNS 1.9 2.6
S0MK1LS 1.89 3.4
S0MK1PNS 1.93 1.6
S6MK 1.89 3
S6MK0.5LS 1.89 3.2
S6MK0.5PNS 1.88 3
S6MK1LS 1.89 3.3
S6MK1PNS 1.9 2.2
S10MK 1.89 3.3
S10MK0.5LS 1.89 3.2
S10MK0.5PNS 1.89 2.7
S10MK1LS 1.88 3.4
S10MK1PNS 1.89 2.5
S20MK 1.87 3.1
S20MK0.5LS 1.87 3.4
S20MK0.5PNS 1.89 3
S20MK1LS 1.87 3.2
S20MK1PNS 1.89 3
3.1.2. Setting times
In Figure 2 it can be seen that the addition of LS slowed down the setting time of the
fresh grouts. LS caused the strongest delays, especially at the largest dosage tested (Figure
2). The high values of setting times of the binding materials is a frequent inconvenient that
arises when using superplasticizers [39,40] and can be due to the interference of the SPs with
the growth of the hydration products of the pozzolanic reaction and/or with the carbonation
process for pure lime systems.
Capítulo I. Parte A
96
Figure 2. Setting time of the different grouts
The setting times did not adjust to a clear pattern. For some grouts, the increasing amount
of MK resulted in shorter values of setting times (for example, samples with the largest
dosages of LS, Figure 2). This was in line with previous works with other pozzolanic
admixtures [14].
For PNS, on average, the increasing amounts of MK, from 6 to 20 wt %, resulted in a
setting time delay. The water availability may account on this fact. These fresh grouts were
prepared with a fixed water/lime ratio 1:1, obtained from the amount of mixing water
required for the control group yielding a consistency of 175 mm (measured in the flow table
test). Keeping constant the water/lime ratio and owing to the small particle size of the
pozzolanic admixture, the increasing percentages of MK reduced the water availability for
the pozzolanic reaction, which should account for a rapid hardening of the fresh mixture.
The main conclusions are that LS caused strong delays in the setting times of lime–MK
grouts, whereas PNS was found to be more appropriate when taking into account this
parameter. The observed delays for both SPs could reasonably be managed in practical
applications.
3.1.3. Adsorption
Adsorption isotherms were done in order to measure the affinity of PNS and LS for the
binder particles in both air lime and MK–air lime media. Particles of air lime and MK–air
lime were dispersed in an aqueous media, in which the polymers were then incorporated.
After a stirring time, it is expected that some of the polymer molecules had been attached to
Capítulo I. Parte A
97
the particles, whereas some others remain free in the solution, showing a different affinity
for the absorbent substrate. Experimental results (Figure 3) showed that PNS was better
retained than LS in the tested lime media. In the presence of different percentages of MK
only slight differences could be observed for each one of the SPs.
Mathematical treatment of experimental adsorption data have been collected in Table 3.
The maximum sorption capacity (qm) of PNS was 51.2 mg·g−1 for plain lime and 44.7 mg·g−1
for lime with 20 wt % of MK. For LS, the maximum sorption capacity value was 32.1 mg·g−1
for plain lime and 29.1 mg·g−1 for lime with 20 wt % of MK (Table 3). Both SPs showed a
better adjustment to a Freundlich model, following a multilayer adsorption model (Table 3)
[41,42]. These experimental results, as well as the molecular architecture of the two SPs, can
be related to their dispersing effectiveness.
Figure 3. Adsorption isotherms of the SPs onto lime pastes (6, 10, and 20 wt % of
pozzolanic admixture). PNS adsorption (top); LS adsorption (bottom).
Capítulo I. Parte A
98
Table 3. Results of adsorption isotherms onto air lime suspensions at different MK
percentages: Langmuir and Freundlich adsorption parameters for both SPs.
Polynaphthalene sulfonate (PNS)
Langmuir Freundlich
qm b R2 K 1/n R2
S0MK 51.2 0.00011 0.7738 0.01210 0.8658 0.9757
S6MK 46.9 0.00011 0.8019 0.01215 0.8582 0.9768
S10MK 43.4 0.00012 0.8469 0.01247 0.8490 0.9775
S20MK 44.7 0.00010 0.7458 0.00986 0.8708 0.9766
Lignosulfonate (LS)
Langmuir Freundlich
qm b R2 K 1/n R2
S0MK 32.1 0.00016 0.9509 0.01983 0.7812 0.9775
S6MK 28.7 0.00018 0.9242 0.01837 0.7830 0.9735
S10MK 31.5 0.00015 0.9647 0.01582 0.7977 0.9825
S20MK 29.1 0.00015 0.9400 0.01346 0.8076 0.9809
The molecular architecture of the LS (Figure 4), with a branchy structure, suggests the
steric hindrance as the predominant mechanism. In this work, LS was seen to be less adsorbed
[23,43] and to show slightly less plasticizing effects than PNS, which is a linear–shaped SP,
but with higher anionic charge density [44,45] (Figure 4). LS has been reported to easily form
Ca2+ complexes [46,47] and in a previous work the higher ability of LS to bind Ca2+ ions has
been established [14]. The formation of these LS–Ca2+ complexes prevented some LS
molecules from being attached to the portlandite, C–S–H, C–S–A–H or C–A–H particles.
Furthermore, considering that the anchorage of the polymer onto the active particles takes
place by means of favourable electrostatic interaction on the double ionic layer, the higher
anionic charge, the more intense the adsorption of the polymer. These facts could explain the
lower adsorption of this LS polymer. The adsorption of the polyelectrolyte onto the active
particles has been reported to be critical for the dispersing action [47], so that PNS showed
higher adsorption and better effectiveness as a dispersing agent in the tested systems..
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99
Figure 4. Molecular structure of the two tested polymers: PNS (left) and LS (right)
Finally, the strong Ca2+ complexation of the LS would explain its influence on the setting
time, preventing lime grouts from carbonation, or even from C–S–H formation.
Figure 5. Compressive strength results of the grouts. PNS (top); and LS (bottom).
CH2H
H
SO3-Na+
n
SO3-Na+
SO3-Na+
SO3-Na+
Capítulo I. Parte A
100
3.2. Hardened state properties
3.2.1. Mechanical strength
Carbonation has a significant influence in the hardening process along time in lime–
based systems [48,49]. The mechanical strengths increase over time due to the carbonation
process, resulting in the formation of CaCO3. Accordingly, on average, the highest values of
compressive strength were obtained at long term curing times, usually after one curing year
(Figure 5). For plain lime mortars (0% MK), the addition of the SPs caused a drop in the
compressive strength values (Figure 5), which can be ascribed to the interference with the
lime carbonation process.
The pozzolanic reaction that takes place between CH particles and reactive MK was
responsible for the observed mechanical strength improvement at short term in the presence
of pozzolanic admixture. This reaction yields C–S–H, C–S–A–H, and C–A–H, according to
the data referred in literature [49,50,51] (Figure 5).
The average value of compressive strength was 2.5 MPa for PNS–samples, whereas 1.8
MPa was determined for LS–samples. In the current study, sample S20MK0.5PNS offered
the largest values, reaching 4.8 MPa after 182 curing days.
Flexural strength values were also measured (Figure 6). The stiffening of the sample due
to the C–S–H formation caused a decrease in the flexural strength when MK was
incorporated. Polymers were seen to confer different flexural resistance: LS increased the
flexural strength, particularly in samples with the highest MK proportions, whereas PNS
generally involved a reduction in the flexural strength. Differences in the extent of
carbonation and/or pozzolanic reaction could explain these findings, as will be discussed
below.
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101
Figure 6. Flexural strength results of the grouts. PNS (top); and LS (bottom).
3.2.2. TG–DTA, FTIR–ATR and XRD studies
The rate of carbonation and the pozzolanic reaction at the different curing times of the
SPs–MK–lime mortars was followed by TG–DTA, FTIR–ATR, and XRD experiments.
Previous works also correlated the structure of the materials with the TG measurements [52].
Figure 7 and Figure 8 depict the percentages of Ca(OH)2 and CaCO3 calculated from TG
(weight loss due to dehydroxylation of portlandite at ca. 450 °C, and weight loss owing to
the calcite decomposition at ca. 800 °C [53]. The weight loss between 25–300 °C (Table 4)
was assigned to the dehydration processes of the calcium silicate (C–S–H), calcium
silicoaluminate (C–S–A–H) and calcium aluminate (C–A–H) hydrated phases derived from
the pozzolanic reaction, according to some authors [14,54,55,56], and also to residual
dehydration of adsorbed water.
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102
Figure 7. Percentages of Ca(OH)2 for mortars at different curing times.
Figure 8. Percentages of CaCO3 for mortars at different curing times.
For example, the addition of 20% MK (sample S20MK) to the plain lime (sample S0MK)
provoked the reduction in the Ca(OH)2 content and the increase in the amount of C–S–H, C–
A–H and C–S–A–H phases generated by the pozzolanic reaction, as proven by the mass loss
increment between 25–300 °C (Table 4). Pozzolanic compounds were identified at the early
stages of curing (7 and 28 days) (Table 4), in line with previous results [54]. The highest
percentages of CaCO3 were found in sample S20MK studied at 91 days and in S0MK and
S6MK samples after 365 days. It can be determined that the pozzolanic reaction took place
mainly at early stages of curing (7 and 28 days), whereas the carbonation process was
significant at longer curing times. This fact could represent a practical advantage in materials
used as grouts which will be in contact with water [49,55].
0 5 10 15 20
7 days
28 days
91 days
182 days
365 days
Portlandite (%)
Curing days S20MK1LS
S20MK1PNS
S20MK
S0MK1LS
S0MK1PNS
S0MK
0 20 40 60 80 100
7 días
28 días
91 días
182 días
365 días
Calcite (%)
Curing days
S20MK1LS
S20MK1PNS
S20MK
S0MK1LS
S0MK1PNS
S0MK
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Table 4. TG results of weight loss between 25–300ºC, assigned to dehydration of
pozzolanic compounds.
Samples Weight loss (%)
7 days 28 days 91 days 182 days 365 days
S0MK (Plain lime) 0.38 0.74 0.45 0.30 0.31
S20MK 1.03 0.88 0.57 0.73 0.66
S20MK1PNS 0.76 1.00 0.75 0.93 0.58
S20MK1LS 0.51 0.70 0.61 0.76 0.80
Regarding the presence of SPs, the carbonation rate was lower for lime–MK mortars
with LS in comparison with samples with PNS. The presence of LS hindered the carbonation
process, resulting in higher amounts of unreacted Ca(OH)2 and correspondingly lower
amounts of CaCO3 formed. This was confirmed by the FTIR spectra of lime–based samples
containing the highest percentages of LS (1%) and MK (20%) (S20MK1LS). These spectra
showed an intense and sharp absorption band at 3600 cm−1 ascribed to –OH groups of
portlandite, which remained after 91 days (Figure 9, dotted area on the left side of the figure).
Conversely, the samples containing PNS (for example S20MK1PNS) did not show the band
ascribed to portlandite (Figure 9), due to the higher extent of the carbonation process.
Absorption bands at ca. 1400 cm−1, 875 cm−1, and 712 cm−1 were respectively assigned to ν3
asymmetric CO3 stretching, ν2 asymmetric CO3 deformation, and ν4 symmetric CO3
deformation modes [57], and associated to the presence of calcium carbonate (calcite). These
results matched with those provided by thermal analysis and were also helpful to justify the
compressive strength experiments (values of 1.3 MPa for S20MK1LS and 3.1 MPa for
S20MK1PNS at 91 curing days due to a larger carbonation process for the latter).
Figure 9. FTIR spectra of different samples after 91 curing days.
Capítulo I. Parte A
104
With respect to the pozzolanic reaction extent, for the higher percentages of MK (20%),
the formation of C–S–H, C–S–A–H, and C–A–H compounds, according to the TG values,
was also favoured on average for PNS–bearing samples (weight loss of 0.99% for
S20MK1PNS sample tested at 28 days and 0.93% at 182 days) in comparison with LS
samples (weight loss of.0.70% for S20MK1LS sample tested at 28 days and 0.76% at 182
days) (Table 4). These results explain the higher compressive strengths observed for PNS
samples. At the same time, the increase in the stiffening of the sample could result in poorer
flexural strengths. The formation of these hydraulic compounds was detected in samples with
MK by FTIR measurements. However, spectroscopic results did not offer clear evidence
about the comparative rate of formation of hydrated pozzolanic compounds (silicate bands at
ca. 1000 cm−1, which revealed the presence of C–S–H compounds, the dotted area on the
right part of Figure 9) [14,27,35].
In spite of these evidences of the pozzolanic reaction, the identification of the crystalline
aluminate and/or silicate phases in the XRD diffractograms was hardly possible. Traces of
stratlingite Ca2Al(AlSi)O2(OH)10·2.25H2O and cowlesite CaAl2Si3O10.6H2O were only
detected in samples S20MK and S20MK1PNS (with PNS as SP) (diffraction patterns not
shown). Both hydrated compounds have been reported to improve the mechanical
strengthening of the samples [54]. The relatively low ratio of pozzolanic admixture, the
curing conditions (room temperature and low relative humidity) and the low crystallinity of
these hydrated compounds would explain their difficult identification by XRD.
For example, in previous studies larger mass ratios MK:lime were used and curing
conditions that favour the pozzolanic reaction (high T/HR) were applied [11,54,58]. In these
works, calcium silicate hydrate gel (CSH), stratlingite (C2ASH8), tetracalcium aluminate
hydrate (C4AH13), monocarboaluminate (C4AC−CH11), katoite (Ca3Al2(SiO4)(OH)8), and
calcium aluminium hydroxide hydrate (Ca2Al(OH)7·6.5H2O) were identified as phases
formed after the lime–MK reaction.
3.2.3. Porosity measurements
The consumption of CH, due to the carbonation progression, and the formation of C–S–
H, C–S–A–H, and C–A–H phases gave rise to a refinement of the pore structure, which was
studied by mercury intrusion porosimetry (MIP). This technique has been applied in cement–
based materials [59]. The addition of MK (S20MK) to the plain lime mortars (S0MK)
reduced the mean pore size diameter from 0.83 µm to 0.56 µm in samples tested at 91 curing
days (depicted as an example) (Figure 10a). This mean pore size reduction was ascribed to
Capítulo I. Parte A
105
the occurrence of the pozzolanic reaction and also to the filler effect of MK. The filler effect
of MK was previously studied, showing that the addition of MK in ordinary Portland cement
(OPC) exhibited an important reduction of the permeability and of the porosity when
compared with control samples (plain OPC mortars) [60]. In lime mortars this filler effect
has also be reported to take place after the incorporation of other pozzolanic compounds, like
NS [14,35]. The formation of the new phases by pozzolanic reaction could also contribute to
this pore size reduction. These results are in line with the increase in compressive strength
observed for MK4 mortars.
The addition of high dosages of PNS to lime mortars did not provoke changes regarding
the mean pore size diameter (Figure 10a) but samples showed higher porosity values in the
experiments after 91 curing days and, subsequently, lower compressive strength values than
those reported for SP–free MK–lime mortars (3.1 MPa for S20MK1PNS vs. 3.4 MPa for
S20MK) (Figure 5). The dosage of 0.5% of PNS (S20MK0.5PNS), however, yielded
hardened grouts of lower porosity, thus providing higher compressive strength values (4
MPa). The adjustment of the dosage of the SP appears to be imperative to guarantee the
mechanical performance of the grouts.
Figure 10. Pore size distribution of different samples tested after 91 curing days. (a)
control sample, 20 S20MK and samples with PNS, (b) control sample, S20MK and samples
with LS, (c) comparison between samples with 0.5 wt. % of PNS and LS.
Capítulo I. Parte A
106
Furthermore, in comparison with LS–bearing grouts (Figure 10b), a higher population
of pores in the pore range 0.1–0.01 µm was observed for the grouts containing PNS (Figure
10a). This pore range has been ascribed to the C–S–H pores [61], confirming the extent of
the pozzolanic reaction in the presence of PNS.
The incorporation of the LS as a superplasticizer caused an increase in the mean pore
size diameter of the grouts (0.56 µm for sample S20MK; 0.68 µm for sample S20MK0.5LS;
0.83 µm for sample S20MK1LS, as depicted in Figure 10b for 91–aged samples). This
increase in the pore size explained the compressive strength fall (3.4 MPa for S20MK vs. 3
MPa for S20MK0.5LS vs. 1.3 MPa for sample S20MK1LS) (Figure 5). This fact can be
partially related to the air content excess found for this additive in the fresh grouts (Table 2).
The graphical comparison between the pore size distributions of the grouts with both SPs
clearly depicts the increment in the main pore size diameter and also in the area under the
curve for LS mortars (Figure 10c). At the same time, the smallest diameter pore population
related to the C–S–H formation was higher for PNS grouts.
3.3. Durability experiments
3.3.1. Freezing–thawing
Figure 11. Alteration degrees of grouts after freeze–thaw cycles.
Capítulo I. Parte A
107
The control group samples subjected to frost resistance test (freezing–thawing F–T
cycles) underwent serious decay leading to the total destruction of the sample after just one
cycle (Figure 11), in agreement with the poor frost resistance of pure air lime mortars [62].
Fitting itself to a dosage–response pattern, the incorporation of MK clearly enhanced the F–
T durability of the grouts. It can be observed that S20MK sample can endure up to 24 F–T
cycles displaying serious decays only in the last cycle (Figure 11). The positive F–T
endurance provided by the pozzolanic admixture included in lime mortars is in line with the
reported incorporation of NS [35]. Nunes and Slizkova [63] assigned this favourable
behaviour of mortars comprising of lime + MK to the enhancement of the pozzolanic reaction
in wet conditions. Another concomitant factor is the reduction in the mean pore size diameter
observed for MK–lime grouts compared with plain lime samples. The decrease in the mean
pore size hindered the absorption of liquid water, preventing its later freezing and expansion
damage and, consequently, increasing the durability of this type of mortar. Table 5 collects
the numerical values corresponding to the different damages observed in the tested samples
after 5, 10, 15, and 20 F–T cycles. The numerical value 5 corresponds to the total decay of
the specimen and appears marked in red in the Table. Beyond this value, the specimen was
totally destroyed and no longer tested.
Table 5. Visual alteration after 5, 10, 15, and 20 freezing–thawing (FT) cycles showing
numerical values of the damage scale *.
Number of FT Cycles
Samples 5 10 15 20
S20MK1LS 2 4 5 –
S10MK1LS 5 – – –
S6MK1LS 5 – – –
S20MK0.5LS 3 5 – –
S10MK0.5LS 3 5 – –
S6MK0.5LS 4 5 – –
S20MK1PNS 2 3 4 5
S10MK1PNS 3 5 – –
S6MK1PNS 4 5 – –
S20MK0.5PNS 0 2 4 5
S10MK0.5PNS 3 5 – –
S6MK0.5PNS 5 – – –
S0MK0.5PNS 5 – – –
S20MK 0 1 2 3
S10MK 3 4 5 –
S6MK 5 – – –
S0MK (control group) 5 – – –
Capítulo I. Parte A
108
Compressive strength values (after 7 and 14 F–T cycles) were found to remain
appreciable (2.5 and 2.4 MPa, respectively) for grouts with the largest additions of MK
(S20MK). In contrast, the flexural strength was significantly affected (values below 0.6
MPa). The fissures observed on S20MK mainly appeared on the side faces. The compressive
strength is parallel to the longitudinal cracks so it is unaffected. However, the flexural
strength is significantly affected by the cracks [63].
The addition of PNS in the MK–air lime was only slightly detrimental for the durability
of the samples, i.e., S20MK1PNS grouts suffered total decay after 19 F–T cycles, and
S20MK0.5PNS (with lower porosity) after 20 cycles. Figure 12 showed images of three
hardened grouts, S20MK, S20MK0.5PNS, and S20MK1PNS, after 10 F–T cycles. The
fissures are clearly visible in S20MK0.5PNS and in S20MK1PNS.
Conversely, the use of LS significantly harmed the F–T durability of the grouts, with
total decay after only 10 and 12 F–T cycles, respectively, for S20MK0.5LS and S20MK1LS
grouts. The highest increase in mean pore size of the LS–bearing samples, providing a higher
absorption of liquid water during the durability test, may contribute to clarifying this
experimental finding.
In order to visually compare the durability performance with both SPs, Figure 12
depicted images corresponding to samples S20MK0.5PNS, S20MK0.5LS, S20MK1PNS,
and S20MK1LS after 10 F–T cycles. As mentioned before, at that stage, the highest decay
was found for S20MK0.5LS. Important deterioration was also observed for S20MK1LS and
S20MK1PNS, in which surface fissures were obvious.
Figure 12. Appearance of different grouts after 10 freeze–thaw cycles.
Capítulo I. Parte A
109
3.3.2. Magnesium sulfate attack
Figure 13. Alteration degrees of grouts after sulfate attack cycles.
The assessment of the resistance of sulphate attack of MgSO4 was also carried out.
Results are gathered in Figure 13. Table 6 displays the numerical values corresponding to the
different damages observed in the tested samples after 5, 10, 15, 20 and 25 sulphate attack
cycles. The numerical value 5 corresponds to the total decay of the specimen and appears
marked in red in the Table. Beyond this value, the specimen was totally destroyed and no
longer tested.
Opposite to F–T resistance, the increase in wt % MK addition damage the sulphate attack
resistance of the grouts. Whilst samples with 6 wt % MK (S6MK) lasted 27 cycles with an
intermediate alteration degree (degree 3), samples with 10 wt %MK (S10MK) only lasted 12
cycles before reaching a complete decay, and samples with 20 wt % (S20MK) reached a total
decay after 6 cycles.
In this sense, Figure 14 showed images of S6MK, S10MK and S20MK grouts after 5
sulphate cycles. In the sample S20MK, the spalling of the superficial part of the specimens
and partial disintegration took place [35]. This result suggested the presence of sulphate
compounds at the surfaces [35,62,63,64].
Capítulo I. Parte A
110
Table 6. Visual alteration after 5, 10, 15, 20, and 25 magnesium sulphate attack cycles,
showing the numerical values of the damage scale * Number of Sulphate Attack Cycles
Samples 5 10 15 20 25
S20MK1LS 1 3 3 4 5
S10MK1LS 4 4 4 5 –
S6MK1LS 4 5 – – –
S20MK0.5LS 0 1 2 3 3
S10MK0.5LS 1 2 4 4 4
S6MK0.5LS 1 2 4 4 4
S20MK1PNS 4 5 – – –
S10MK1PNS 1 4 4 5 –
S6MK1PNS 0 1 2 3 4
S20MK0.5PNS 2 5 – – –
S10MK0.5PNS 1 2 4 4 5
S6MK0.5PNS 3 5 – – –
S0MK0.5PNS 0 2 4 5 –
S20MK 3 5 – – –
S10MK 4 4 5 – –
S6MK 1 2 2 3 3
S0MK (control group) 1 5 – – –
Figure 14. Appearance of different grouts after 5 sulfate crystallization cycles.
Capítulo I. Parte A
111
The PNS addition in a 0.5% dosage yielded the highest tolerance to the sulphate attack
for S10MK0.5PNS. When the percentage of PNS was increased to 1%, the highest endurance
was found for the lower percentage of MK (S6MK1PNS). In the case of the addition of LS
on MK–lime mortars, a linear behaviour was observed: the larger the amount of MK, the
higher the durability against sulphate attack cycles. To illustrate these results, Figure 14
depicts images corresponding to samples S6MK1PNS, S6MK1LS, S20MK1PNS,
S20MK1LS after 5 sulphate attack cycles. Severe decays and losses of a part of the mortars
were observed for grouts S20MK1PNS and S6MK1LS at this stage.
In the same line of the detrimental presence of MK, owing to the enhancement of C–S–
H, C–S–A–H, and C–A–H phases when PNS was present, PNS–bearing grouts showed a
worse sulphate attack resistance. The literature has shown that Mg2+ ions (in case of
magnesium sulphate attack) cause decalcification of C–S–H, increasing the degree of
alteration [65]. This is in line with the observed stronger damage in samples S20MK and
S20MK1PNS, which presented a large amount of C–S–H, as discussed before.
A detailed examination by XRD of the grouts after three cycles of the sulphate attack
revealed the formation of expansive hydrated compounds and soluble salts, such as
hexahydrite and gypsum (MgSO4·6H2O and CaSO4·2H2O). These compounds were
responsible for the degradation of the samples. Furthermore, it was seen that the lower the
amount of uncarbonated portlandite, the stronger the formation of degradation salts. The
presence of portlandite hindered the formation of these sulphates, possibly by the
precipitation of magnesium hydroxide (brucite) that was also detected in some of the XRD
patterns.
A quantitative phase analysis was carried out by means of a Rietveld refinement of the
XRD patterns with TOPAS software. As an example, comparative percentages of the samples
with 20% MK are collected in Table 7.
Table 7. Results of the Rietveld quantitative phase analysis of the XRD after 3 sulfate
attack cycles. Percentages of the different phases.
Samples Phase (wt. %)
Calcite Portlandite Brucite Quartz Gypsum Hexahydrite
S20MK 86.2 – – 0.7 5.6 7.5
S20MK1PNS 82.6 0.4 1.0 0.4 10.3 5.3
S20MK1LS 81.1 4.3 3.0 0.4 9.1 2.1
Capítulo I. Parte A
112
These results are in complete agreement with the presence of portlandite reported in
Figure 7. Samples with the lowest percentages of portlandite (S20MK and S20MK1PNS)
showed the highest percentages of expansive and soluble sulphate salts. The grout with the
highest percentage of portlandite (S20MK1LS) yielded the lower amount of expansive
hexahydrite. At the same time, this grout showed the highest percentage of brucite,
confirming the protective role of the portlandite, which trapped Mg2+ ions delaying their
decay activity [66,67].
Although in previous works the growth of expansive phases, such as ettringite and
thaumasite, had been reported to constitute an important mechanism of degradation [64] of
cementitious samples subjected to sulphate attack, in this work there was no evidence of the
formation of these compounds in the tested grouts.
Two different mechanisms took place concerning the durability of the tested grouts. The
refinement of the pore structure caused by the presence of MK and PNS (filler effect and
pozzolanic reaction) enhanced the frost resistance of the mortars by hindering the water
access. However, the appearance of C–S–H, C–A–H, and C–S–A–H impaired the sulphate
attack resistance that caused decalcification of these phases. The presence of Ca(OH)2 had a
protective effect delaying the decay induced by Mg2+ by precipitation of Mg(OH)2. The
sulphate attack was seen to be strongly dependent of the chemical and mineralogical
composition of the grouts [64,68].
Capítulo I. Parte A
113
4. Conclusions
The fluidity of the lime grouts that also contained MK as a pozzolanic admixture was
clearly increased upon the addition of the two tested polymer–based superplasticizers (LS
and PNS). Among the two tested polymers, PNS showed higher dispersing effect than LS on
account of its higher adsorption onto portlandite, C–S–H, C–S–A–H, and C–A–H particles.
The higher anionic charge of the polyelectrolyte PNS and its linear molecular architecture
explained its better adsorption. Setting times were less affected for PNS addition than for LS
incorporation. LS was seen to cause delays in the setting time.
The pozzolanic reaction was favoured in grouts with PNS, consequently the highest
values of compressive strength were reached when this polymer was employed, i.e., 4.8 MPa
after 182 days in samples with 20% MK and 0.5% PNS.
The incorporation of MK enhanced the freezing–thawing durability of the grouts due to
the decrease in the mean pore size that consequently hampered the absorption of liquid water
and reduced the damage by freezing and expansion of the retained water in pores. The
addition of PNS on lime grouts slightly decreased the F–T durability of the grouts (from
enduring 24 F–T (0% PNS) to 19 F–T or 20 F–T, with 0.5 and 1 wt % of PNS, respectively),
so that the enhancement in fluidity, compressive strength, and frost resistance provided by
this polymer supports its use for lime–based grouts.
However, the formation of C–S–H, C–S–A–H, and C–A–H was preferred in the presence
of PNS as polymer, and the appearance of these phases results in a weaker resistance against
sulphate attack. Grouts were altered by decalcification of hydrated phases and by formation
of hexahydrite and gypsum. A protective role of portlandite against magnesium sulphate
attack was clearly identified. Accordingly, the polymer LS, which preserve a significant
amount of Ca(OH)2, could be an alternative for the obtaining of grouts requiring high
sulphate attack resistance.
Author Contributions
Data curation, conceptualización and formal analysis: main contributor J.F.G.–S;
assisted by J.I.A., A.D. and J.M.F.; funding acquisition: J.I.A.; investigation: only
contribution by J.F.G.–S.; methodology: J.F.G.–S. and Í.N.–B.; project administration:
Í.N.–B.; supervision: J.I.A.; validation and visualization: Í.N.–B. and R.S.; writing—
preliminar, draft, review and editing: main contributor J.F.G.–S. ,assisted by A.D. J.M.F.
and J.I.A.
Capítulo I. Parte A
114
Funding
This study was funded by Spanish Ministry of Economy and Competitiveness
(MINECO), grant number MAT2015–70728–P, and by the Government of Navarra
(Gobierno de Navarra) under the title “Ayudas a Centros tecnológicos y Organismos de
investigación y difusión de conocimientos para la realización de proyectos de I+D para el
año 2018”, grant number Exp. 0011–1383–2018–000005, project PC065 RECURBAN. The
second author thanks the Friends of the University of Navarra, Inc., for a pre–doctoral grant.
Acknowledgments
The authors thank the technical support provided by Cristina Luzuriaga.
Conflicts of Interest
The authors declare no conflict of interest.
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Capítulo I. Parte B
123
Combination of Polymeric Superplasticizers, Water Repellents and
Pozzolanic Agents to Improve Air Lime–Based Grouts for Historic
Masonry Repair
Jesús Fidel González–Sánchez1, Burcu Taşcı2, José M. Fernández1, Íñigo Navarro–Blasco1,
José Ignacio Álvarez1*
1 MATCH Research Group, Chemistry Department, School of Sciences, University of Navarra, 31008
Pamplona, Spain; [email protected], [email protected], [email protected], [email protected]
2 Deparment of Architecture, Izmir Katip Çelebi University, 35620 Izmir, Turkey; [email protected]
* Correspondence: [email protected] or [email protected] ; Tel.: +34948425600
Received: 12 March 2020 / Revised: 7 April 2020 / Accepted: 10 April 2020 / Published: 11 April 2020
Abstract
This paper presents the experimental procedure to develop air lime–based injection grouts,
including polymeric superplasticizers, a water repellent agent and pozzolanic agents as
additives. Our research focuses on the development of grouts to improve various
characteristics simultaneously by combining different additions and admixtures. Aiming to
improve the injectability of the grouts, in this study, different polymeric superplasticizers
were added, namely polycarboxylated–ether derivative (PCE), polynaphthalene sulfonate
(PNS) and condensate of melamine–formaldehyde sulfonate (SMFC). As a water–repellent
agent, sodium oleate was used to reduce the water absorption. The enhancement of the
strength and setting time was intended by using microsilica and metakaolin as pozzolanic
mineral additions. Compatibility between the different admixtures and action mechanism of
the different polymers were studied by means of zeta potential and adsorption isotherms
measurements. Diverse grout mixtures were produced and investigated by assessing their
injectability, fluidity, stability, compressive strength, hydrophobicity and durability. This
research led to several suitable mixtures produced by using more than one component, to
enhance efficiency and to provide better performance of grouts. According to the results,
the grout composed of air lime, metakaolin, sodium oleate and PCE was found to be the
most effective composition, improving the mechanical strength, injectability and
hydrophobicity.
Keywords: polymeric superplasticizers; zeta potential; adsorption isotherms; steric
hindrance; grouts; injectability; hydrorepellency; freeze–thaw cycles
Capítulo I. Parte B
124
1. Introduction
One of the most widely used methods addressed to repair different masonry defects and
cavities in the preservation of the Built Heritage is the injection of grouts [1,2,3]. Grouts,
fluid mixtures made of water, binder and additives, must properly flow—under an
appropriate pressure—into a masonry wall in a fresh state [4,5]. The literature has pointed
out some requirements for grouts in fresh state, such as high penetrability (i.e., injectability)
and good stability of the suspension (meaning no, or at least limited, segregation and
bleeding) [6,7]. In addition, the grout must be chemically compatible with the ancient
masonry, in order to prevent the historic structure from damages caused, for example, by
high contents in soluble salts. Mechanical compatibility is another requirement; for instance,
repair materials with too–high stiffness are not compatible with the old masonry [8,9].
Taking into account these aspects, natural hydraulic lime (NHL) and hydraulic lime (as
obtained by air lime with pozzolana) have been the most widely used binders for repair grouts
of the Architectural Heritage, as they offer suitable chemical and mechanical compatibility
[1,3,4,6,7,9,10,11,12,13,14]. Pure air lime grouts face up to the poor water retention,
excessive drying and subsequent shrinkage [9,15] and have been mainly tested for non–
structural applications [14], whereas cement–based grouts or organic grouts are not
chemically compatible, and excessive stiffness is also observed for the former [16].
Therefore, one of the main challenges concerning the research on these materials is the
design of tailored grouts [4]. Additives and admixtures are very useful to enhance different
properties of the grouts: For example, polymers behaving as superplasticizers would promote
injectability, as a critical parameter for the applicability of the grouts, which will also improve
due to the mixing water reduction in the final hardened microstructure [11,12,14,17]; water–
repellent agents would impart hydrophobicity to the hardened grouts, reducing the water
uptake [18,19,20]; and pozzolans additions would increase the mechanical resistance and
accelerate the setting time [11,15,21,22,23].
Some previous works have highlighted the advantages of these additives/admixtures,
individually added to binding materials. For example, various advantages of using polymers
acting as superplasticizers (SPs) in lime grouts have been ascertained
[6,7,10,11,12,13,14,17,22,23,24,25]. As polymeric admixtures, the superplasticizers increase
the fluidity of the fresh grouts, promote suitable injectability and improve the workability at
a constant water/binder ratio. When these polymers are added into the grout mixture, they
prevent particles from agglomeration acting as dispersing agents and thus reducing the water
Capítulo I. Parte B
125
demand [21,26]. Most works have addressed lime–based grouts with the addition of a
superplasticizer (commonly polycarboxylated ether) [11,12,14,17,23,25,27]. Much more
limited information has been produced on the effect in lime grouts of other SPs such as
polynaphthalene sulfonate (PNS) and poly–melamine sulfonate (SMFC), commonly tested
in the cement chemistry [24,25,28].
The use of water–repellents is of importance to minimize the uptake of water in grouts
and mortars. The access of water to the inner part of hardened grouts and mortars is largely
detrimental for the structural integrity of the masonry: Water dissolves soluble salts, giving
rise to efflorescences. Furthermore, it takes part in freeze–thaw cycles, provoking severe
mechanical damages to the masonry. Hydrophobicity imparted by water–repellent
admixtures would enhance the long–term resistance of the grouts. Dual organic compounds,
with a polar moiety (usually a carboxylic group) and a hydrophobic tail, such as calcium
stearate and calcium oleate, have been studied [18,19,20].
The use of pozzolans has been widely reported, and some studies have described the
possibility of improving resistance, accelerating setting times and permitting hardening—
even when CO2 is scarcely available—by adding pozzolana to mixtures [29], which is
noteworthy for injection grouts applied in deep fissures and cavities with restricted CO2
access. Metakaolin (MK) has been one of the most widespread studied pozzolans, although
nanosilica has also been the target in some research works [21,30,31,32]. MK is usually
processed by calcination of high–purity kaolin clay at temperatures ranging between 650 and
800 °C. It contains silica and alumina in an active form, and they react with the calcium
hydroxide of the air lime (Ca(OH)2, CH), yielding hydrated calcium silicate (C–S–H) phases,
and also C2ASH8 and C4AH13 as hydrated silico–aluminate and hydrated aluminate phases,
respectively. The filler effect of metakaolin, together with the production of new hydrated
phases, results in improved air lime–based grouts’ properties, such as setting time and
compressive strength, while also preventing hardened grouts from microcracking [21]. The
effect of the increasing replacement of NHL by metakaolin has also been studied [15].
Another pozzolana tested in binding materials is microsilica (MS), generally comprised of
amorphous silicon dioxide as a fine powder. The material is a product of the silicon and
ferrosilicon, and it is produced in smelting industries. Studies with MS in concrete showed
favorable results for strength–supporting sulphate exposure [33,34].
A promising way of modulating the characteristics of the grouts is the simultaneous
combination of different admixtures and mineral additions. Only a few works have dealt with
Capítulo I. Parte B
126
the obtainment of grouts by combining, for example, a superplasticizer and a water retainer
[9,16,21], or a superplasticizer together with a pozzolana (metakaolin, nanosilica or silica
fume) [17,25], but a systematic study on quaternary mixtures, analyzing the effect of air lime
as binder with polymeric superplasticizers, a water–repellent agent and pozzolanic additions,
is not available.
Accordingly, the context and the rationale of the current work is that synergistic
simultaneous combinations between air lime, a superplasticizer, a water–repellent agent and
pozzolana would make it possible to obtain tailored injection grouts suitable for restoration
of the Built Heritage.
The following raw materials were used for the combinations: calcitic air lime, three
different polymer–based admixtures, which are superplasticizers: polycarboxylate ether
(PCE), polynaphthalene sulfonate (PNS) and poly–melamine sulfonate (SMFC); a water–
repellent agent (sodium oleate); and two types of pozzolanic addition (metakaolin and
microsilica). Fresh state properties of the grouts, such as injectability, bleeding and fluidity
(as measured by the slump test), were determined. Action mechanisms, interactions and
compatibility between the tested admixtures were assessed by measuring zeta potential of the
suspensions and adsorption isotherms of the admixtures. Hardened state was also assessed,
by evaluating the compressive strengths, carbonation rate, hydrophobicity and pore structure,
and the durability of the grouts was finally studied by exposing the samples to freezing–
thawing cycles. The influence of the different additives/admixtures of the grout compositions
on these parameters is later discussed.
2. Materials and Methods
2.1. Materials and Composition of the Grouts
Mixing proportion of the grouts was 1:3 binder/aggregate weight ratio, according to
previous prescriptions [1]. Binder was CL–90 hydrated calcitic lime (Cal Industrial S.A.
Navarra, Spain) (CaO percentage 68.53%, with major impurities of MgO (3.29%), SO3
(1.37%) and SiO2 (1.03%)). Mean particle size was 10 μm (less than 10% > 50 μm). A very
fine limestone aggregate, with particle size lower than 2 mm, was used and supplied by CTH
(Huarte, Navarra, Spain).
For the different mixtures, the following components were added, with respect to lime,
when necessary:
Capítulo I. Parte B
127
• Polymer–based superplasticizer (SP) (two different dosages 0.5% and 1% by weight
of lime (bwol)): polycarboxylate ether (PCE), commercialized by BASF as Melflux;
condensate of melamine–formaldehyde sulfonate (SMFC), commercialized by BASF
as Melment F10 (Ludwigshafen, Germany); polynaphthalene sulfonate (PNS),
commercialized by FOSROC International as Conplast SP340 Fa (Fosroc Euco S.A.,
Izurtza, Spain).
• Water–repellent agent (0.5% bwol): sodium oleate (O), provided as a commercial
product: HISA A 2388 N by ADI–Center–S.L.U (Barcelona, Spain).
• Pozzolanic additions (20% bwol): Metakaolin (MK) (Metaver, supplied by,
NEWCHEM, Pfäffikon, Switzerland) and microsilica (MS), supplied by ULMEN
Europa (Castellón, Spain).
The first polymer–based superplasticizer used was PCE (Figure 1A), which consists of
one main linear backbone with side carboxylate and ether groups. The carboxylate groups
are the anchoring groups by which the adsorption of these admixtures to cement particles
takes place [17,26].
The second polymeric SP belongs to the family of the sulfonated melamine
formaldehyde condensates (SMFC) (Figure 1B). In this synthetic polymer, each repeating
unit contains one sulfonate group. The condensation number (n) is usually in the 50–60 range,
giving a molecular weight in the order of 12,000–15,000 [35].
The third employed polymer was PNS (Figure 1C), in which its molecular structure is
characterized by a hydrophobic moiety (naphthalene) and a hydrophilic part (sulfonate
groups).
Figure 1. Structures of different superplasticizer a) PCE b) SMFC c) PNS.
AR
OM
OH
(EO)n
Me
R
OH
n mM = Metal
Me = Methyl
EO = Oxyethylene
R = Me, H
B
N+
N
N+
NH
CH2
SO3
-
NHCH2H NH CH2 OH
Na+
n
C
H
CH2
H
SO3-Na
+
n
Capítulo I. Parte B
128
Table 1. Characteristics of the polymers.
Admixture Mw
(Da)
Anionic
charge
density
(meq g–1)
Elemental composition
C (%) H (%) O (%) N (%) S (%) Na (%)
PCE 8000 0.43±0.05 47.62±0.80 7.65±0.13 42.2±0.05 – – 2.53±0.01
SMCF 12302 2.26±0.04 20.80±0.04 3.71±0.05 31.83±0.30 23.60±0.24 10.7±0.12 9.36±0.20
PNS 8620 2.44±0.07 43.92±0.46 3.79±0.01 29.03±0.45 – 12.3±0.19 10.96±0.03
Oleate n.d.* 3.32±0.13 69.97±0.03 10.50±0.01 11.30±0.20 – – 8.30±0.21
* not determined
In some previous works, the most relevant properties of these polymers, such as
molecular weight, anionic charge density and elemental composition, as well as the methods
to assess these values, were reported [11,27]. The molecular weights (Mw) of the polymers
were determined by size–exclusion chromatography. Anionic charge densities of each one of
these polyelectrolytes were obtained by titration, using the positively charged Poly–
DADMAC (acid–base titration for oleate). A LECO analyzer (LECO Corporation, St Joseph,
MI, USA) was used to determine the elemental composition of the polymers. Table 1 gathers
these values.
Sodium oleate was added as a water–repellent agent (O) (see characteristics in Table 1).
This compound is characterized by a long non–polar hydrocarbon chain and a polar
carboxylate group at one end, having a bipolar nature. Therefore, it may be adsorbed and
concentrate at the air–paste interface, usually in the air bubble surface. This fact causes
reinforcement of the air bubbles and avoids coalescence [27]. This admixture has also been
reported in the scientific literature on cement mortars as AEAs [29,30,31,32]. The dosage
was 0.5% of the total dried mortar weight, in agreement with a previous work that reported
the enhancement lime–based mortars at that dosage [27].
Specific surface areas, as measured by the BET method after N2 adsorption isotherms
(ASAP 2020, Micromeritics, Norcross, GA, USA), for MK and MS were of 20.00 and 15.70
m2·g−1, respectively. According to the supplier, microsilica particles are spherical, main
range of primary particle sizes between 0.2 and 1 µm, and the MS composition is at least
85% SiO2 content, with low carbon content [36]. The average particle sizes in aqueous
suspensions were of ca. 3.9 µm for MK and 380 µm for MS (particle size distribution
determined by laser diffraction in a Malvern Mastersizer, Malvern Instruments, Ltd.,
Capítulo I. Parte B
129
Malvern, UK, depicted in Figure 2), evidencing a clear agglomeration of the MS in
comparison with the particle size of the primary particles [11,36].
With the aim of assessing the effect of the different admixtures in the properties of
grouts, particularly in the injectability, a 31% of mixing water was established for all samples.
This value was reached after carrying out an adjustment of the water demand of the control
mortar (additives/admixtures–free) to obtain a spread flow diameter of 185 mm as measured
in the flow table test. The different compositions of the 24 prepared and tested mixes are
collected in Table 2.
2.2. Preparation Procedure and Curing Conditions
The grouts were prepared mixing the powdered hydrated calcitic lime, the sand and,
when necessary, the pozzolanic addition and the solid admixtures (water–repellent and
superplasticizers) for 5 min, using a solid–admixtures mixer BL–8–CA (Lleal, S.A., Spain).
After this step, the mixing water was added and mixed for 90 s, at low speed, and adjusted
according to UNE–EN 196–1, in a Proeti ETI 26.0072 (Proeti, Madrid, Spain) mixer [37].
Prismatic molds of 40 × 40 × 160 mm were used for casting fresh grouts. Standard EN
196–1 was followed for the filling in two layers and for the compaction using an automatic
compactor (IBERTEST iB32–045E–1, S.A.E. Ibertest, Madrid, Spain), with the aim of
removing the air bubbles present in the mixture. Molds were stored at lab conditions (20 °C
and 60% RH), and hardened grouts were demolded 5 days later. Hardened state properties
were studied after different curing ages: 7, 28, 91, 182 and 365 days. Representativeness of
the results was guaranteed by testing three replicates of the grouts per each curing time.
Figure 2. Particle size distribution of the pozzolans
Capítulo I. Parte B
130
Table 2. Composition of the grouts (% values).
Name Lime Sand Pozzolanic addition* Water
repellent*
Oleate
Superplasticizer*
Microsilica Metakaolin PCE SMFC PNS
Control samples
(without
polymeric
superplasticizers)
C 25 75 – – – – – –
C–MS 25 75 20 – – – – –
C–MK 25 75 – 20 – – – –
C–O 25 75 – – 0.5 – – –
C–O–MS 25 75 20 – 0.5 – – –
C–O–MK 25 75 – 20 0.5 – – –
Samples without
pozzolanic
addition
O–PCE0.5 25 75 – – 0.5 0.5 – –
O–SMFC0.5 25 75 – – 0.5 – 0.5 –
O–PNS0.5 25 75 – – 0.5 – – 0.5
O–PCE1 25 75 – – 0.5 1.0 – –
O–SMFC1 25 75 – – 0.5 – 1.0 –
O–PNS1 25 75 – – 0.5 – – 1.0
Samples with
Microsilica
O–MS–
PCE0.5 25 75 20 – 0.5 0.5 – –
O–MS–
SMFC0.5 25 75 20 – 0.5 – 0.5 –
O–MS–
PNS0.5 25 75 20 – 0.5 – – 0.5
O–MS–
PCE1 25 75 20 – 0.5 1.0 – –
O–MS–
SMFC1 25 75 20 – 0.5 – 1.0 –
O–MS–
PNS1 25 75 20 – 0.5 – – 1.0
Samples with
Metakaolin
O–MK–
PCE0.5 25 75 – 20 0.5 0.5 – –
O–MK–
SMFC0.5 25 75 – 20 0.5 – 0.5 –
O–MK–
PNS0.5 25 75 – 20 0.5 – – 0.5
O–MK–
PCE1 25 75 – 20 0.5 1.0 – –
O–MK–
SMFC1 25 75 – 20 0.5 – 1.0 –
O–MK–
PNS1 25 75 – 20 0.5 – – 1.0
* % by weight of lime
2.3. Fresh–State Tests and Analyses
For all the following tests, at least three replicates were carried out for each one of the
performed tests, and each one of the grouts’ compositions, so that the depicted values are an
average value of all the recorded measurements.
• The flow table test (according to the EN 1015–3 [38]) was followed, to monitor the
slump flow measurements, after 15 strokes of the flow table. The larger the spread
diameter, the higher the fluidity of the grout.
Capítulo I. Parte B
131
• Workability was determined as the period in which the degree of stiffness of the grout
hinders the penetration of a piston. Workability can be related to the setting time of
the grouting mixture (the shorter the workability time, the shorter the setting time).
According to the standard EN 1015–9 [39], every 15 min, a probe was slowly
introduced into the fresh grout, scoring the weight, which was gradually increasing
due to the hardening of the grout. When this weight reached 1500 g, the assay was
concluded.
• A Zeta potential electroacoustic analyzer (ZetaProbe Analyzer, Colloidal Dynamics,
Ponte Vedra Beach, FL, USA) was used to determine the surface charge of the
suspensions of the air lime with the additives. Two batches of experiments were
carried out:
(a) Initial media of air lime, water and, when necessary, pozzolanic additives and
sodium oleate were prepared by following the same compositions detailed in
Table 2. Solutions of polymer–based superplasticizers (1% w/w) were then used
as titrant media, and zeta potential values were continuously monitored.
(b) Initial media of air lime, water and, when necessary, pozzolanic additives and SP
were prepared by following the same compositions detailed in Table 2. Solution
of sodium oleate (1% w/w) was, in this case, used as titrant media, monitoring
the zeta potential values.
• Adsorption isotherms were obtained after carrying out different sorption assays.
Different batches of flasks were prepared: one, with 5 g of air lime per 25 mL of
water; two more batches with also pozzolanic additive (either MS or MK, 20 wt.%
with respect to the lime). In some flasks, when required, pre–adsorption of some
admixtures was also carried out incorporating either SP or oleate (1 wt.% or 0.5 wt.%
with respect to the lime, according to the proportions reported in Table 2) and mixing
the dispersions for 30 min. The adsorption of the admixtures, either sodium oleate or
SPs, was studied adding increasing amounts of the admixture (0.0125, 0.0250,
0.0375, 0.0500, 0.1000, 0.1500, 0.2000 g) to the different flasks. Dispersions were
magnetically stirred for 30 min and then centrifuged at 8000× g for 15 min. The
supernatant was collected and filtered (0.45 µm PTFE filters). The difference between
the initial (added) and final concentration (remaining solution concentration) was
deemed to be the admixture adsorbed amount. UV–VIS spectrophotometry was used
to measure the concentration of the admixture in the solution (maxima at λ = 221, 222
Capítulo I. Parte B
132
and 296 nm for PCE, SMFC and PNS). The mathematical fitting of the adsorption
data was calculated for Langmuir and Freundlich models.
• Bleeding test refers to the determination of a water layer that could appear on the
surface with a clear separation line between water and grout [4]. Bleeding tests were
carried out in a graduated cylinder, where grout was placed, and the accumulation of
bled water and the expansion volume were measured over 15, 30, 45, 60, 120 and 180
min. The tests were performed according to EN 447 and adapting of ASTM C940
[40,41]. Final bleeding (after 180 min) should be lower than 5%.
• Grouts must be suitable for injection through a syringe or tubing, to fill internal cracks
and voids. An injectability test was carried out by injecting the grout at constant
pressure to a vertically held column, from its bottom part (column was a transparent
methacrylate tube height 390 mm and inner diameter 21 mm) (see experimental setup
in Figure S1, Supplementary Materials). The column was filled with granular material
whose characteristics are explained below (Table 3). This test is an adaptation of the
sand column test (EN 1771: Determination of Injectability Using the Sand Column
Test [42]), to be used for injection grouts. Injectability of a grouting mixture in a
capillary network under predefined pressure is defined by the distance traveled by the
grout as a function of time according to EN 1771. In this work, (according to the
recommendations reported in Evaluation of Lime–Based Hydraulic Injection Grouts
for the Conservation of Architectural Surfaces [43]), the material suggested in the
standard for achieving a flow into a 0.2 mm crack in concrete is replaced by crushed
travertine with grain sizes of 2–4 mm, a size that simulates an approximately 0.3–0.6
mm crack width. Each grout was prepared by mixing for exactly 3 min, using the
same procedure adopted in the fluidity tests. The pressure used for filling the cylinders
(0.075 MPa) was constant due to the use of an equipment of injection known as
“pressure pot”, for 60 s. The time required for the complete filling of the cylinders
was recorded.
Table 3. Porous media characteristics (travertine).
Characteristic Value
d (90) 3.8 mm
d (10) 2.9 mm
Porous media porosity 47%
Water Absorption 6.6%
Capítulo I. Parte B
133
For these tests, high water/binder ratio of 1.24 was applied constantly due to the high
water demand of the air lime and to the use of pozzolans [32,44].
Several characteristics of the porous media were determined (Table 3): (a) parameters
d(90) and d(10), which are respectively the diameter through which 90% and 10% of
the total mass pass; (b) the total porosity, which was evaluated by measuring the
volume of water which could be filled inside each cylinder, to know the available
voids inside the column; and (c) the water absorption of the travertine.
During injectability tests, identical conditions were applied to the mixing procedure, and
environmental conditions were kept constant. Pre–wetting was not applied due to the low
water absorption of the travertine (6.6%) and to the detrimental effect on the adherence
between the filler and the grout and on the mechanical strength reported [6].
Injectability rate was defined for numerical comparison, by using the time for the grout
to reach the top of the cylinder, the quantity of injected grout, height of introduced grout and
amount of the voids with the formula [6] given below:
where I is the grout injectability (s−1), t the grout injection time to fill the injected height
(s), m the injected mass during the injection process (g), ρ the density of grout (g/mL) and
VV is the voids volume of porous media (mL).
After the injectability experiments, the cylindrical methacrylate tubes with the fresh
grouts were laid on a horizontal position and cured under lab conditions for at least 28 days.
Slices extracted from the central part of the columns were cut, to assess the filling of the
voids.
2.4. Hardened–State Tests
• Compressive strengths were measured after 7, 28, 91, 182 and 365 curing days in the
4 × 4 × 16 cm prismatic specimens. A device Proeti ETI 26.0052 (Proeti, Madrid,
Spain) was used at a breaking speed 5–50 KP s−1 with a time interval between 30 and
90 s in the compressive strength tests.
Capítulo I. Parte B
134
• Thermal analysis of the hardened grouts was carried out with a simultaneous TG–
sDTA 851 Mettler Toledo thermoanalyzer device (Schwerzenbach, Switzerland),
using alumina crucibles. Samples were heated from 25 to 1000 °C, at a rate of 10
°C·min−1, under static air atmosphere.
• The porous structure of the hardened grouts was studied by Mercury Intrusion
Porosimetry (MIP), using a Micromeritics AutoPore IV 9500 equipment
(Micromeritics Instrument Corporation, Norcross, GA, USA) (pressure range
0.0015–207 MPa).
• The evaluation of the wettability of the hardened grouts was performed by measuring
hydrophobicity through the static water contact angle of the samples, with an
equipment OCA 15EC (DataPhysics Instruments GmbH, Filderstadt, Germany). Five
water droplets at five different points of 5 μL were put onto the surface of the
hardened grouts, and the reported results are averages of these measurements.
2.5. Durability
Prismatic specimens of the hardened grouts–prepared and cured 28 days as described
before—were tested to assess the durability in the face of freezing–thawing cycles. The
cycles for the evaluation of the frost resistance consisted of water immersion of the samples
for 24 h and a subsequent freezing at −10°C for 24 h. For these experiments, a CARAVELL
521–102 freezer (Caravell Ltd., Buckingham, UK) was used.
The structural integrity of the samples was visually assessed after the finishing of each
freeze–thaw cycle, according to a previously reported criterion [22], which ascribes the
following alteration state of the treated specimens:
• None: alteration for those samples with no evidence of decay.
• Scarce: for samples showing a slight degree of deterioration (some thin, short,
shallow cracks on the surface of the specimens).
• Moderate: for altered samples, showing several deeper cracks.
• Large: for heavily altered specimens presenting deep cracks and a certain degree of
swelling.
• Very large for samples with severe decay, large deep cracks, partial weight loss and
large swelling.
Capítulo I. Parte B
135
• Total for destroyed samples, with only some parts remaining.
3. Results
3.1. Properties of the Fresh Grouts
3.1.1. Fluidity (Spread Diameter)
All mixtures without superplasticizers (control samples) presented similar fluidity values
as compared to that of the control grouts (sample C), as measured by the spread diameter in
the flow table test (Figure 3). The pozzolanic additions showed a gradual spread reduction of
the fresh grouts, the pattern being: free–pozzolan > microsilica > metakaolin. This finding
may be explained by considering the increased water demand of the samples with pozzolanic
addition due to the high specific surface area of the pozzolans. This is in line with the
observed spread diameter reduction in air lime pastes with pozzolanic agents [11].
Differences between the two pozzolanic additions can be ascribed to the different particle
size. In spite of the relatively small surface area differences and to the similarities in reactivity
and particle size reported in the literature for these additions [45,46], in the current work, as
observed in Figure 2, MS particles exhibited a strong tendency to flocculate in the used
aqueous systems, thus giving rise to large and less reactive agglomerates.
The influence of the superplasticizers on the spread values of fresh grouts with these
admixtures was as follows. PCE addition resulted in a sharp fluidity increment, with spread
diameter values higher than 300 mm, irrespective of the mix composition. Thus, PCE was
seen to yield high–fluidity grouts, in good agreement with the previously reported
effectiveness of the polycarboxylate ether derivatives both in lime and cement–based systems
[11,14,26].
Figure 3. Spread diameter values of the different mixtures
Capítulo I. Parte B
136
Figure 4. Zeta potential values of the binary lime + oleate and ternary lime + oleate +
pozzolan systems titrated with the three SPs (PCE, SMFC and PNS).
PNS and SMFC showed a similar behavior in all mixtures, for each one of the two tested
dosages. The effectiveness of the dispersing action, as measured by the spread, was not as
good for these SPs as it was for PCE. The similarity between PNS and SMFC arises from the
likenesses in their linear molecular structure, in line with the general structure of these
admixtures reported by Gelardi et al. [47]. These SPs exhibit mainly an electrostatic repulsion
mechanism, due to their flat adsorption onto the binder particles and to their high anionic
charge density (see Table 1), whereas steric hindrance was seen to play a minor role, as
described by Pérez–Nicolás et al. [27]. This action mechanism has been proven to be less
efficient than the steric hindrance action (prevalent for PCE).
Zeta potential measurements were carried out, upon titration with the different polymer–
based SPs, for the lime–oleate pastes, as well as for the same pastes with the two pozzolanic
additions (Figure 4).
As it can be seen, the pastes initially (before the addition of SPs) yielded positive zeta
potential values (40–50 mV). Pérez–Nicolás et al. [27] indicated that air lime particles
exhibited positive zeta potential values due to the positive charge of the portlandite crystals.
In the presence of pozzolanic addition, the expected formation of C–S–H compounds has
also been observed to yield high positive values of zeta potential: The negative charge caused
by the silanol groups’ (Si–O–H) deprotonation in the C–S–H phases is strongly sheltered by
Capítulo I. Parte B
137
the adsorption of the Ca2+ ions [27], recognized as potential–determining species [48],
leading to a positive overcharging phenomenon.
Consistently, zeta potential values remained practically unaltered after the first 3 to 5
additions of SPs (Figure 4). After that, a dramatic increase in the zeta potential was observed
irrespective of the SP tested, and then a gradual decrease toward lower zeta potential values
was observed. The formation of a second adsorption layer accounts for this finding.
In the presence of the oleate chains, and owed to the addition of the SP, the adsorption
saturation dosage of the first layer was quickly achieved, and a second layer of calcium ions
sheltered the first polymer adsorption layer, thus resulting in a sharp increase of the zeta
potential values. A second layer of adsorbed polymer started on top of the calcium ions’
layer, explaining the gradual decrease as a consequence of the negatively charged polymeric
molecules (due to the deprotonation of the active groups at the alkaline pH) and of the
displacement of the shear plane of the outer Helmholtz layer [11].
In support of these assertions, several experimental findings can be argued:
(i) Adsorption isotherms of sodium oleate onto lime particles (with and without
pozzolanic additives) revealed a very strong adsorption of oleate onto these
particles, making it reasonable achieving the saturation dosage of the first layer
(Figure 5). Almost–negligible adsorption was observed for aqueous suspensions of
pozzolans, confirming the strong influence of Ca2+ ions on the oleate adsorption, in
agreement with the reported values in previous works by Wang, Z. et al. and Wang,
Y. et al. [49,50] that described a sharp oleate adsorption onto minerals in the
presence of calcium cations.
Figure 5. Adsorption isotherms of the oleate onto systems of lime, pozzolans
and lime with pozzolanic additives (MK or MS).
Capítulo I. Parte B
138
Table 4. Parameters of the mathematical adjustment to Langmuir and Freundlich
algorithms for the adsorption isotherms of SPs onto lime with pre–adsorbed oleate.
System SP Langmuir Freundlich
qm (mg g−1) b (dm3 mg− 1) R2
K (mg1 − 1/ndm3/ng−1) 1/n R2
Lime–oleate PCE 43.2 0.00001 0.1091 0.00186 0.8475 0.9485
Lime–oleate SMFC 28.2 0.00026 0.9347 0.01615 0.8239 0.9763
Lime–oleate PNS 36.8 0.00019 0.9056 0.02251 0.7790 0.9530
Notes: qm: maximum sorption capacity. b: the Langmuir constant. K, 1/n: the Freundlich
constants. R2: correlation coefficient of the linear regression.
(ii) Adsorption isotherms of the superplasticizers onto lime particles, in which oleate
was previously adsorbed, also showed the ability of the SPs to be adsorbed in a
similar amount to the one that took place in the plain lime systems (Figure 6). This
adsorption onto lime particles in which oleate molecules were pre–adsorbed can
only be explained by assuming a double–layer adsorption. Isotherms also fit well
into a Freundlich model (see high R2 values in Table 4).
(iii) Zeta potential curves obtained for lime systems (with or without pozzolanic agent)
with pre–adsorbed superplasticizer, upon titration with a sodium oleate solution,
were totally different (Figure 7): All curves showed a slight and continuous increase
toward more positive values, without any sharp change in the curves. The zeta
potential curves followed the same pattern as that of the SP–free systems titrated
with sodium oleate. These curves could correspond to a simple monolayer
adsorption process, in which oleate was adsorbed onto (a) free active sites and (b)
in the sites previously occupied by SP molecules, which were removed due to a
competition process.
Figure 6. Adsorption isotherms of the three tested SPs onto systems of lime and lime
with pre–adsorbed oleate.
Capítulo I. Parte B
139
Figure 7. Zeta potential of simple lime systems, binary systems of lime with pozzolanic
additives (MS, left diagram; MK, right diagram), and ternary systems with pozzolanic
additives and pre–adsorbed SP. All the systems were titrated with a sodium oleate (1 wt. %)
solution.
This assumption was later confirmed by adsorption isotherms studies of oleate in
lime systems with pre–adsorbed superplasticizer. It was seen that all added oleate
remained fully adsorbed, whereas the concentration of SP in the supernatant solution
increased as more oleate was added. For example, this system yielded a 100.0 ±
0.9% of PCE in solution (that is, all the PCE was released), whereas the adsorption
of PCE in a system with pre–adsorbed oleate resulted in a lower non–adsorbed
polymer percentage of 92.94 ± 0.2% (that is, 7.06% of PCE remained adsorbed),
thus confirming, as explained in (ii), the double–layer adsorption. This strong and
competitive adsorption of oleate can be understood when considering its higher
anionic charge density in comparison with the SPs.
(iv) Furthermore, the literature has described the multilayer adsorption of polymers onto
mineral particles, as, for example, in the case of oleate (forming calcium dioleate
layers) [50] and in the case of other superplasticizers [27,51].
Different patterns were observed in the zeta potential curves from the point of highest
value onward, upon further additions of the three tested SPs, as can be seen in Figure 4:
(a) For PCE, zeta potential moved slightly toward lower positive values. The adsorption
of the PCE (depicted in this second part of the curves of zeta potential) did not cause
a substantial surface charge modification, confirming the weak influence of the
anionic charge of this SP (which was the lowest, as reported in Table 1). The strong
steric hindrance of the side chains of this polymer is more effective than the
electrostatic repulsions of the negatively charged carboxylated groups. The
Capítulo I. Parte B
140
predominant effect of the steric hindrance in this polymeric SP was confirmed by
its high impact in fluidity (Figure 3), while it simultaneously did not dramatically
modify the surface charge of the particles. The literature agrees about the prevalence
of the steric hindrance mechanism for similar polymer molecules [51,52,53].
(b) For SMFC and PNS, the adsorption of the SP caused a clear decrease in the zeta
potential values (sharper in the case of SMFC), finally resulting in a charge reversal
into negative values of the zeta potential (Figure 4). The action mechanism of these
two polymers can be linked to the electrostatic repulsions, particularly under
alkaline conditions that fostered the ionization of the sulfonic groups [51,54]. The
dosage at which the IEP was achieved would be the optimum dosage of the SP.
SMFC inverted the sign of the surface charge at lower dosages and should be
expected to be more effective than PNS. The higher molecular weight of this SMFC
polymer (Table 1) contributes to enhance the predominantly efficient steric
repulsions, thus explaining these experimental findings.
3.1.2. Workability
The use of PCE induced noticeable changes in workability for almost all mixtures
(Figure 8). The addition of the PCE in microsilica–bearing samples gave rise to a delay in
the stiffening of the grouts. This well–known effect has been pointed out in previous works
and can be ascribed to the attachment of the PCE onto binding particles, which hinders their
irreversible agglomeration, avoiding the early hardening of the grouts [11,55]. This delay
was not observed for samples with MK at the highest dosage of PCE and can be explained
by considering the fast pozzolanic reaction of the MK, as compared with microsilica.
Stiffening time was shortened when SMFC and PNS were added as SPs. In almost all cases,
for these two SPs (SMFC and PNS), the use of a 0.5% dosage yielded a sharper shortening
of the stiffening time, which is directly linked to the poorer dispersing action of the lowest
SP dosage due to the lower amount of adsorbed polymers.
Capítulo I. Parte B
141
Figure 8. Workability of the different mixtures
3.1.3. Bleeding
Bleeding of a fresh grout affects the quality of injection, since it causes clogging during
application. A high bleeding value is an indicator of the absence of stability of the grout and
can be due to the presence of admixtures of different hydrophilic character in the mixture.
This segregation could increase along time, at least in the initial steps of the process. During
the grout injection, bleeding undermines the effectiveness of the grout, because the upper
part of the pores cannot be filled, due to the excess of water [4]. To assess the stability of the
designed lime grouts, percentages of volumetric changes and bleeding of the different
mixtures were determined (Table 5). All assayed samples presented very small volumetric
changes, always below 1% (results not shown). The bleeding percentages were also low, and
the obtained results fell within the tolerable limits (below the threshold value of 5%), as
reported elsewhere [13,40].
Different percentages of segregation were obtained for each group of studied mortars.
Samples without pozzolanic addition exhibited the lowest bleeding values. Samples with
pozzolanic additions yielded higher bleeding values, although all of them were below the
limit value. Accordingly, the designed grouts do not present excess of free water and can be
considered as stable [12].
PCE as admixture with pozzolanic materials presented the lowest bleeding percentages,
except for samples with microsilica. This fact can also be related to the setting time delay,
since PCE hindered the irreversible agglomeration of the binding particles, thus causing
segregation [11].
Capítulo I. Parte B
142
Table 5. Bleeding and injectability values (fresh grouts).
Sample Bleeding * (%) Injectability (s−1)
Control samples (without polymeric superplasticizers)
C – 0.006
C–MS <1% 0.016
C–MK <1% 0
C–O <1% 0.005
C–O–MS 2 0
C–O–MK 2 0
Samples without pozzolanic addition
O–PCE0.5 1 0.04
O–SMFC0.5 <1% 0.015
O–PNS0.5 2 0.022
O–PCE1 2 0.05
O–SMFC1 <1% 0.036
O–PNS1 2 0.033
Samples with microsilica
O–MS–PCE0.5 <1% 0
O–MS–SMFC0.5 <1% 0
O–MS–PNS0.5 <1% 0
O–MS–PCE1 4 0
O–MS–SMFC1 2 0
O–MS–PNS1 4 0
Samples with metakaolin
O–MK–PCE0.5 <1% 0.059
O–MK–SMFC0.5 1 0.005
O–MK–PNS0.5 2 0.01
O–MK–PCE1 <1% 0.08
O–MK–SMFC1 4 0.022
O–MK–PNS1 4 0
* Values obtained according to EN 447 three hours after initial mixing.
3.1.4. Injectability
Water/binder ratio, the type and percentage of superplasticizer, the mixing procedure,
grain size, pore size, total porosity and water absorption capacity of the filling material can
be mentioned among the parameters influencing the quality of a grout injection [1,6,13,15].
Historic masonries were simulated by using methacrylate cylinders filled with travertine,
in order to reproduce the inner part of historic walls. Cylinders were filled with the 2–4 mm
fraction travertine type. With the aim of providing a reliable model of injectability, the use
of just stable filling materials was selected rather than using filling material combinations.
Furthermore, we did ascertain that filling particles had the same size distribution and their
water absorption capacities were also the same to each other, avoiding variability due to the
moisture content of the particles [1,56].
Capítulo I. Parte B
143
Figure 9. Injection height values and time of injection in the cylindrical columns for the
different grouts
The injectability was measured for the designed grouts, including the two dosages of the
SPs. The time of filling and the height reached by the grouts, upward from the bottom of the
cylinders, were measured and are displayed in Figure 9. The injectability (s−1) was calculated
as detailed in Section 2.3 and values are collected in Table 5.
Grouts without microsilica addition (control samples) were able to flow through the
column, at least partially. Plain lime grout (C) reached a height of 55 mm, and oleate–lime
grout (C–O) went up to a height of 100 mm, both taking ca. 16 s in the excursion. Slump
measurements showed very similar results for these two grouts, although injectability differs,
possibly owing to the air–entraining action of the oleate, since small air bubbles could
contribute to enhance the injectability, as previously reported [18].
The addition of pozzolans did complicate the injectability of grouts. The mixture C–MK
reached 25 mm in 16 s, in good agreement with the fluidity observed during the flow table
test (Figure 3). However, the simultaneous presence of oleate and metakaolin hindered the
injectability of the grout. On the other hand, the addition of microsilica was fully detrimental
for the injectability of the grouts. All the microsilica–containing mixtures, including the
controls (C–MS and C–O–MS), were incapable of flowing through the column. The
microsilica particles acted as a barrier, because of their well–known cohesive forces,
preventing the grout injection [57]. This finding is in very good agreement with the large
Capítulo I. Parte B
144
particle size of the microsilica agglomerates earlier measured (Figure 2), complicating the
achievement of the necessary yield stress at the injection front and making it difficult to flow
through the fine voids of the travertine particles filling the cylinder. In the end, this resulted
in a blockage of the grout penetration and in an injectability obstruction [6]. Accordingly, the
addition of a maximum 10% by weight dosage of this kind of pozzolanic materials is
recommended to improve injectability and to increase mechanical strength in the hardened
state [25].
The addition of the three tested polymeric superplasticizers resulted in different
performances with respect to the injectability of the grouts. The simultaneous presence of
sodium oleate appeared not to foster the injectability (as is the case for samples with PNS
and SMFC in comparison with pure air lime grout control and oleate–containing control).
This result is an expression of a certain incompatibility between the two admixtures (water–
repellent and superplasticizer): As discussed previously, in Section 3.1.2, the strong
adsorption of oleate onto lime particles restricts the attachment of the superplasticizers onto
these particles, thus reducing the SP effectiveness. This fact seems to be of the utmost
importance for the two superplasticizers whose action mechanism is mainly based on
electrostatic repulsions (PNS and SMFC). The dosage of 1% (O–SMFC1 and O–PNS1)
showed very similar values to those published by other authors (which were between 0.016
and 0.038 s−1) [1,6,12,44].
For PCE, its electrosteric repulsion mechanism allowed a better effectiveness, even with
a reduced number of attached molecules of SP. In addition, the favorable plasticizing effect
of non–adsorbed molecules of polycarboxylate ether derivatives, which remain in the
interstitial solution, must be considered, in line with previous findings [58,59,60].
The addition of metakaolin depicted a very different performance pattern between the
SPs, according to their respective main action mechanisms. The addition of MK worsened
the injectability for samples with SMFC and PNS. These results are in line with those
obtained from the flow table test and can be explained by considering the increase in the
water demand due to the pozzolanic agent and the consumption of SP due to the progressive
formation of C–S–H phases. The latter effect has been well described for cement–based
materials in the case of flat polymeric admixtures [58,61].
Capítulo I. Parte B
145
Figure 10. Section images of cylinders filled with grouts after 28 curing days.
Capítulo I. Parte B
146
Conversely, for grouts with PCE, the presence of MK enhanced the injectability, and the
grout filled the whole column in a very short period (less than 10 s). The addition of a material
with high surface area led to an increase in the anchorage active sites for PCE. The branched
molecular architecture of this polymer, together with the recognized activity of the non–
attached molecules, clearly improved the injectability of these PCE–MK grouts. The
effectiveness of the dispersing action of polycarboxylate ethers in lime–based systems has
been noticed in previous works [11,14,17,21] and confirmed in the current research. Grouts,
including PCE as superplasticizer, exhibited the highest injectability values in all cases. The
O–MK–PCE grout showed the best injectability, 0.08 s−1, which is larger than results reported
by other authors [1,6,12,44].
In general, the increase in the dosage of the superplasticizer enhanced the grout injection.
A dosage increase of up to 1% of PNS and SMFC in the C–O mixtures caused the rate of
injectability to double (Table 5). Additionally, the low measured bleeding (below the limit
of 5%) guaranteed the absence of the instability phenomena that are pointed out in the
literature due to an excessive dosage of SP [6].
After 28 curing days, slices from the central part of the columns were extracted and
scrutinized, to assess the filling of the voids (Figure 10).
3.2. Hardened Grout Properties
3.2.1. Compressive Strength
During the hardening process, plain lime–based systems exhibited an increase in
mechanical properties, thanks to the carbonation process in which CaCO3 is formed over
time. Therefore, the values of compressive strength after long–term curing times—182 and
365 days—were greater on average (Figure 11). Pozzolanic reaction and formation of C–S–
H phases in grouts with pozzolans also contributed to the strength of the hardened grouts.
The addition of metakaolin increased the compressive strength, and the highest
mechanical strength was obtained in the control mixture O–MK after 365 curing days (Figure
11). Microsilica was not so effective in increasing the strength. Its pozzolanic activity was
lower as compared with MK, due to the larger particle size of the microsilica. TG–DTA
analysis (Figure S2, Supplementary Materials) confirmed the differences: The percentages
of Ca(OH)2 were lower for MK–bearing grouts (values after 182–365 curing days were on
average below 4%, whereas samples with microsilica exhibited higher percentages of
Ca(OH)2), suggesting a greater consumption of Ca(OH)2 during the pozzolanic reaction with
Capítulo I. Parte B
147
MK. A Pearson correlation (coefficient 0.654, p < 0.01**) between the compressive strength
and the percentage of portlandite was established: The higher the percentage of portlandite,
the lower the compressive strength (Figure 12). It can be seen that most samples with
percentages of portlandite below 4% yielded compressive strengths higher than 3 MPa,
whereas samples with Ca(OH)2 percentages > 4%, in general, resulted in compressive
strengths below 3 MPa.
The use of superplasticizers was, in general, favorable in order to increase the final
mechanical strength of the grouts, PCE and SMFC, yielding the highest values. This finding
is ascribed to the refinement of the pore structure caused by the superplasticizer, especially
by PCE. The assessment of the pore size distributions (Figure 13) of the grouts showed the
following:
Figure 11. Compressive strength of grouts at different curing times (SP dosages: 0.5 and
1%)
Capítulo I. Parte B
148
Figure 12. Correlation between compressive strength and portlandite (% calculated
from TG results) in grouts after 182 and 365 curing days.
• The addition of the pozzolanic additive (microsilica or metakaolin) reduced porosity
by about 1 μm in diameter, due to the filling effect of the microsilica and the
pozzolanic reaction (Figure 8, reduction in the area under the curve of the mercury
differential intrusion).
• The addition of PCE caused a sharp drop in the number of pores, of about 1 μm. In
addition, the main pore size shifted toward lower diameters (between 0.5 and 0.8 μm).
Figure 13. Pore size distributions of different paste samples after 365 days of curing
Capítulo I. Parte B
149
3.2.2. Hydrophobicity
The static water contact angle and the time for the water–drop absorption are shown in
Table 6. Measurements were carried out in grouts after 365 curing days. This parameter
provides information about the real effect of the water–repellent agent, sodium oleate, and
its compatibility with the SPs.
Table 6. Results of the static water contact angle measurements (WCA) and of the time
interval for the water drop absorption.
Sample WCA
Time interval for the full absorption of the
drop of water
t < 5s 5s < t < 10s t > 10s
Control samples
(without polymeric
superplasticizers)
C –
C–MS –
C–MK –
C–O 84±2.1
C–O–MS 59±2.1
C–O–MK 35±2.3
Samples without
pozzolanic addition
O–PCE0.5 70±3.1
O–SMFC0.5 86±2.6
O–PNS0.5 105±2.8
O–PCE1 68±2.1
O–SMFC1 54±2.9
O–PNS1 40±3.2
Samples with
Microsilica
O–MS–PCE0.5
44±2.5
O–MS–SMFC0.5
72±2.2
O–MS–PNS0.5
83±2.6
O–MS–PCE1
98±2.8
O–MS–SMFC1
40±2.4
O–MS–PNS1 37±2.5
Samples with
Metakaolin
O–MK–PCE0.5
44±3.4
O–MK–SMFC0.5
112±2.0
O–MK–PNS0.5
89±2.8
O–MK–PCE1
124±2.5
O–MK–SMFC1
44±2.7
O–MK–PNS1
56±2.4
Capítulo I. Parte B
150
One of the intended characteristics of these grouts formulations was the hydrorepellency.
The hydrorepellency is a superficial phenomenon caused by the tensioactive character of the
sodium oleate. During the mixing process in an aqueous dispersion, the hydrophobic (non–
polar) part of the molecule is oriented toward the aerial phase, whereas the polar segment is
in the aqueous system. The combined effect of the pore size distribution (with small pores)
and the active water–repellent agent led to suitable hydrorepellency. The O–MK–PCE1 grout
exhibited the best hydrorepellency, thanks to its low total porosity (see Figure 13) and to the
availability of molecules of the water–repellent agent (even assuming that most of the oleate
molecules will be adsorbed onto lime particles). In this sense, previous discussions in Section
3.1.2, on the zeta potential values and on the adsorption of the SPs in the designed grouts,
have shown the lowest interaction, and consequently the best compatibility, between the
sodium oleate and the PCE. On the other hand, polymers SMFC and PNS yielded higher
adsorption values onto lime particles, as reported in Figure 6.
3.2.2. Durability
The control grout formulations without a water–repellent agent were fully decayed after
just one F–T cycle (total destruction of the specimens). The addition of sodium oleate
definitely improved the freeze–thaw durability of the grouts, in accordance with our previous
report on the positive effect of this admixture in lime–based mortars [18]. For instance, the
control sample (C–O) resisted up to 18 freezing–thawing cycles (Figure 14).
The presence of pozzolanic admixtures in the grouts displayed different results: The
addition of microsilica resulted in an adverse effect on the freeze–thaw durability, as can be
seen in the control samples with microsilica (sample C–O–MS), showing serious decay after
just two cycles (Figure 14). Conversely, the addition of metakaolin improved the resistance
in the face of freezing–thawing cycles. According to the results, the reduction in the mean
pore size prevented the absorption of liquid water, blocking its later freezing and expansion
damage and thus providing better resistance against freezing–thawing cycles.
Metakaolin–containing samples showed a better durability when treated with SPs.
Formulations with PCE and SMFC as SPs yielded the highest resistances.
Capítulo I. Parte B
151
Figure 14. Values in the damage scale of grouts after freeze–thaw cycles.
4. Conclusions
Quaternary mixtures of air lime, polymer–based superplasticizers, a water–repellent
agent and pozzolanic additives were studied as grouts to be used as repair materials for Built
Heritage. The compatibility between the different admixtures was assessed.
Results showed that PCE was much more effective in increasing both the injectability
and fluidity of the grouts than SMFC and PNS. The action mechanism of this polymeric
superplasticizer was confirmed to be mainly steric, whereas SMFC and PNS acted through
an electrostatic repulsion mechanism. The molecular architecture of these polymers was
critical to explain their performance.
Interactions with sodium oleate were found: The adsorption of sodium oleate onto lime
particles was evident and caused a reduction in the superplasticizing effectiveness of the SPs,
particularly of SMFC and PNS, as proved by the zeta potential measurements and adsorption
Capítulo I. Parte B
152
isotherms. At the same time, the large adsorption of SMFC and PNS onto the oleate layer
reduced the hydrorepellency of the treated grouts, as confirmed by the static water contact
angle. The use of PCE was seen to be more favorable in terms of the highest injectability and
hydrorepellency.
As for the pozzolanic additives, metakaolin imparted better characteristics to the grouts
than microsilica, particularly in combination with SPs: higher injectability, better adherence
and wrapping of the particles during injection, as well as higher mechanical strengths.
Durability, in the face of freezing–thawing cycles, was also outstandingly increased due to
the presence of MK. Microsilica showed a marked tendency to agglomerate in aqueous
dispersions, which was strongly detrimental for the injectability of the grouts prepared with
this pozzolanic additive. Besides, low mechanical strengths and poor durability were
observed for grouts, including MS.
According to the results, the grout composed of air lime, metakaolin, sodium oleate and
PCE, in its largest dosage of 1 wt.%, was found to be the most effective composition,
improving the mechanical strength, the injectability and the hydrophobicity.
Supplementary Materials
The following are available online at https://www.mdpi.com/2073–4360/12/4/887/s1,
Figure S1: Setup of the injectability determination. Graduated methacrylate column filled
with travertine porous medium; Figure S2: Percentages of portlandite (Ca(OH)2) of grouts at
different curing times (TG results).
Author Contributions
Conceptualization, J.I.A. and Í.N.–B.; data curation: J.M.F.; formal analysis: J.F.G.–S.
and B.T.; funding acquisition: J.I.A.; investigation: J.F.G.–S. and B.T.; methodology:
J.F.G.–S., Í.N.–B. and B.T.; project administration: J.I.A. and Í.N.–B.; supervision: J.I.A.;
validation: J.M.F.; visualization: Í.N.–B.; writing—original draft: Í.N.–B. and J.I.A.;
writing—review and editing: J.M.F. and J.I.A. All authors have read and agreed to the
published version of the manuscript.
Funding
This study was funded by Spanish Ministry of Economy and Competitiveness
(MINECO), grant number MAT2015–70728–P. The first author thanks the Friends of the
University of Navarra, Inc., for a pre–doctoral grant.
Capítulo I. Parte B
153
Acknowledgments
The authors thank the technical support provided by Cristina Luzuriaga and Marta
Yárnoz.
Conflicts of Interest
The authors declare no conflict of interest.
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(http://creativecommons.org/licenses/by/4.0/).
Capítulo II: Desarrollo de
morteros de cal con actividad
fotocatalítica mejorada y
autolimpiables
Improvement of the depolluting and self–cleaning
abilities of air lime mortars with dispersing admixtures
Enviado a Journal of Cleaner Production (En proceso de revisión,
octubre, 2020)
Capítulo II
163
Improvement of the depolluting and self–cleaning abilities of air lime mortars with dispersing admixtures
J.F. González–Sáncheza, B. Taşcıb, J.M. Fernándeza, Í. Navarro–Blascoa, J.I.
Alvareza*
a Materials and Cultural Heritage, MATCH, Research Group, Department of Chemistry, University of
Navarra, 31008 Pamplona, Spain
b Deparment of Architecture, Izmir Katip Çelebi University, 35620 Izmir, Turkey *Author to whom correspondence should be addressed.
Abstract
The aim of this study is to develop new durable air lime mortars with enhanced
photocatalytic depolluting and self–cleaning abilities. Nanosilica, as pozzolanic mineral
admixture, was used to improve the strength of mortars, whereas nanotitania (TiO2) was
added to impart photocatalytic properties. At the same time, five different dispersing
admixtures –superplasticizers– were added in bulk to the mortars to enhance the
photocatalytic activity by reducing the rate of charge carrier recombination. Four
polycarboxylate–based derivatives and a polynaphthalene sulfonate were tested aiming
to achieve an efficient charge separation. In order to increase the lasting of the mortars
subjected to water movements, sodium oleate was also added as a water repellent agent.
Since the photoinduced hydrophilicity, responsible for the self–cleaning effect, might be
affected by the water repellent, the compatibility between this admixture and the
photocatalytic performance of the nanotitania was also investigated. Results showed that
photocatalytic activity was improved due to the action of the superplasticizers as indicated
by an average 33% increase of NO degradation, which is significant to the depolluting
activity of these mortars. Furthermore, these mortars also showed a greatly reduced
release of intermediate toxic compounds, mainly NO2: the selectivity factor (NOx/NO)
reached values up to 87%. The self–cleaning ability, studied through dye degradation, of
the mortars with SPs was also enhanced around 1.2 times. Three of the polycarboxylate–
based superplasticizers enhanced the photosensitization of the dye under visible light
irradiation, resulting in faster decolouring kinetics. In connection with the self–cleaning
performance, these same SPs preserved the photoinduced hydrophilicity of the lime
mortars, reaching good wettability of the surface of the mortars (water contact angles of
ca. 10º), even in the presence of the sodium oleate, proving the compatible characteristics
of the admixtures and allowing obtaining a new range of actively depolluting lime
mortars.
Keywords: lime mortar, photocatalyst, superplasticizer, depolluting, TiO2,
dispersion, NO removal, selectivity, self–cleaning
Capítulo II
164
1. Introduction
Air pollution is known to be responsible for a wide range of problems and some
efforts are being devoted to resolve these issues by taking practical steps (Khin et al.,
2012). Chemical compounds such as NOx, SOx, CO, H2S, NH3, other nitrogenous
compounds, sulphur–containing compounds, hydrocarbons, and volatile organic
compounds (VOC) (benzene, toluene, etc.) have been reported as the most common
hazardous components (Colls, 2002). Concerning the building materials, a severe
problem generated by the air pollution is the deposition of atmospheric particles on the
surface of these materials. Deposited particles can form black crusts (sulphated carbon
particles) giving rise to aesthetic and structural problems in both modern and historical
buildings (Krishnan et al., 2018; Pozo–Antonio and Dionísio, 2017). The removal of these
dirt deposits also involves the use of chemicals, which implies both environmental and
economic concerns (Dalton et al., 2002; Pérez–Nicolás et al., 2015; Pozo–Antonio and
Dionísio, 2017).
To deal with these challenging problems, different strategies may be adopted to
reduce the atmospheric pollutants and to avoid the dirt accumulation and the subsequent
deterioration of the construction materials (Luna et al., 2019; Pérez–Nicolás et al., 2018;
Saeli et al., 2018; Xia et al., 2020). Among them, the design of photocatalytic materials
with depolluting and self–cleaning abilities is one of the most promising ways (Krishnan
et al., 2018; Luna et al., 2020; Munafò et al., 2015).
The use of photocatalysts offers several advantages and they have been efficiently
employed for the removal of gaseous contaminants with harmless end products (Folli et
al., 2012; Lucas et al., 2013; Pérez–Nicolás et al., 2018). Due to the superficial character
of the photocatalytic reaction, photocatalytic agents must be immobilized onto supporting
materials, so that their incorporation in construction materials is one valuable option. The
exposed areas of building materials facilitate the interaction between atmospheric
pollutants and photocatalysts, reducing the concentration of pollutants in the surrounding
environment (Luna et al., 2019). At the same time, the reaction of the photocatalytic
oxidation degrades the dirt deposits allowing the building material to display a self–
cleaning behaviour. This self–cleaning effect is also observed in superhydrophilic
surfaces (i.e. water contact angle < 5º), in which the drops of water spread on the surface
of the material, giving rise to a flowing aqueous film capable to sweep along dirt and dust
Capítulo II
165
(Son et al., 2012). Under illumination, photocatalysts exhibit photo–induced
hydrophilicity, which, together with their photocatalytic activity, yield proved self–
cleaning materials.
These building materials, incorporating photocatalysts, may become a good
alternative for their use as new construction materials. In that way, the recourse to
construction materials will reduce the air pollutants in the vicinity of the buildings and
will circumvent the problems related to the dirt and dust accumulation, preserving the
structure from damages, keeping the aesthetics of the edifications, particularly those of
the Cultural Heritage, and leading to a reduction in maintenance and cleaning costs
(Kapridaki et al., 2019, 2018; Luna et al., 2019; Pérez–Nicolás et al., 2018).
Binders, such as mortars, renders or grouts have been explored as materials able to
accommodate photocatalysts added in bulk (Pérez–Nicolás et al., 2017; Ruot et al., 2009).
The current work pursues the design of new lime mortars, which are attracting the interest
of the scientific community as valuable repair materials (Azeiteiro et al., 2014; Salavessa
et al., 2013). The use of lime mortar exhibits several potential advantages in terms of the
sustainable development of these materials: their production has been reported to produce
less environmental footprints, due to the lower energy consumption and CO2 emissions
as compared with cement (Giosuè et al., 2020). Some lime–based binders have been
found to be carbon–negative building materials, such as hemp–lime concrete (Arehart et
al., 2020; Arrigoni et al., 2017; Walker and Pavía, 2014). In addition, the use of pozzolans
(to partially substitute the binder) has been said to have a positive effect in lowering the
initial emissions associated to the binder production (Arehart et al., 2020). The local
availability, low processing level, capacity of recovering traditional methods of
construction and the healthy nature of the compound can be also mentioned as
advantageous aspects of the use of lime (Arrigoni et al., 2017; Deng et al., 2020; Orsini
and Marrone, 2019).
Furthermore, the use of lime may be effective in reducing the NO2 release, which has
been pointed out as one of the drawbacks of the photocatalytic NO abatement (Bloh et
al., 2014). NO2 toxicity is higher than that of the NO and may act as precursor of other
harmful components (Yang et al., 2018). The proved high NO2 adsorption of the
portlandite confirms the lime binders as suitable hosting matrices for photocatalysts, with
ability to increase the selectivity of the NO removal (Kaja et al., 2019; Krou et al., 2013;
Zhang et al., 2008).
Capítulo II
166
These mortars could be applied as one–coat rendering mortars, as multilayer renders
or as repointing materials, both in modern buildings and in repair works of the Cultural
Heritage (Giosuè et al., 2018). These new finishing mortars, which offer large exposed
areas favourable for depolluting purposes, should at the same time be durable and show
self–cleaning ability. To this aim, different admixtures were simultaneously combined,
including: (i) a mineral admixture (pozzolanic nanosilica), which imparts strength and
durability (Nunes et al., 2016; Sharma et al., 2019; Tsardaka and Stefanidou, 2020); (ii)
a waterproofing agent (sodium oleate) to reduce the water absorption and the detrimental
effects of the water movements (Falchi et al., 2015; Silva et al., 2020); (iii) nano–TiO2,
as the most popular metallic oxide semiconductor photocatalyst (Crupi et al., 2018; Folli
et al., 2012; Haider et al., 2019; Zouzelka and Rathousky, 2017); and (iv) a
superplasticizer (up to five different ones were assayed: four based on polycarboxylated–
ether derivatives and one poly–naphthalene sulfonate (Padovnik et al., 2016; Padovnik
and Bokan–Bosiljkov, 2020; Puertas et al., 2005; Silva et al., 2019)). The compatibility
between all these types of admixtures in lime mortars is still pending of investigation and
some of the admixtures might ruin the expected action of others. This work focuses on
the two main following approaches.
On the one hand, the role of the superplasticizers was studied. These dispersing
admixtures were added with the purpose of overcoming one of the problems related to
the use of photocatalysts, which is the charge carriers recombination that results in a low
quantum efficiency and, accordingly, a low depolluting effectiveness (Araña et al., 2019;
Mamaghani et al., 2017; Wang et al., 2020; Zouzelka and Rathousky, 2017). The
electron–positive hole coupling due to the proximity between active sites is a major
drawback, jeopardizing the efficiency of the photocatalytic action. This vicinity may be
a direct consequence of the choice of nano–sized compounds, which, while being positive
because they offer more surface area and thus active sites, show at the same time a sharp
trend to agglomerate reducing the catalyst area (Yang et al., 2019). The calcium–rich and
highly alkaline environment in lime or cement mortars has been reported as an additional
factor explaining the efficiency drop, since the precipitation of Ca(OH)2 and CaCO3 may
cover TiO2 active sites (Yang et al., 2019). The use of suitable superplasticizers can lead
to an effective separation of the TiO2 nanoparticles increasing the photocatalytic activity.
Compatible superplasticizers with air lime, which had been proved to be effective when
applied as coatings (Pérez–Nicolás et al., 2018), were thus tested in these new mortars in
Capítulo II
167
bulk addition. The depolluting activity of the mortars was assessed by monitoring the
NOx degradation in a closed reactor. The release of intermediate toxic NO2 was also
assessed and the selectivity values of the new mortars were calculated. The self–cleaning
ability was determined by dye–degradation measurements.
On the other hand, the influence of the admixtures, particularly the waterproofing
one, on the photo–induced hydrophilicity was also investigated. Whilst the addition of a
water repellent is positive for increasing the frost resistance of the mortars (Falchi et al.,
2013; Nunes and Slížková, 2016; Silva et al., 2020) by reducing the water uptake (Silva
et al., 2020), its presence might interfere with the hydrophilicity, thus endangering the
self–cleaning performance. The compatibility between the presence of the waterproofing
admixture and the self–cleaning effect was therefore studied.
2. Materials and methods
2.1 Materials and composition of mixtures
For the preparation of the mortars, hydrated calcitic lime CL–90 was used as a
powder, supplied by CALINSA, Navarra, Spain. This lime has a composition of 68.5%
CaO, 3.3% MgO, 1.4% SO3 and 1.0% SiO2. The particle size of the powdered lime was
10 µm (less than 10% > 50 µm). A fine limestone aggregate supplied by CTH (Huarte,
Navarra, Spain) with a particle size less than 2 mm was also used. The granulometry of
the aggregate has been published elsewhere (González–Sánchez et al., 2020). The
preparation of the mortars was made using a 1 to 3 weight ratio of binder/aggregate
(González–Sánchez et al., 2020; Izaguirre et al., 2011). In order to separate and to identify
the effect of the admixtures, the mixing water was fixed at a 28% water/lime ratio, which
was the percentage able to yield a settlement diameter of 165 mm in the control sample,
as measured in the flow table test according to EN 1015–3 (European Committee for
Standardization, 2006). Slump values of each sample are provided in the Supplementary
material (Table S1).
The following admixtures were also added to prepare the mortars. Detailed
composition of each one of the samples is displayed in Table 1. The percentages of
admixtures are expressed by weight of lime (bwol):
- Pozzolanic mineral admixture (20%): Nanosilica (NS), provided by ULMEN
Europa S.L. as a colloidal superplasticizer–free silica suspension. Fig. 1 shows the TEM
Capítulo II
168
analysis of this admixture, with average particle size of ca. 50 nm and a specific surface
area of ca. 500 m2g−1 (established by BET nitrogen adsorption isotherms) (Fernández et
al., 2013; Navarro–Blasco et al., 2014).
- Waterproofing agent (0.5%): Sodium oleate (O) (HISA A 2388 N). Its molecular
weight is of 304 g mol−1. The structure and performance of this admixture has been
described by Izaguirre et al. (Izaguirre et al., 2009).
- Photocatalytic agent (T) (2.5%): Nanoparticles of bare titanium dioxide (TiO2) as
photocatalyst supplied by Aeroxide P25, Evonik. The particle size of the photocatalyst,
21 nm, was ascertained in previous research by Pérez–Nicolás et al. (Pérez–Nicolás et
al., 2017). Fig. 1 shows the TEM micrograph of this admixture, evidencing its tendency
to agglomerate.
- Superplasticizer (SP) (added in two different dosages 0.5% and 1%): the following
superplasticizers were tested: four different polycarboxylate–based polymers and a
polynaphthalene sulfonate PNS were used. A complete characterization of their structures
was reported in previous works (González–Sánchez et al., 2020; Pérez–Nicolás et al.,
2018). Most relevant characteristics are summarised below:
▪ PCE–A, molar mass 8.00 × 103 and specific anionic charge 920 μeq/g
▪ PCE–B, molar mass 4.60 × 104 and specific anionic charge 1695 μeq/g
▪ PCE–C, molar mass 3.84 × 104 and specific anionic charge 2740 μeq/g
▪ PCE–D, molar mass 3.16 × 104 and specific anionic charge 1895 μeq/g
▪ PNS, molar mass 1.40× 105 and specific anionic charge 4089 μeq/g
Fig. 1. TEM micrographs of nanosilica (left) and nano–TiO2 (right)
200 nm 200 nm
Capítulo II
169
Table 1. Assayed samples: percentage composition of the admixtures
Name Nanosilica
(NS)
Sodium
oleate
(O)
TiO2
(T) Superplasticizer
Control L – – – –
Superplasticizer–free
samples
O–T – 0.5 2.5 –
NS–T 20 – 2.5 –
O–NS–T 20 0.5 2.5 –
PCE–A
A0.5 – 0.5 2.5 0.5
A1 – 0.5 2.5 1.0
A0.5–NS 20 0.5 2.5 0.5
A1–NS 20 0.5 2.5 1.0
PCE–B
B0.5 – 0.5 2.5 0.5
B1 – 0.5 2.5 1.0
B0.5–NS 20 0.5 2.5 0.5
B1–NS 20 0.5 2.5 1.0
PCE–C
C0.5 – 0.5 2.5 0.5
C1 – 0.5 2.5 1.0
C0.5–NS 20 0.5 2.5 0.5
C1–NS 20 0.5 2.5 1.0
PCE–D
D0.5 – 0.5 2.5 0.5
D1 – 0.5 2.5 1.0
D0.5–NS 20 0.5 2.5 0.5
D1–NS 20 0.5 2.5 1.0
PNS
P0.5 – 0.5 2.5 0.5
P1 – 0.5 2.5 1.0
P0.5–NS 20 0.5 2.5 0.5
P1–NS 20 0.5 2.5 1.0
2.2. Preparation of mixtures
All raw materials (lime, aggregates and admixtures) in the planned proportions were
mixed for 5 minutes in a solid–admixtures mixer BL–8–CA (Lleal, S.A.). Afterwards, the
resulting mixture was poured into a different Proeti ETI 26.0072 mixer (Proeti) and water
was added and mixed for 90 s at low speed and adjusted according to EN 196–1
(European Committee for Standardization, 2005).
Cylindrical moulds (36 mm height and 40 mm diameter) were used to prepare the
hardened specimens. Samples were demoulded after 7 days and stored at the same curing
conditions (20 °C and 60% RH). Curing ages were 28 and 91 days. Properties were then
investigated. At least three replicates of the mortars were tested per curing age and per
studied property to obtain representative results. For some analyses, a Struers cutting–
Capítulo II
170
polishing machine was used to obtain slices (10 mm height and 40 mm diameter) from
the cylindrical specimens.
2.3. Methods
2.3.1. Photocatalytic activity: NOx abatement
A continuous flow experiment adapted from an ISO standard method was used for
this assay (Draft International Standard, 2007). The experimental system includes a
cylindrical photoreactor (height 12 cm; diameter 14 cm), fed by a 0.78 L min–1 flow of
nitrogen and air with an initial concentration of 500 ppb NO and around 20 ppb of NO2
(in all instances accurately and continuously monitored by means of a Environnement
AC32M chemiluminescence detector). The conditions established were 50 ± 5% RH and
25 ± 2 °C; a Osram Ultra Vitalux 300W lamp was used to supply UV–vis illumination
(Corrêa, 2015). The nominal irradiance of the lamp after 1 h and at a 0.5 m of distance
was of 41.4 W m–2 (780–380 nm), 13.6 W m–2 (400–315 nm) and 3.0 W m–2 (315–280
nm). This lamp combines visible, UVA and UVB radiation achieving a good simulation
of the solar light (Heikkilä et al., 2009; Prieto and Lagaron, 2020). Experiments were
carried out for each sample discs of 91 days–cured mortars, with 25.14 cm2 of total
exposed area. While the sample was inside of the reactor and the lamp off, the NOx stream
was flowed 10 minutes to stabilize the NO concentration in the reactor, allowing reaching
the adsorption equilibrium between the NOx and the sample. Afterwards, lamp was
switched on for 30 minutes. Then, the lamp was turned off for 10 min, allowing the
recovering of the initial NO concentration value. The selectivity values were calculated
as the percentage ratio NOx/NO, high values meaning a very limited NO2 release and thus
a more effective total NOx degradation. The error analysis of the experimental, expressed
in terms of relative standard deviation, was calculated under reproducibility conditions at
3.3% from the repeated analysis of at least three identical samples.
2.3.2. Determination of density, air content, workable life, pore size distribution
and compressive strength of the mortars
In the fresh mixtures, the density, air content and workable life (expressed as the time
needed to reach stiffness in the mortar) were determined according to the standards EN
1015–6, 1015–7 and 1015–9 (European Committee for Standardization, 1999a, 1999b,
1999c). Values are collected in the Supplementary material (Table S1).
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Mercury intrusion porosimetry (MIP) measurements (Micromeritics–AutoPoreIV–
9500) were used to establish the pore size distributions of the hardened mortars after both
curing ages. The samples were analyzed at a pressure range of 0.0015–207 MPa.
A Proeti ETI 26.0052 compression machine (Proeti) was utilized to measure
compressive strengths after 28 and 91 curing days of the cylindrical mortars. The assays
were executed at a breaking speed 5–50 KP s−1 and a time interval between 30 and 90 s.
2.3.3 Adsorption of the superplasticizers and zeta potential studies
In order to know the adsorption of superplasticizers on TiO2, a batch adsorption
experiment was carried out. Five reference samples with 10 mg of each SP and 5
suspensions with the same amount of SP plus 500 mg of TiO2 were prepared and made
up to a final volume of 50 mL. Samples were mechanically stirred for 30 min to reach the
adsorption equilibrium and were subsequently centrifuged for 2 hours at 8000 rpm in a
Heraeus Biofuge Stratos wobbler. Then the supernatant was taken, and the total organic
carbon (TOC) was determined in a TOC–L Shimadzu total organic carbon analyzer. The
adsorbed amount of superplasticizer was thus calculated as the difference between the
TOC content of the reference samples and the TOC content of the supernatant of the
suspensions.
The surface charge of the different suspensions was monitored with a Zeta potential
electroacoustic analyzer (ZetaProbe Analyzer, Colloidal Dynamics). First, two different
initial media were prepared: one, a suspension in water of nano–TiO2 particles; other, a
mixture of air lime, water, NS and sodium oleate, using the same relative compositions
described in Table 1. The media were stirred for 30 minutes. Then, different polymer–
based superplasticizers solutions (1% w/v) were used as titrant media solutions, and zeta
potential values were continuously monitored.
2.3.4. TG studies
The rate of carbonation of hardened samples was analyzed at 28 and 91 curing days
using alumina crucibles by thermogravimetric studies in a simultaneous TG–sDTA 851
Mettler Toledo thermoanalyzer device at 10 °C min−1 heating rate, under static air
atmosphere were heated from 25 until 1000 °C. The percentages of weight losses around
450–480ºC were ascribed to the dehydroxylation of the uncarbonated portlandite,
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whereas the weight losses at ca. 800–900ºC were attributed to the CO2 release from the
calcium carbonate.
2.3.5. Self–cleaning test
The self–cleaning capacity of all samples was evaluated by studying the dye
degradation in mortars exposed to UV–Vis light. First, the surface of the discs was stained
with three layers of an aqueous solution of organic dye rhodamine B (1 mM) applied by
brushing. Samples were left to dry in an oven at 50ºC for 60 min. Subsequently, the discs
were irradiated under the Osram Ultra Vitalux UV–Vis lamp of 300 W (data of irradiance
above mentioned in section 2.3.1). The photodegradation activity (discoloration of the
surface of the mortars) was evaluated at 5 time intervals (5, 20, 80, 140 and 310 min)
using a Konica–Minolta CM–2300d colorimeter. Measurements were carried out in 9
circular regions (diameter 3 mm) for each stained sample surface and the color variations
over time were obtained by the chromatic coordinates a∗ and b∗. With the data obtained,
the normalized color change (ΔCn) as Chroma variation was calculated with follow
equation (Fornasini et al., 2019):
∆Cn=√[at
*– a0*]
2+[bt
*– b0*]
2
[aC* – a0
*]2+[bC
* – bt*]
2
Eq. 1
where at* and bt
* are the coordinates at irradiation time t, whereas aC
* and bC* are
measured on the clean stones before staining with dye. The value 1 would correspond to
the complete dye degradation. Finally, results were reported as a function of the
irradiation time.
2.3.6. Surface wettability and photo–induced hydrophilicity
The evaluation of the surface wettability and the photo–induced hydrophilicity of the
different samples was performed by OCA 15EC (DataPhysics Instruments GmbH)
equipment measuring the static water contact angle (CA) of the samples under
illumination with an Osram Ultra Vitalux 300 W lamp at 0, 1, 3, 5, 8 and 30 min. Onto
the surface of the hardened grouts, five water droplets of 5 μL were put at five different
points, and the results were expressed as averages of these measurements.
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3 Results and discussion
3.1 NOX removal: effect of the porosity and role of the superplasticizers
The photocatalytic activity was assessed by monitoring the nitrogen oxide removal
ability of the mortars after 91 curing days in a closed reactor and the percentages of the
NO and NOx removal are depicted in Fig. 2. The patterns of the NO abatement tests
followed the general trend reported in previous works (Ângelo et al., 2013; Pérez–Nicolás
et al., 2018, 2017) and showed in Fig. S1 (Supplementary material): in dark conditions,
NO values were left to stabilize in order to drive out the adsorption phenomenon. When
the light was switch on, a quick decrease of the NO concentration was observed. Samples
reached a plateau, which also showed a slight trend to decrease along the time of
exposure, unlike some other results that showed a tendency to increase (Jin et al., 2019).
This finding can be ascribed to an unsaturation state, suggesting the presence of free
active sites able to degrade more NO molecules.
As it can be seen in Fig. 2, the addition of nano–TiO2 dramatically increased the NO
abatement, in comparison with the control sample (L, TiO2–free). The removal of up to
6% of NO in the control sample is ascribed to the photolysis of the pollutant and to the
sorption and conversion of NO into nitrous acid (not measured) (Gandolfo et al., 2015;
Zouzelka and Rathousky, 2017). The addition of the nano–structured photocatalytic
admixture increased the NO removal up to a 28–37% range (for samples O–T, NS–T and
O–NS–T). In comparison with these samples, the use of dispersing admixtures sharply
increased by 33% the NO abatement (the percentage of NO removal of mortars with
superplasticizers was 44% on average). These results are a clear evidence of the
usefulness of incorporating superplasticizers in lime mortars to achieve a suitable
dispersion of the TiO2 within the binding matrix, which had been highlighted as one of
the challenging issues in cementitious binders (Yang et al., 2019).
3.1.1 Influence of the porosity
The effect of the pozzolanic admixture addition was found to be dependent on the
composition of the mortar. For superplasticizer–free mortars, the addition of NS enhanced
the NO removal. The refinement of the pore structure accounts for this finding: the
prevalence of capillary pores between 10 and 100 nm for samples with NS (NS–T and
O–NS–T) was observed in the pore size distribution graphs of mortars (91 curing days),
depicted in Fig. 3. These pores act as a booster of the photocatalytic activity, as stated by
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Kaja et al. (Kaja et al., 2019) in cement mortars with TiO2. These authors concluded that
the formation of capillary pores in the range 10–50 nm was critical for the enhancement
of the photocatalytic activity (NO abatement). In the current work, sample O–T without
NS showed negligible porosity in that pore range, yielding a 28% of NO removal. The
samples with NS (NS–T and O–NS–T), with an outstanding increase of capillary pores
in that pore range, increased the NO degradation 1.2–1.3 times.
Fig. 2. NO and NOx abatements for different samples under UV–Vis irradiation.
Fig. 3. Pore size distribution of superplasticizer free samples (91 curing days)
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However, in samples with SPs, the addition of nanosilica depicted a different
performance. As a general tendency, samples with NS showed a lower abatement of NO.
The porous structure contributes to clarify the reasons for this lesser performance. The
addition of SPs modified the pore size distribution of the samples with NS, nano–TiO2
and lime. Instead of increasing the capillary pores between 0.01 and 0.1 m (as reported
for air lime mortars with additions of NS (Duran et al., 2014)), the presence of the
superplasticizers caused a clear decrease of the population of the pores of this range
(Fig.4). This fact had been also observed in previous works dealing with NS and SPs
(Fernández et al., 2013; Pérez–Nicolás et al., 2016) and was related to the inhibition of
the pozzolanic reaction (Pérez–Nicolás et al., 2016) and to the enhancement of the filling
effect of the better dispersed NS within the network of lime particles (Alvarez et al.,
2013).
Fig. 4. Pore size distribution of samples with different superplasticizers (91 curing days)
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Due to the use of the same ratio of mixing water (see section 2.1), the presence of a
high–range water–reducer, a superplasticizer, produced a large excess of water which in
the end evaporated causing the main contribution to the pore size distribution (main pore
peak at around 1–2 m, which is larger than the most commonly reported 0.5 to 0.8 m
for air lime mortars (Duran et al., 2018; González–Sánchez et al., 2020; Lanas and
Alvarez, 2003; Martínez–García et al., 2019; Santos et al., 2018). This shift of the critical
pore diameter towards higher diameters and the increase observed in the area under the
curve (meaning higher total porosity) influenced the values of the compressive strengths,
which showed an expected drop with respect to the control (Fig. S2, Supplementary
material).
Despite the absence of the capillary pores from 10 to 100 nm, the NO abatement of
these mortars was higher than that of the SP–free mortars (Fig. 2). This finding can be
tentatively ascribed to the combination of the following factors: (i) the macroporosity
increase, in line with the work by Sugrañez et al. (Sugrañez et al., 2013). These authors
reported a better photoactivity and thus a higher NO degradation in cement mortars with
a greater amount of macropores > 2 m. (ii) The achievement of an efficient TiO2
dispersion because of the presence of superplasticizing admixture.
Concerning the first factor and ruling out the influence of the 10–100 nanometer–
sized pores for samples with SP, the effect of the total porosity was considered. Fig. 5
shows the influence of the total porosity on the NO abatement (photocatalytic activity)
for SP–free samples and for mortars with each one of the tested SPs. The results showed
that there is not a clear correlation between these parameters. A high porosity of the
mortar (> 38%) does not necessarily involve a high NO removal rate. This can be noticed,
for example, for P1–NS, P0.5 and samples with PCE–D, which despite their high total
porosity exhibited NO removal percentages around 40% or lower. The opposite is also
true. Mortars with comparatively lower total porosity (32–36%) yielded high NO
abatements (> 45%). The small slopes of the correlations confirm the absence of any
significant influence of the total porosity.
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Fig. 5. Influence of the total porosity (%) on the NO abatement (%).
The influence of the macroporosity (pores ≥ 1 m) on the NO abatement was also
studied. The graph is presented in Fig. 6 and a significant correlation (p < 0.01) was
identified altogether, confirming the influence of the macropores ≥ 1 m in the
photoactivity of mortars with TiO2 (Sugrañez et al., 2013).
However, it is evident that the changes in the region of the critical pore diameter and
in the macropores within the mortars with SPs are not enough by themselves to explain
the photocatalytic activity. There are some acute NO abatement differences among
samples with similar macroporosity ≥ 1 m, as for example samples P1–NS and B0.5
(with 29 and 43%, respectively, of NO abatement) or samples B1–NS and C1 (46 and
57% of NO abatement). Therefore, the second factor, i.e. the effectiveness of the
dispersion of TiO2 by the SPs, should be then taken into account to understand the
photocatalytic activity of the tested mortars. Their different performance can be explained
considering the molecular architecture of the superplasticizers and their action
mechanism.
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Fig. 6. Influence of the macroporosity (pores ≥ 1 m, expressed as volume of
intruded mercury per g of mortar) on the NO abatement (%).
3.1.2 Role of the superplasticizers
The NO degradation values (Fig. 2) showed that the lime mortars with
polycarboxylate–based SPs yielded better NO removal percentages than those with PNS.
Mortars with PCEs resulted in average NO removal values from 43% to 50%, while the
average percentage for PNS was 39%. Among the tested PCEs, PCE–C degraded on
average 50% of NO resulting in the highest rates of NO removal. PCE–A and PCE–B
exhibited the same NO abatement percentages (average values of 45%), while the
percentage for PCE–D was slightly lower (43%).
The different polymeric structures of the superplasticizers have a clear influence on
their dispersing ability. In previous studies it has been pointed out that the
polynaphthalene sulfonate has a linear structure, with a high number of anionic groups
(sulfonates), which allow the PNS to be strongly adsorbed onto the different particles
(Crépy et al., 2014; Duran et al., 2018; González–Sánchez et al., 2020; Mezhov et al.,
2020). In Fig. 7 the zeta potential titration of aqueous nano–TiO2 dispersion with PNS
depicts the charge reversal phenomenon (according to the double–layer model, from
positive zeta potential values to negative ones) caused by the intense PNS adsorption
(experimentally determined to be 77% onto these particles) and its high anionic charge
density.
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Fig. 7. Zeta potential titration of aqueous nano–TiO2 dispersion with different
superplasticizers.
On the other hand, polycarboxylated etherified admixtures have been described as
branched polymers, with a main central backbone with ionizable carboxylate groups and
side chains with variable length (Fediuk et al., 2019; Puertas et al., 2005; Zhang and
Kong, 2015). As reported in the characteristics of the admixtures (section 2.1), the anionic
charge densities of these PCEs are lower than that of the PNS, explaining why their zeta
potential curves are quite different. In Fig. 8, the charge reversal only took place for PCE–
C, which has more carboxylate groups than the other PCEs and shows more adsorption
onto TiO2 particles (76%) than, for example PCE–B and PCE–D (adsorption values of
68% and 52%, respectively). However, the IEP (isoelectric point) for PCE–C was
achieved after the addition of higher amounts of SP in comparison with the PNS. The
addition of PCE–A hardly changed zeta potential because, despite its 79% of adsorption,
its low number of carboxylate groups did not substantially modify the charge at the
surface of the TiO2 particles.
It is generally assumed that at pH of the assay the adsorption is mainly driven by
hydrogen bonds between the SPs and the TiOH/TiOH2+ groups at the surface of the TiO2
particles, influenced also by the molecular weight of the polymers (Liao et al., 2009; Liufu
et al., 2005).
However, the effectiveness in the dispersion of the nano–TiO2 particles in the
complex medium of the fresh lime mortar does not depend mainly on an electrostatic
repulsion action. Whilst the PNS shows a working mechanism based on electrostatic
repulsions (Mezhov et al., 2020), the PCEs have been proved to combine electrostatic
repulsions with steric hindrance (Baltazar et al., 2013; Crépy et al., 2014; Puertas et al.,
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2005; Silva et al., 2019). In the literature, this electro–steric working mechanism was
confirmed by far as more efficient in lime systems than the mere electrostatic one
(González–Sánchez et al., 2020; Yoshioka et al., 1997). Other factors that must be
considered are the anchorage of the SPs onto the particles, necessary for an effective
dispersion (Plank and Yu, 2010; Seabra et al., 2007), and the length of the side chains of
the PCEs, responsible for the steric–based dispersion (Plank and Yu, 2010; Yoshioka et
al., 1997).
Fig. 8 shows the zeta potential curves of titration of the simulated complex systems
of the mortars (lime, nano–TiO2, sodium oleate and NS) with the SPs. In order to interpret
these curves, it must be considered that, at the highly alkaline pH of the lime medium, the
following groups at the surface of the different particles should be ionized and thus
negatively charged: portlandite particles, C–S–H phases (formed because of the
pozzolanic reaction between lime and NS) and the nano–TiO2 particles. These negatives
surfaces were expected to be strongly sheltered by a layer of positive calcium counter–
ions, potential determining ions, explaining the positive zeta potential values at the
beginning of the experiment and promoting the adsorption of negatively charged
polymers (González–Sánchez et al., 2020; Pérez–Nicolás et al., 2018; Plank and Winter,
2008; Zhang and Kong, 2015). As it can be observed in Fig. 8, the gradual addition of
PNS led to the systems to reach the IEP (zeta potential 0 mV). This finding implies that
the dispersing action of PNS was inhibited, because this admixture has an electrostatic
working mechanism (Mezhov et al., 2020).
Fig. 8. Zeta potential curves of titration of the simulated complex systems of the
mortars (lime, nano–TiO2, sodium oleate and NS) with different SPs.
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Among the PCEs, Fig. 8 shows, in agreement with the higher NO abatement observed
in mortars with this SP, how the titration with PCE–C allowed obtaining faster the
positive zeta potential responsible for the electrostatic stabilization of the particles, which,
together with the steric hindrance, is responsible for the effective dispersion in the assayed
media. Similarly, the poorest photocatalytic performance in mortars with PCEs was
observed for PCE–D, which in Fig. 8 was seen to present a severe delay (high amount of
SP required) in the achievement of the stabilization of the particles.
The length of the lateral chains varies as follows: PCE–D > PCE–C > PCE–A ≈ PCE–
B (L. Dvorkin, N. Lushnikova, M. Sonebi, 2017). The best efficiency in NO removal of
the mortars with PCE–C can be thus explained considering its better dispersion of the
TiO2 particles in the lime systems due to (i) its higher adsorption onto these particles, (ii)
its highest anionic charge among the PCEs at the pH of the mortar and (iii) the noticeable
length of its side chains (including 45 units of ethylene oxide) (Pérez–Nicolás et al.,
2018).
In spite of having longer side chains (52 units), PCE–D showed lower adsorption and
lower anionic charge, thus being not as effective as the PCE–C. The mortars with PCE–
A and PCE–B showed similar NO abatement results due to the steric hindrance
similarities. The highest adsorption of the PCE–A, mainly due to its lower molecular
weight, resulted in a slightly faster stabilization of the nanoparticles.
These results were, in terms of dispersion of TiO2, different from those reported in
the work by Pérez–Nicolás et al. (Pérez–Nicolás et al., 2018), who confirmed a better
dispersion of PCE–B and PCE–D. In the cited paper, the systems were aqueous coatings
of TiO2, whereas in the current work the SPs were added in bulk to a complex lime matrix,
including nano–TiO2, a water repellent, aggregate, lime particles and NS in some cases.
There is agreement in the relatively poor performance of PNS: its working mechanism is
not adequate for lime systems and due to its flat adsorption the growing of hydration and
carbonation compounds (deposits of calcium hydroxide, C–S–H, calcium carbonate)
inactivates its dispersion ability (Pérez–Nicolás et al., 2018; Puertas et al., 2005).
The effect of the dosage of the superplasticizers was clear only for PCE–B and PCE–
C, while for the others (PCE–A, PCE–D and PNS) there was not a clear dosage–response
pattern. The complexity of the system, including different adsorption surfaces (lime
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particles, aggregate, TiO2, in some cases nanosilica and the presence of oleate molecules)
may explain this outcome (Plank and Winter, 2008).
Another outstanding issue considering the depolluting activity of these new mortars
is the fact that the use of SPs also resulted in the enhancement of selectivity values
(percentage ratio NOx/NO), which were found to be 81.2% on average (Fig. 9). SP–free
mortars yielded a mean value of 74.2% of selectivity, whereas a clear enhancement was
seen for the mortars with the different SPs: 79.1% for PCE–A, 78.8% for PCE–B, 81.4%
for PCE–C, 85.1% for PCE–D and 81.7% for PNS. From an environmental point of view,
these results are relevant, since only a small fraction of the oxidized NO was released as
NO2 (a more dangerous and toxic pollutant) (Yang et al., 2019; Zouzelka and Rathousky,
2017). These selectivity values are close to the ones achieved by new synthesized
photocatalysts designed to reduce the NO2 release (Yuan et al., 2020) and much higher
than other selectivity values reported in the literature (Ambre et al., 2016; Balbuena et al.,
2016; Luna et al., 2020). It must be highlighted that these high selectivity values were
achieved in samples almost fully carbonated, which is a valuable result due to the reported
decrease in selectivity as a consequence of the carbonation of either cement or lime (Kaja
et al., 2019). According to the thermogravimetric analyses, the degree of carbonation after
91 curing days was 88.4 for SP–free samples, whereas reached a 92.3% for mortars with
SPs.
Two main reasons can be argued to explain the low release of NO2. On the one hand,
the chemical composition of the binding matrix. This composition includes the presence
of alkaline–earth metallic ion (Ca2+–rich system) that have been reported to adsorb NO2
molecules (Papailias et al., 2018, 2017; Pérez–Nicolás et al., 2015) and, through a
chemisorption process onto calcium carbonate, give rise to the formation of nitrate (Lu et
al., 2020); furthermore, the alkaline pH of the lime mortar matrix is able to allow the
disproportionation of the NO2 (Araña et al., 2019) and enhances the interaction with TiO2,
yielding an improvement of the photocatalytic performance (Jin et al., 2019). The increase
in the surface adsorbed water, promoted by the alkaline hydrolysis of TiO2 and by the
hydrophilicity of the Ca(OH)2 in this matrix, has been also reported to enhance the
selectivity (Yang et al., 2018, 2017). On the other hand, the presence of SPs provides an
effective TiO2 separation, offering more possibilities for NO2 adsorption and degradation
onto TiO2 particles (Sivachandiran et al., 2013). This was particularly significant to
explain the differences with the free–SP mortars.
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Fig. 9. Selectivity values (Percentage ratio NOx/NO) of different samples
3.2. Self–cleaning effect and photoinduced hydrophilicity
3.2.1 Self–cleaning performance
The degradation of the dye deposited onto the surface of the mortars was studied as
indicative of the self–cleaning ability of the mortars. In order to monitor the decoloring
efficiency, each one the superficially stained mortars (mortars after 91 curing days) were
compared with their bared counterparts and accordingly the influence of the porosity was
to a certain extent attenuated. The discoloration along time under light exposure was
recorded and expressed as Cn (Fig. 10). The mortars without nano–TiO2 were able to
degrade the dye (Rhodamine B, RhB) up to ca. 44%. This result can be ascribed to the
self–degradation of the dye (Fornasini et al., 2019), also fostered by the alkaline
hydrolysis of the pigment (Zhan et al., 2000; Zhu et al., 2012) due to the alkaline
conditions of the lime mortars. The TiO2–free mortars were able to discolor the dye in
values slightly higher to those reported for limestones (Fornasini et al., 2019) because of
the latter aforementioned reason.
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Fig. 10. Normalized chroma change (ΔCn) (discoloration) for Rh–B stained
samples at different UV–Vis exposure time.
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In the presence of the nano–TiO2 admixture, the lime mortars were able to remove
up to ca. 54% of the RhB over 310 minutes. The increase in the dye degradation (10%)
and the final values were in line with previously reported self–cleaning abilities of TiO2–
bearing coatings (Fornasini et al., 2019). The role of the superplasticizers was also
studied. The PCEs B, C and D enhanced the discoloration of the stained mortar, reaching
degradation values close to 70% in many samples, and therefore increasing the self–
cleaning ability of the lime mortars with respect to the SP–free specimens. In comparison
with TiO2–free mortars and SP–free mortars, respectively, PCE–B enhanced the
decoloring efficiency 1.43 and 1.20 times on average; PCE–C, 1.45 and 1.22 times; and
PCE–D 1.40 and 1.18 times. As in the case of the photocatalytic activity (NO
abatements), the mortars with PCE–C yielded the highest efficiency, followed by PCE–
B and PCE–D. The decomposition kinetics was also fast for these samples, yielding
between 30 and 45% of dye degradation in just 20 minutes of illumination. Samples
without SPs only reached 11% of discoloration in the same period of time.
Mortars with PNS also yielded dye degradation in the range of 60–70%, except for
the poor performance of sample P1–NS (with just 50% of discoloration). It should be
borne in mind that this sample also showed a low percentage of nitric oxide abatement
(Fig. 10). These results can be attributed to the fact that this sample presented the lowest
macroporosity (pores > 1 m) among all the SP–bearing mortars. For the other samples
with PNS, in comparison with the samples with PCEs B, C and D, the kinetics of the
degradation was more sluggish. After 20 minutes of irradiation, only a 15% to 30% of the
dye was degraded.
The PCE–A, in spite of yielding acceptable NO abatement results, did not show a
good self–cleaning performance: only the mortar A0.5–NS was able to degrade up to 60%
of RhB. The degradation kinetics for these samples was extremely sluggish (on average
just 15% of degradation during the first 20 min of irradiation). The reasons for such
performance are not easy to elucidate. Since the pore size distribution of these mortars
was similar to the others with PCEs (Fig. 4) and the NO abatement was effective (Fig. 2),
some interaction of the dye with the superplasticizer should be hypothesized as the cause
of the poor self–cleaning performance. Under the UV–Vis irradiation, two ways of the
Rhodamine B degradation can be expected (Ahmed et al., 2017; Bera et al., 2020; Lei et
al., 2005; Rochkind et al., 2015): 1) a photosensitization mechanism, in which the dye
absorbs the visible light reaching the excited state. The excited dye then transfers charge
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to the conduction band of the semiconductor (TiO2 in this case) and reactive ·OH radical
is formed, which in its turn is responsible for the degradation of the dye molecule (Wu et
al., 1998); 2) a direct photocatalytic mechanism, in which upon absorption of the UV
photons by the TiO2, charge separation and formation of active species occur on the
semiconductor, which will end up degrading the dye.
The second mechanism with TiO2 only works under UV irradiation which energy
matches the band–gap of the semiconductor, and understandably the NO abatement is
also strictly dependent on it (Rochkind et al., 2015; Yu et al., 2014). However, if the first
mechanism prevails, some interference during the charge donation from the dye to the
semiconductor might block the dye degradation, and this reason can be postulated to
explain the poor self–cleaning performance of the mortars with PCE–A.
In support of this argument, besides the similarities of the PCE–A mortars concerning
the pore size distribution and the NO abatement, two additional considerations can be
made: i) previous works have shown that the contribution of the visible light illumination
to the dye degradation is higher than that of the UV light, irrespective of the intensity of
the irradiation, remarking the significance of the photosensitization process in the
decoloring efficiency (Kuo and Ho, 2001; Rochkind et al., 2015; Wu et al., 1998); ii) the
experimental results show the differences in the kinetics of the degradation, being the
PCE–A mortars the samples with the slowest degradation. With undoped TiO2, the
photosensitization has been confirmed to exhibit faster discoloration kinetics than that of
the direct photocatalytic degradation (Rochkind et al., 2015), so that the later appears as
prevalent in PCE–A mortars (slow kinetics), whereas the former is predominant in the
others PCEs (fast kinetics) that present good self–cleaning performance.
The different adsorption of this polymer onto semiconductor particles and its low
molecular weight might explain its favored interstitial arrangement between the adsorbed
dye and the semiconductor, hindering the photosensitization mechanism of RhB
degradation (Ojani et al., 2012).
3.2.2 Photoinduced hydrophilicity
In order to have a full understanding of the influence of the different admixtures in
the self–cleaning performance of the lime mortars, the second approach of this research
work was to study if the addition of the waterproofing admixture (sodium oleate) might
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interfere with that property. Specifically, the waterproofing agent could reduce the
photoinduced hydrophilicity due to its water repelling action. The 91 days–aged lime
mortars were thus exposed to irradiation and the static water contact angle (CA) was
monitored at different times. Fig. 11 shows the different graphs of the surface wettability
of the mortars for each one of the tested SPs.
At time 0 min, before the irradiation, the waterproofing effect of the sodium oleate
was evident. Whilst the CA could not be determined for the control mortar (admixture–
free lime mortar) due to the instantaneous absorption of the water drop, the addition of
sodium oleate (sample O) sharply increased the CA of the mortar (up to ca. 80º), proving
the hydro–repellency imparted to the mortars by the oleate.
The addition of other admixtures, such as NS and nano–TiO2 increased the
wettability of the mortars, because, among other reasons, the dilution effect of the oleate
(owing to the addition of 20% and 2.5% of NS and TiO2 by weight of lime). For example,
in photocatalyst–bearing mortar O–NS–T, continuous irradiation for 30 minutes caused
a CA moderate drop from 51º to 37º (i.e. a 27% of CA reduction).
The presence of the superplasticizers in the mortars induced noticeable changes in
the CA values, proving their ability to favor the photoinduced hydrophilicity owing to the
dispersion of TiO2 active sites. CA values below 10º were obtained for some samples
after 30 min of irradiation. The lowest CA values were achieved in mortars with PCE–D,
PCE–C and PNS. On average, the percentages of CA reduction for samples before and
after 30 minutes irradiation of UV–Vis light were ca. 44% for PCE–A and PCE–B, ca.
52% for PCE–C and PNS and ca. 64% for PCE–D.
Fig. 12 shows the profile of the water drops after deposition on the surface of
different mortars after 3 minutes of irradiation, allowing observing the different
wettability of the specimens.
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Fig. 11. Static water contact angle (CA) as a function of the UV–Vis irradiation
time of different samples.
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Fig. 12. Water droplets over different samples after 3 minutes of UV–Vis irradiation:
(a) O–NS–T, (b) A0.5–NS, (c) B0.5–NS, (d) C0.5–NS, (e) D0.5–NS and (f) P0.5–NS.
These results are in line with previous results on photocatalytic coatings (Fornasini
et al., 2019), and they confirm that the incorporation of superplasticizers in bulk allowed
the preservation of the photoinduced hydrophilicity in these lime mortars, favoring their
self–cleaning characteristics. While the addition of a waterproofing agent is well known
to increase the durability of the lime mortars (Atahan et al., 2008; Izaguirre et al., 2010;
Silva et al., 2020), its presence has now been proved not to be detrimental for the
photoinduced hydrophilicity and thus for the self–cleaning properties of the lime mortars.
4 Conclusions
The specific target of this paper was to enhance the photocatalytic activity and self–
cleaning effect of lime mortars by using dispersing admixtures. The air lime mortars were
modified upon addition of a waterproofing agent, a pozzolanic admixture (nanosilica), a
photocatalytic agent (nano–TiO2) and superplasticizers.
This study has shown that the addition of superplasticizers resulted in a clear increase
of the depolluting action of the lime mortars by 33% on average as compared with SP–
free mortars, reaching a 44% of NO removal. In these mortars, the effect of the pore size
distribution was ascertained and a certain influence of the macropores > 1 m was
identified. The effective charge carrier separation was found to enhance the photoactivity,
particularly for mortars with PCE–C as superplasticizer. The polycarboxylated SPs
a)
b)
c)
d)
e)
f)
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190
increased more the photoactivity than PNS and this was related to the molecular
architecture of the SPs, which dominated the adsorption of the SPs, the anionic charge
density and the length of the side chains of the PCEs.
NO2 formation as a very toxic intermediate was hampered in a good degree due to
the use of dispersing admixtures, which provided selectivity values as high as 87%. These
good results were ascribed to the positive effect of the superplasticizers and to the
characteristics of the lime matrix, with alkaline pH and alkaline–earth ions.
The self–cleaning ability of the mortars was also improved by the addition of SPs.
The study of the degradation of the Rhodamine B dye deposited onto the surface of the
mortars showed ca. 70% of discoloration after 310 minutes of UV–Vis irradiation With
respect to TiO2–free mortars, PCEs B, C and D enhanced the decoloring efficiency 1.43
times on average. With respect to SP–free TiO2–bearing mortars, the enhancement of the
self–cleaning ability was 1.20 times. The poor performance of PCE–A concerning the
self–cleaning activity was related to the interference with the photosensitization
mechanism of the dye degradation, which was found to be fostered by the other PCEs
that displayed faster kinetics of degradation. This mechanism is dependent on the visible
light excitation of the dye and explains why, despite the poor self–cleaning performance,
the NO abatement of PCE–A samples (UV dependent) was high.
This study demonstrated that the presence of SPs also enhanced the photoinduced
hydrophilicity of the mortars, a mechanism that favors the self–cleaning action. The
presence of a waterproofing agent, sodium oleate, added to increase the durability of the
lime mortars, was compatible with the photoinduced wettability of the surface.
Particularly PCE–D, PCE–C and PNS fostered the achievement of low CA values (ca.
10º) during the irradiation of the mortars.
Due to the positive enhancement of the depolluting and self–cleaning abilities of the
air lime mortars, further studies might be addressed to adjust the dosages and the
water/lime ratio of these mortars depending on their final application as rendering mortars
(one–coat, multilayer or repointing mortars), allowing to obtain interesting
photocatalytically active and self–cleaning materials.
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191
Author Contributions
J.F. González–Sánchez: Main contributor in Investigation, Data curation, Formal
analysis, Writing– Original draft preparation, Writing– Reviewing and Editing. B. Taşcı:
Data curation, Formal analysis. J.M. Fernández: Writing– Reviewing and Editing. Í.
Navarro–Blasco: Investigation, Conceptualization, Visualization, Validation, Project
administration. J.I. Alvarez: Methodology, Supervision, Writing– Reviewing and
Editing, Funding acquisition.
Funding
This study was funded by Spanish Ministry of Economy and Competitiveness
(MINECO), grant number MAT2015–70728–P. The first author thanks the Friends of the
University of Navarra, Inc., for a pre–doctoral grant.
Acknowledgments
The authors thank the technical support provided by Cristina Luzuriaga.
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Capítulo III: Diseño y
obtención de morteros de
revoco con fisuración reducida
y adherencia mejorada Improving lime–Based rendering mortars with
admixtures
Enviado a Construction and Building Materials (En proceso de
revisión, octubre, 2020)
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Improving lime-based rendering mortars with admixtures
J.F. González-Sánchez, J.M. Fernández, Í. Navarro-Blasco and J.I. Alvarez *
MATCH Research Group, Chemistry Department, School of Sciences, University of Navarra, 31008
Pamplona, Spain.
*Author to whom correspondence should be addressed.
ABSTRACT
The present work presents focuses on the use of different admixtures for the
development of rendering lime-based mortars with improved adhesion and durability, as
well as reduction of cracking. To this aim, combinations of an adhesion improver
(ethylene-vinyl acetate copolymer, EVA), a water repellent agent (sodium oleate), a
viscosity enhancer (a starch derivative) and a mineral admixture (pozzolanic addition of
nanosilica or metakaolin) were tested. The renders were applied on four different
substrates (sandstone, limestone, granite and brick) to assess their performance.
The influence of the admixtures’ combination on fluidity, stiffening time,
adhesion, cracking, compressive strength, pore structure, frost resistance and durability
against magnesium sulfate attack was evaluated. The EVA admixture was seen to enhance
the adhesion when used in combination with oleate, metakaolin and starch. This
combination also led to a minimized cracking. Opposite trends between adhesion and
cracking were observed as a function of the porosity of the substrates and of the presence
of small-sized capillary pores.
The interferences with the carbonation accounted for the drops observed in
compressive strength for the nanosilica-free tested renders; nanosilica-containing renders
showed good compressive performance, due to the filling effect of the admixture and to
the C-S-H formation. The use of most of the admixtures’ combinations was seen to clearly
enhance the durability of the renders, in the face of freezing-thawing cycles as well as
sulfate attack, proving the applicability of these lime-based renders for repair works of
the Cultural Heritage and for new Civil Engineering applications.
Keywords: renders, air-lime, EVA latex, starch, pozzolanic addition, multiple
admixtures, improved adhesion.
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1. INTRODUCTION
When cement renders were applied for cultural heritage repair works, scientific
literature highlighted several drawbacks and incompatibilities [1–3]: efflorescence
phenomena due to the formation of large amounts of soluble salts due to the migration of
alkaline ions, low permeability to water vapor and a coefficient of thermal expansion
higher than most masonry.
The use of lime as binding material for repairing renders helps to improve the
appearance of historical buildings and it is also of great importance since renders act as a
sacrificial layer to protect and preserve old masonry. Rendering mortars could be used
directly to adhere to the vertical wall of the building without other binding materials [4].
The knowledge about the composition and performance of lime renders is basic to repair
and renovate the existing ancient renders as well as to design new compatible ones.
The choice of lime as binder can be upheld according to previous studies [4–9]. It
has been widely applied in the internal or external decoration of buildings due to its
unique aesthetics, texture, high compatibility with the external insulation system,
excellent weather resistance and durability [4]. It is considered one of the healthiest and
most environmentally friendly materials used in modern civil buildings compared to
organic coatings, ceramic tiles, and natural stones [10].
However, the lime-based renders also show some drawbacks: for example, high
sensitivity to deterioration processes due to low internal cohesion and high porosity that
could provoke bad adherence. Furthermore, these factors lead to high rates of water
absorption and a subsequent low mechanical resistance, thus enhancing the susceptibility
to several damaging actions, frost and salt crystallization often being mentioned as the
most damaging [11–14].
In addition, the lime-based renders are very sensitive towards cracking. The
phenomenon of cracking is really heterogeneous and is dependent on drying, hydration
(for renders with hydraulic phases) and creep. Stresses induced by shrinkage due to the
restricted drying can be highlighted as one of the main factors causing cracks, which can
be aggravated under severe drying conditions [14,15]. Shear stresses generated at the
render/substrate interface are also a source of cracking. As a consequence, diffusivity and
permeability to water and to other vapor and liquid compounds is sharply increased,
affecting adhesion and giving rise to detachments [15].
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The use of lime as binder entails also some well-known limitations, such as a
much more extended setting time, slow placement and hardening process. The mix design
(that is, binder: aggregate ratio, water content, lime characteristics, type of aggregate,
etc.) influences the carbonation process, involving changes in the microstructure of the
mortar and affecting the setting and hardening of the render. Since the pore structure is
strongly changed, the mechanical properties and the water transport behavior are also
modified [16].
The incorporation of mineral admixtures to the render mix such as pozzolanic
materials (as brick dust, nanosilica or metakaolin [17,18]), admixtures such as water
repellent materials (natural or artificial resins, wax and animal fat), viscosity modifiers
and fibers appear as an alternative to combat the mentioned drawbacks of the lime
renders, increasing their durability and turning these renders into suitable repair materials
for the Built Heritage [19–22].
Among the pozzolans, Alvarez et al. [18] found that the addition of nanosilica
(NS) to a lime-based binding material changed dramatically the distribution of the
mesopores. Besides, the NS incorporation induced C-S-H development, giving rise to an
enriched population of gel pores (< 10 nm), including the microporous range. These two
facts led to an improvement of the compressive strength of air lime mortars [18].
The use of metakaolin imparts considerable strength and the necessary workability
in the fresh state. Vavrijuk et al. [23] observed that mortars achieved higher compressive
and bond strengths more quickly.
Literature has shown the positive effect of waterproofing agents for lime-based
mortars [10,24]. Sodium oleate and other anionic surfactants are the most commonly used
admixtures. The improvement was especially remarkable in terms of reduction of the
water absorption through capillarity, and the subsequent durability enhancement of the
material in the face of freezing–thawing cycles. Furthermore, Izaguirre et al. [10]
confirmed that the maximum compressive strengths were reached in a shorter period of
time.
The use of rheology-modifying admixtures is also worth of consideration for lime-
based binding systems. Literature has reported viscosity modifiers for binding systems
such as starches and their derivatives (ethers and esters), cellulose (also etherified,
hydroxypropylmethyl, hydroxyethyl and carboxymethyl cellulose) or some other
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biopolymers ethers (like chitosan or guar gum) [1,25,26]. The thickening action can be
understood considering the water retention ability of these polymeric molecules and the
entanglement between the chains. In lime systems some of these admixtures have shown
a dosage-response pattern also improving the adherence of the binding material [27]. The
water retention may be useful to slow down the drying and thus to minimize the cracking.
It should be noted that the lime render adhesion on a substrate depends on moisture
and open porosity at the substrate/mortar interface [28]. Furthermore, the main modes of
failure in mortar/substrate systems are [15,29]: tensile cracking through the thickness of
the mortar and peeling or shearing at the interface between the two materials. Cracking
due to drying of the coating mortars depends largely on the boundary conditions (external
RH, wind speed, etc.) and the substrate (roughness, Young's modulus, etc.). If the water
absorption of the substrate is too high, the mortar can dry out, which is unfavorable
especially for hydraulic binders since it hinders hydraulic reactions. A proposed solution
to avoid this effect is the substrate humidification before applying the mortar. The use of
admixtures may also help to control the drying and to enhance the adherence.
Some admixtures, such as methylcellulose and ethylene-vinyl acetate (EVA)
copolymer, are currently used as modifiers in Portland cement and concrete for improving
the adherences [27,30]. Methylcellulose contributes to flexural strength, improves the
dispersion and stability of hydration products, thus reducing the strength regression of
cement in late stage [30]. The addition of EVA to concrete and mortar increases flexural
strength because the active groups in their molecules can also react with the cations of
cement hydration products to improve the physical structure of the mortar [27,31,32].
EVA can be formulated as redispersible powders to form a latex dispersion responsible
for its properties [33]. EVA also improves the adhesion between the aggregates and the
matrix of the cementitious material, reduce the modulus of elasticity of the concrete and
improve its ability to absorb stresses under variable temperature conditions [34–36].
To obtain lime-based renders that can be efficiently used as repair materials for
the Architectural Heritage, the design of mixes with combination of multiple admixtures
is explored in the current work. Air lime as binding phase is combined with: a pozzolanic
addition (to increase the strength, durability and binding capacity), a waterproofing agent
(sodium oleate, to reduce the water absorption), a viscosity enhancer (a starch derivative,
to improve the application in the fresh state and to avoid a damaging quick drying) and
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an adhesion booster (ethylene-vinyl acetate copolymer). The compatibility between the
admixtures in the light of their effects is investigated. The renders are applied onto four
different substrates (sandstone, limestone, granite and brick), analyzing the effect on
adherence, cracking and durability among other properties. The final performance of the
renders is assessed providing valuable information about the potential use of these mixes
as repair materials.
2. MATERIALS AND METHODS
2.1. Materials and composition of the renders
Air lime supplied by CALINSA, Spain, CL 90-S class, in powder, was used for
preparing the renders. Mean particle size was 10 μm (less than 10% > 50 μm). The lime
presented a CaO percentage of 68.53%, with major impurities of MgO (3.29%), SO3
(1.37%) and SiO2 (1.03%). As aggregate, a very fine limestone with particle size lower
than 2 mm, supplied by CTH (Huarte, Navarra, Spain) was used, its chemical composition
was 52.83% (CaO), 2.28% (MgO), 1.14% (Fe2O3 + Al2O3), 0.57% (SO3), 0.49% (SiO2),
0.07% (Na2O), 0.05% (K2O), 43.10% (ignition loss) [17] and its particle size distribution
is displayed in Fig.1. Mixing proportion of renders was 1:3 binder/aggregate (air
lime/aggregate) weight ratio [25].
The detailed composition of the different combinations of admixtures for the
renders was reported in Table 1.
Fig. 1 Grain size distribution of the aggregate
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Table 1. Percentage of admixtures of the renders expressed as percentage by
weight of lime
The description of the different admixtures was:
• Mineral admixture (pozzolanic addition, 20% by weight of lime, bwol):
nanosilica (NS) or metakaolin (MK). Nanosilica (NS) (ULMEN Europa
S.L.) was supplied as a colloidal, superplasticizer-free silica suspension,
solid/liquid ratio of 0.28 and pH = 9.68. Its specific surface area was ca.
500 m2 g− 1 and the average particle size was around 50 nm. Metakaolin
(MK) (Metaver, supplied by, NEWCHEM, Pfäffikon, Switzerland), with a
specific surface area of 20 m2 g− 1 and the average particle sizes in aqueous
suspensions were of ca. 3.9 µm. Detailed characterization of these two
mineral admixtures has been published elsewhere [37].
Name
Pozzolanic addition Water
repellent
(sodium
oleate)
Rheology
modifier
(starch
derivative)
Adhesion
enhancer
(EVA) Nanosilica Metakaolin
Control samples
C - - - - -
C-NS 20 - - - -
C-MK - 20 - - -
C-O - - 0.5 - -
C-O-NS 20 - 0.5 - -
C-O-MK - 20 0.5 - -
Samples without
rheology modifier
O-E5 - - 0.5 - 5
O-E10 - - 0.5 - 10
O-NS-E5 20 - 0.5 - 5
O-NS-E10 20 - 0.5 - 10
O-MK-E5 - 20 0.5 - 5
O-MK-E10 - 20 0.5 - 10
Samples with
rheology modifier
O-S-E5 - - 0.5 0.5 5
O-S-E10 - - 0.5 0.5 10
O-NS-S-E5 20 - 0.5 0.5 5
O-NS-S-E10 20 - 0.5 0.5 10
O-MK-S-E5 - 20 0.5 0.5 5
O-MK-S-E10 - 20 0.5 0.5 10
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• Water-repellent agent (0.5% bwol): Sodium oleate (O), provided by
HISA A 2388 N from ADI-Center-S.L.U. The well-known structure of
the oleate, with its long non-polar hydrocarbon chain and its polar
carboxylate group at one end, has been also reported elsewhere [10,24].
• Rheology modifier (0.5% bwol): a potato starch derivative Casaplast
KO09 (S). This is an etherified starch with a high degree of substitution
and soluble in cold water according to the datasheet of the supplier. Due
to its non-ionic character, it is highly compatible with bivalent ions such
as calcium and magnesium. It has been applied as a thickener for
cement and plaster mortars [25]. X-ray diffraction (XRD), FTIR and
TG-DTA studies were used to characterize the structure and functional
groups of the admixture. XRD experiments were performed in a Bruker
D8 Advance diffractometer with a Cu Kα1 radiation, from 5° to 70°
(2θ), 1 s per step, and a step size of 0.04°. The infrared spectra
(Shimadzu IRAffinity-1S apparatus) were registered at 100 scans over
a wavelength range of 4000–600 cm−1, with resolution of 4 cm−1. TG-
DTA (851e Mettler Toledo thermoanalyzer device) used alumina
crucible, temperature range from 25 to 1000 °C, 10 °C·min− 1 heating
rate, and under static air atmosphere.
• Adhesion enhancer (5% and 10% bwol): Elotex MP 2080 (E) which is
a water-redispersible powder of ethylene-vinyl acetate copolymers
(EVA). It has hydrophobic properties suitable for dry mineral mortars
based on calcium or cement according to the producer. The
characterization of the admixture was also carried out using the
methods reported above for the rheology modifier.
2.2. Preparation of the renders
The dry raw materials, lime and aggregate, and the required amounts of the
pozzolan and of the chemical admixtures were blended for 5 min using a solid mixer BL-
8-CA (Lleal S.A.). The fluidity of the fresh samples was adjusted to a slump of
145 ± 10 mm [38] in line with the slump values for single-coat renders [39], and thus the
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amount of mixing water was set accordingly. Dry components and mixing water were
mixed in a Proeti ETI 26.0072 mixer for 90 s at low speed mixer. Fresh state properties
were determined as described below.
Fresh renders were cast in cylindrical moulds (40 mm of diameter and 36 mm of
height). Molds were stored at lab conditions (20 °C and 60% RH), and hardened samples
were demolded 7 days later. Hardened state properties were studied after 28 and 91 curing
days. To guarantee the representativeness of the results, three replicates of the samples
per each curing time and per measured property were tested.
2.3 Fresh state
For the fresh state of the renders, the following properties were studied according
to the quoted standardized methods:
• Density and air content, both data being recorded using a receptacle of 1 dm3
previously weighed, which, after being filled with fresh mortar, was
weighed again to obtain the density [40]. In a specific device, the entrained
air was removed and replaced by a measurable amount of introduced water,
which allowed us to determine the air content [41].
Fig. 2. Photographs of the different substrates
Sandstone
Limestone
Granite
Brick
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• Water retention capacity, determined by weighing absorbent materials
placed on the fresh sample before and after 5 min of contact under pressure
[42]; workable life, obtained from a specific device provided with a bradawl,
which pushed the fresh sample until the strength exerted to introduce it into
the sample was larger than 15 N (EN1015-9) [43].
• And finally, the evolution of the renders when applied on different
substrates was assessed. This last test consisted of spreading a monolayer of
fresh render of ca. 15 mm according to EN 998-1 [44] on four different pre-
wetted substrates (limestone, sandstone, granite and brick) and observing
any developments (cracking and/or detachments) at different ages after
application: 1 day, 2 days, 1 week, 1 month and 2 months. The substrates
were prepared cutting them in monoliths of 5x5 cm, with a thickness of 3
cm (Fig. 2). The pre-wetting was carried out spraying tap water on the
surface of the substrates. Semi-quantitative mineralogical composition of
the stony substrates was ascertained by X-ray diffraction:
− sandstone (density 2.30 g cm-3, from Lleida, Spain, 41% dolomite, 39%
calcite, 20% quartz);
− limestone (density 2.67 g cm-3, type Marbella, from Murcia, Spain, 100%
calcite);
− granite (density 2.72 g cm-3, from Porriño, Spain, 26% pyroxene, 22%
andesine, 17% albite, 15% microcline, 11% quartz, 9% calcite);
− brick (density 1.14 g cm-3, 39% quartz, 32% diopside, 18% feldspar, 11%
calcium aluminosilicate).
Fig. 3. Pore size distribution of the different substrates
Capítulo III
212
The total porosity of the substrates was determined by Mercury Intrusion
Porosimetry (MIP) with a Micromeritics Auto Pore IV 9500 equipment (Micromeritics
Instrument Corporation) (pressure range 0.0015–207 MPa). The obtained results were as
follows: 20.85% (sandstone), 8.61% (limestone), 1.69% (granite) and 35.8% (brick). It
can be seen that brick was a really porous substrate but with a ribbed surface. Sandstone
and limestone exhibited a lower porosity and granite was a really low porous substrate.
The analyses of the pore size distribution of these substrates depicted the patterns
displayed in Fig. 3. The area under the curve perfectly matches the values of the total
porosity of the substrates. A detailed effect of these pore size distributions is presented
below in section 3.3.
2.4 Hardened-State
• The adherence of the plaster was determined according to the UNE 83 22 EX
regulation [45].
• Compressive strengths were measured after 28 and 91 curing days in the
cylindrical specimens. The rate of loading was 50 N s− 1 in a device Proeti ETI
26.0052.
• The pore size distribution was ascertained by MIP as indicated in the previous
section.
• The thermal decomposition of the hardened samples was monitored by TG-DTA
analysis (equipment and conditions detailed above).
• The permeance was obtained to get the permeability value, according to the
standard EN1015-19 [46]. The permeance is the water vapor flow that passes
through one area unit under equilibrium conditions for each unit of vapor pressure
difference on both sides of the plaster. Then, water vapor permeability is
calculated as the result of multiplying permeance by the thickness of the test
specimen.
• Water absorption through capillarity of different renders according to the
EN1015-18 standard [47].
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213
• Frost resistance was determined by means of freezing-thawing cycles. The cycles
consisted of water immersion of the samples for 24 h and subsequently freezing
at −10 °C for 24 h (CARAVELL 521-102 freezer).
• For the assessment of the sulfate attack resistance, the monolithic samples were
completely submerged in an aqueous solution saturated with MgSO4 at 20 °C and
95% RH for 24 h. After this step, the samples were dried in an oven at 65 °C for
24 h and submerged in water for 24 h at 20 °C and 95% RH. To conclude the
cycle, the specimens were again dried as described above. The cycles were
continuously repeated until the destruction of the specimens or a maximum of 28
cycles.
3. RESULTS AND DISCUSSION
3.1. Characterization of the admixtures
The starch derivative (S) and the ethylene-vinyl acetate copolymer (EVA)
admixtures were fully characterized. The other admixtures, including the mineral
admixtures (nanosilica and metakaolin), and the sodium oleate had already been
previously characterized as reported elsewhere [24].
The XRD pattern of the starch-based admixture is displayed in Fig. 4. The
halo between 15 and 25º 2 corresponds to the modified starch, due to its amorphous
condition. The sharp diffraction peaks identified in the pattern are ascribed to sodium
sulfate (files of the ICDD PDF 89-4751, thenardite, anhydrous sodium sulfate
mineral, and PDF 37-1465, sodium sulfate). This salt was most likely added during
the obtaining of the starch derivative as coadjuvant of the cross-linking process [48–
52].
Fig. 4. XRD pattern from starch
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214
Table 2. FTIR band assignment for starch (Casaplast KO09) and EVA (Elotex MP 2080).
Starch EVA
Wavenumber
(cm 1)
Functional
group
Wavenumber
(cm 1)
Functional
group
Wavenumber
(cm 1)
Functional
group
3403-3200 O-H
3630-3620 CH2
(ethylene) 1105-1100 C-O
2926-2860 C-H 2900-2850 C-H 1030-1020 C-O, CH3
1683-1500 C=O 1730-1725 C=O 950-940 C-C
1156-937 C-O 1470-1440 C=O 750-720 CH2
(ethylene)
1083-1023 O-C 1370-1380 CH3 635-625 O-C-O
1275-1200 C-O 610-600 C=O
The FTIR spectrum of this admixture is shown in Fig. 5 and the assignment of
the absorption bands is summarized in Table 2. The predominant presence of –OH
groups is denoted by the intense absorption band at 3403 cm-1
The XRD pattern of the adhesion enhancer Elotex MP 2080 (E) shows the halo
suggesting the amorphous character of the polymer between 18 and 28º 2 (Fig. 6). Some
crystalline peaks are also observed associated with inorganic materials (dolomite,
kaolinite, and calcite) and diethylene glycol. These materials are usually added as
anticaking agents to redispersible powders formulation in order to prevent the adhesion
between polymer particles during manufacturing, transporting and storage, some of them
with surfactant properties [25,30,53]. FTIR spectrum of the EVA admixture (Fig. 5) was
recorded and the bands assignment (Table 2) was ascribed to the specific peaks of
polyethylene and polyvinyl acetate.
Fig. 5. FTIR spectra obtained for starch (Casaplast KO09) and EVA (Elotex MP 2080)
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215
Fig. 6. XRD pattern of EVA admixture
The content of copolymer vinyl acetate and the ethylene part of the EVA was
ascertained by thermogravimetric (TG) analysis. The TG curve (Fig. 7) shows the weight
loss as a function of temperature. Several sections that can be associated with the presence
of the different components of the sample were observed: a) from room temperature to
280 ºC, the sample undergoes no thermal transformation; b) thermal decomposition
occurs in two stages: the first one ends around 400 ºC and the second one runs from 400
ºC to 500 ºC, approximately; c) at elevated temperatures, 500 - 600 ºC, the entire sample
burnt.The first step of the curve corresponds to the loss of vinyl acetate in the copolymer,
while the second decomposition is due to that of the ethylene part [54]. The quantification
of the TG weight loss of the decomposition process allows to determine the vinyl acetate
content of the EVA copolymer. For the calculation, it must be taken into account that the
decomposition of the acetate groups takes place through the formation of acetic acid [54].
Percentage of 65.3% vinyl acetate and 34.7% ethylene part were obtained.
Fig. 7. TG curve of EVA admixture
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216
3.2. Effect of the admixtures on the mixing water requirements
As shown in Table 3, the use of EVA admixture (abbreviated E in the
nomenclature of the samples) slightly increases fluidity compared with the control sample
or with the samples with just the waterproofing agent, sodium oleate, with the same
percentage of mixing water. This result can be explained due to the “ball bearing” effect
of the polymer particles, to the entrained air and to the dispersing effect of the tensioactive
compounds used in the formulation of the EVA powders [55]. The presence of small
amounts of surfactants, a common practice during the EVA preparation, was seen to have
a clear influence in the fresh state performance of the renders with EVA: the entrained air
increased particularly at high EVA dosages.
The addition of pozzolans increased the water demand to reach the set flow value:
this is a well-known effect of the pozzolans which can be ascribed to the large surface
area and to their reactivity. Among the pozzolans, the water requirements were higher for
nanosilica, as proved by the control samples (C-NS and C-MK) and by the samples with
EVA and starch.
The sharpest increase of the water demand was observed upon the addition of the
viscosity enhancer, the starch derivative. This admixture behaves as a thickening agent
that requires a greater amount of water to achieve a certain level of fluidity [25,56].
Regarding the workable life, the adhesion enhancer at 5% does not substantially
modify this value in comparison to the plain control sample (C), as observed for samples
O-E5 and O-MK-E5. At the largest dosage tested in the current work (10%), regardless
of whether another additive is in the mixture, the stiffening time increased. In these cases,
the admixture at larger dosages exhibited a water retaining action that can be related to
the hydrophilicity of the particles of the polymeric latexes at a colloidal state, to the
increase in viscosity of the aqueous phase and to the inhibition of the water loss due to
the filling and sealing effect of the polymers [33–35].
The effect of the starch-based admixture on the workable life of the samples was
pronounced owing to the water retaining action of the starch derivative molecules and to
the viscosity increase of the liquid phase [25],which can be ascribed to the high number
of –OH functional groups (absorption band identified in the FTIR spectrum Fig. 5).
Capítulo III
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Table 3. Mixing water, fluidity and workability, air content and bulk density
values of different mixtures (spread values as measured by the flow table test, which were
fixed at 145 ± 10 mm).
*percentages with respect to the weight of lime and sand
These groups enhance the water retention and the cross-linking phenomenon
(and thus the viscosity) between polymer chains by hydrogen bonds. Therefore, the
drying of the renders was delayed and the access of CO2 to the inner part of the
renders hindered, resulting in a prolonged stiffening time. In this sense, it should be
noticed that the water holding capacity of mortars influences drying. More water
retention leads to lower drying rates, both by evaporation and by suction of the
substrate. This effect naturally promotes better hydration, in the case of lime binders
with a hydraulic phase, and can also produce good conditions for carbonation,
ensuring sufficient moisture content, however moderate, for a long period, even in
dry external conditions [57,58].
Sample Mixing water*
(%)
Fluidity
(mm)
Stiffening time
(min)
Air Content
(%)
Density
(g cm-3)
C 28% 148 69 4.4 0.87
C-NS 30% 145 475 2.7 0.91
C-MK 29% 146 117 3.1 0.81
C-O 28% 155 95 3.2 0.90
C-O-NS 28% 135 360 2.6 0.91
C-O-MK 28% 146 330 3.7 0.85
O-E5 28% 155 62 4.9 0.87
O-E10 28% 152 138 5.2 0.86
O-NS-E5 31% 135 747 4.2 0.87
O-NS-E10 33% 135 912 4.8 0.86
O-MK-E5 28% 145 80 4.2 0.81
O-MK-E10 30% 146 139 4.6 0.80
O-S-E5 37% 147 1435 4.0 0.86
O-S-E10 39% 147 2540 4.2 0.88
O-NS-S-E5 37% 141 1342 4.2 0.86
O-NS-S-E10 39% 142 1749 5.2 0.88
O-MK-S-E5 37% 135 1440 3.6 0.82
O-MK-S-E10 41% 155 1850 3.6 0.81
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3.3. Application of the renders on different substrates
Fig. 8 and 9 show the qualitative evaluation of the adherence and cracking
occurrence, respectively, of the renders when applied as one-coat mortar on the four
different substrates: sandstone, limestone, granite and brick. The evaluation of
adherence (Fig. 8) was made observing the applied renders at different ages and
assigning a description following this criterion: (i) detachment in different zones was
assigned to samples that showed detachment of the monolayer render in different
zones; (ii) detachment in the edge for the samples that only showed detachments in
the borders of the substrates; and (iii) good adherence for the samples that presented
no evidences of detachments. It must be pointed out that all the observed detachments
were adhesive failures, confirming the relevance of the mortar-substrate interface.
The cracking assessment (Fig. 9) was made observing the applied renders at
the same ages and comparing the quantity of cracks over the different mixes’
monolayers and the absence of cracking, according to: (i) severe cracking assigned
to the sample that showed the entire or almost full area of monolayer with cracking;
(ii) evident cracking for the samples with many areas showing cracking; (iii)
moderate cracking for renders with limited cracking; and (iv) little or no cracking
for the samples that visually presented no cracking in almost all or all the monolayer
area.
To illustrate the criteria for the qualitative assessment, Fig. 10 shows several
examples of samples with cracking or adherence problems and their qualification.
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219
Fig. 8. Qualitative evaluation of adhesion of lime renders on different substrates: a)
sandstone, b) limestone, c) granite and d) brick
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220
Fig. 9. Qualitative evaluation of the cracking of lime renders on different substrates: a)
sandstone, b) limestone, c) granite and d) brick
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221
Sample O-MK-E10 applied onto sandstone (left) and granite (right) with no cracking. A
small detachment was identified in a corner of the granite specimen.
Sample O-NS-S-E5 onto sandstone, with detachment in the edge (left bottom part) and
evident cracking
Sample O-MK-E10 onto limestone, with good adherence and moderate-evident
cracking
Sample O-MK-S-E5 onto sandstone, with good adherence and no cracking
Fig. 10. Examples of different samples and their qualitative assessment of adherence
and cracking
O-MK-E10
O-NS-S-E5
O-MK-E10
O-MK-S-E5
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222
The evolution of the renders applied on the different substrates showed the
following behavior:
• Sandstone: the control samples with only pozzolanic or starch
admixtures showed in general a moderate to poor adherence. The
cracking was intense particularly in the control samples with oleate (C-
O and C-O-NS, particularly). The EVA admixture induced a good
adherence of the renders and the absence of cracking, as it was observed
in O-MK samples with 5 and 10% adhesion enhancer and also O-MK-
S-E5 mix, which, after 2 months, presented a good visual appearance
(Fig. 11). In comparison with control samples C-MK and C-O-MK, the
EVA admixture in samples with MK increased the adherence and
reduced the cracking.
Fig. 11. Renders without cracking and good adherence after two months applied on
sandstone
Good adherence
O-MK-E5
Good adherence
O-MK-E10
Good adherence
O-MK-S-E5
O-MK-E5
O-MK-E10
O-MK-S-E5
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223
On the contrary, renders without pozzolanic mineral admixture and with the
starch-based admixture failed, showing cracks and adhesive detachments
(O-S-E5 and O-S-E10) (Fig. 12).
Fig. 12. Renders applied on sandstone with cracks and detachment after two months
O-S-E5
O-S-E10
Detachment
O-S-E5
Moderate
cracking
O-S-E10
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224
• Limestone: the application of the renders on the limestone was not as
successful as in the case of the sandstone. Most control samples showed
poor adherence, detachments and evident to intense cracking (Fig. 8 and
9), in line with previous works in the literature [10,18]. Most renders
including admixtures showed cracks and -in some cases- detachments. For
example, renders O-E10 and O-S-E5 evidenced detachments and most of
areas with visible cracking (Fig. 13). Sample O-MK-S-E5, in spite of small
cracks, exhibited the best adherence on this surface (Fig. 13). As in the
case of the sandstone substrate, renders with MK and EVA together
yielded the best performance.
Fig. 13. Photographs of renders applied on limestone after two months.
Good
adherence
O-MK-S-E5
O-MK-S-E5
O-E10
O-S-E5
Detachment
areas
O-E10 O-S-E5
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225
• Granite: all the set of the control samples applied on this substrate showed poor
adherence (Fig. 8). The addition of the EVA admixture clearly enhanced the
adherence. However, the cracking in the renders applied on granite was not as
strong as in other substrates like limestone or brick. The render showing the best
performance was O-MK-S-E10 (no detachments, no cracks, good aesthetic
appearance) (Fig. 14). In comparison with control samples, the simultaneous
presence of pozzolanic agent, starch and EVA enhanced the adherence. The
absence of MK was detrimental, as can be seen in sample O-E10 that showed
cracks on the whole surface as well as material detachment observed in the borders
of the substrate monolith (Fig. 14).
Fig. 14. Photographs of renders applied on granite after two months.
O-MK-S-E10
O-E10
Detachment
O-E10
Intense
cracking
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226
Fig. 15. Photographs of renders applied on brick after two months.
• Brick: the use of any of the admixtures was not favorable for renders applied on
bricks. Severe cracking was observed (Fig. 15). This finding is related to the
absorptivity of the substrate, as will be explained below. The adherence was
however improved due to the addition of the EVA admixture in combination with
MK (see the good adherence of sample O-MK-S-E5), although not so much for
combinations with NS that showed several detachments (Fig. 15).
Quantitative assessment of the bond strength was also carried out (Table 4). In
some cases the equipment was not able to quantify due to the low bond strength of the
material (values indicated as ND, not detected).
Detachment
O-NS-S-E10
Good
adherence
O-MK-S-E5
Detachment,
intense cracking
O-NS-S-E5
Detached fragment due
to the adhesive failure
Detachment
C Intense cracking
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227
Table 4. Adherence of different mixes over different substrates
Sample Sandstone (N/mm2) Limestone (N/mm2) Granite (N/mm2) Brick (N/mm2)
C 0.0067 ND 0.0022 0.0225
C-NS ND ND ND 0.0199
C-MK 0.0065 ND 0.0028 0.0212
C-O ND ND ND 0.0118
C-O-NS ND ND ND 0.0221
C-O-MK ND ND 0.0025 0.0232
O-E5 0.0134 ND 0.0312 0.0223
O-E10 0.0179 ND 0.0202 0.0291
O-NS-E5 ND ND ND 0.0272
O-NS-E10 ND 0.0115 ND 0.0504
O-MK-E5 0.0198 ND 0.0440 0.0857
O-MK-E10 0.0066 0.0462 0.0088 ND
O-S-E5 0.0477 0.0295 ND 0.0091
O-S-E10 0.0163 ND ND 0.0418
O-NS-S-E5 ND ND ND 0.0463
O-NS-S-E10 ND ND ND 0.0481
O-MK-S-E5 ND 0.0092 ND 0.0253
O-MK-S-E10 ND 0.0069 0.0069 0.0207
In line with the results shown in the qualitative assessment of the adherence,
the bond strength evaluation indicates that the addition of the EVA admixture,
particularly in positive combination with MK and starch admixtures, resulted in an
enhancement of the adherence. Table 4 shows the higher values of adherence of most
samples with EVA as compared with the control samples. This was mainly applicable
to renders applied onto brick, in which the bond strength was generally able to be
measured. In renders onto limestone, the adherence of the control samples could not
be detected. Finally, in some control samples applied on granite or sandstone, the
adherence values were in general lower than those measured for samples with EVA
admixture.
The performance of the renders was dependent on two main factors: the pore
structure of the substrate and the combination of the admixtures of the renders. These
two factors are tightly connected with the drying of the renders, which plays a crucial
role to understand the cracking and the bond strength.
Capítulo III
228
The fluctuation of the internal humidity of the render can be attributed to:
i) Drying shrinkage, caused by external evaporation, responsible for the water
removal from the render. This causes significant volumetric shrinkage of the
material.
ii) Chemical shrinkage, only for lime renders with hydraulic components
(either renders prepared with hydraulic limes and/or mixed with pozzolans).
This phenomenon takes place during hydration of hydraulic compounds (the
lower the water/binder ratio, <0.40, the more intense the shrinkage) and
results in strong capillary pressure and changes in free surface energy which
cause cracking.
iii) Water absorption by the substrate, generated when the render interacts with
different porous media [58].
Concerning the characteristics of the substrate, the absorption of water by the
substrate causes a reduction of water in the mortar, which leads to a decrease in its open
porosity [59]. The greater the water absorption of the substrate, the greater the influence
on its open porosity and bulk density. Furthermore, the suction pressure is another factor
that must be considered. This pressure depends on the relative size of the mortar and
substrate pores: the small pores of the substrate exert a suction pressure that induces the
transport of water from the larger pores of the mortar. This mechanism is particularly
significant for air lime mortars, which, in fact, have a higher population of large pores
[60] and, therefore, a higher percentage of the pores of the substrate will be smaller than
those of the mortars and will be able to apply suction pressure. The influence of substrate
absorption on these mortars refers to drying and microstructural changes [57,59,61]. In
all cases, both external evaporation and suction of the substrate promote microstructural
modification along with possible cracks [59].
Measurements of the total porosity values of the substrates used in the current
work obtained by MIP [62] have been reported in section 2.2. Substrates of very high
porosity, like brick, caused intense water suction and then the cracking of the renders was
severe, confirming the close relationship between drying and cracking. On the other hand,
the application of lime renders on a substrate of low porosity, such as granite, minimized
the cracking.
Capítulo III
229
Fig. 16. Comparison of the pore size distributions of the limestone and sandstone.
The apparently contradictory results between sandstone (with higher total porosity
but with renders showing less cracking) and limestone (less porous substrate but renders
with higher cracking) can be clarified considering a detailed analysis of their pore size
distributions (reported for all the substrates in Fig. 3). As it can be seen in Fig. 16, the
limestone presents a marked population of small pores between 0.013 and 1 microns. Due
to their low diameter, the capillary suction was higher and the induced cracking more
intense (as in the case of the brick, the substrate showing the highest pore population <
0.5 microns).
Concerning the cracking, and irrespective of the substrate, the combination
including metakaolin, oleate, 5% EVA and the starch-based admixture (S), was the most
effective in controlling and minimizing the cracking. Even in renders applied on brick,
the formed cracks were smaller than with other combinations of admixtures.
With respect to the adhesion, however, the porosity of the substrate seems to play
a favorable role. The renders applied on brick yielded the highest bond strength values
(Table 4), followed by the renders on sandstone. Large porosity, and thus large textures,
allowed a better interaction at the interface between the render and the stony substrate,
with more surface of anchorage [63]. This mechanical interlocking increases depending
on the ratio between the effective contact surface and the area which could be potentially
bound [64]. This was confirmed by the good adhesion on the brick, which exhibits a
ribbed surface (Fig. 2). Pore size distribution is also important: the predominant capillary
pores of the substrate obstructed the adhesion and led to detachments, such as those
observed for renders onto limestone. This can be explained due to the impossibility of the
Capítulo III
230
render to penetrate the texture and wet the substrate as a consequence of the low size of
these pores [65]. This fact is also confirmed by the poor adhesion results observed in the
low-porosity substrates of the renders including the viscosity enhancer (starch): the
increase in viscosity hindered the penetration of the fresh render in low-sized pores,
causing a contact failure. The mixes that showed the least adherence on different surfaces
were O-NS-E5, O-NS-S-E5, O-NS-S-E10: only the bond strength on the brick surface
was able to be recorded, which can be ascribed to the ribbed surface.
The use of EVA increased the bond strength after application on sandstone,
granite and brick. This admixture was even more effective in combination with MK,
although in this case EVA should be at 5% dosage. The mix with the best adherence on
brick and granite was O-MK-E5. The presence of the rheology modifier slightly worsened
the bond strength values, for the aforementioned reasons. On balance, the combination of
MK, oleate, EVA (5%) and starch may be selected as the most appropriate considering
the cracking attenuation and the good adherence.
3.4. Influence of the admixtures on the compressive strength and pore
structure
In the lime-based renders an increase in mechanical properties over time due to
carbonation should be expected [17,37]. Fig. 17 depicts the values of compressive
strength after two curing times, 28 and 91 days. In samples with pozzolanic agent, the
pozzolanic reaction yielding C-S-H phases also should contribute to the strength of the
renders. This was evident in the set of control samples: renders with either NS or MK (C-
NS and C-MK) exhibited higher values than those of the pure lime (C). The presence of
oleate showed a certain interference with the carbonation (decrease sharper at 91 curing
days). Detailed effects of the individual influence of NS, MK and sodium oleate have
been reported elsewhere [10,37] Except for samples with NS, the admixtures O, S and
EVA caused a drop in the compressive strength values as compared with the control group
of samples, including C, C-MK and C-O-MK renders. This finding can be ascribed to the
interference with the lime carbonation process and/or microstructural modifications
caused by the admixtures. In the case of hydraulic phases, such as those to be developed
by reaction of hydrated lime with MK, the presence of EVA has been reported to interfere
with the continuous formation of C-S-H phases [30].
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231
The degree of carbonation was determined by thermal analysis and it is expressed
-in Fig. 18- as the ratio CaCO3/Ca(OH)2. The higher the ratio, the higher the carbonation
degree of the render. For samples with pozzolanic agent, the increase in the ratio
simultaneously indicates the consumption of free Ca(OH)2 due to the pozzolanic reaction,
as clearly shown in control renders C-NS, C-MK, C-O-NS and C-O-MK. As it can be
seen, the presence of the admixtures O, S and EVA reduced the carbonation degree and/or
the consumption of portlandite during the pozzolanic reaction (as mentioned before, EVA
interferes with the C-S-H formation). Due to the low ionic character of these admixtures,
the interference cannot be ascribed to Ca2+ complexation, unlike other admixtures
[17,25]. Changes in porosity and water retention might account for modifications in the
CO2 access and in the water availability inside the pores required for the fulfilment of the
carbonation and of the pozzolanic reaction [10,25,30].
Figure 17. Compressive strength results of the renders. a) E5%, and b) E10%.
0
1
2
3
4
5
6
7
8
Com
pre
ssiv
e st
ren
gth
(MP
a)
RC28 RC 91 a)
0
1
2
3
4
5
6
7
8
Co
mp
ress
ive
str
en
gth
(MP
a)
RC28 RC 91 b)
Capítulo III
232
Fig. 18. CaCO3/Ca(OH)2 ratio of different samples
Fig. 19 and 20 show the pore size distribution of the renders after 91 days of
curing. All renders showed a main intrusion volume in the range 0.5 to 1 μm pore
diameter (in accordance with previous works on lime-based mortars [66,67], with an
almost unimodal distribution). The addition of EVA in both dosages (5% and 10%)
increased the volume of intruded mercury at the main peak in the range from 0.6 to 0.8
μm. In these samples the surfactant added with EVA increased the air content [30],
accounting for this higher porosity. In all cases, the starch-based admixture shifted the
main pore size towards higher diameters, a finding that can be attributed to the greater
amount of mixing water required because of the addition of the viscosity enhancer.
There was a clear reduction in total porosity (area under the curve) when NS was
added. The filling effect of the NS and its ability to react with calcium hydroxide to form
C-S-H phases explain this finding, which justifies the higher compressive strengths of the
renders with NS (Fig. 17) [see detailed analyses of the pore structure of lime with NS
mortars in 17,24]. Pore size distribution of the renders with NS confirmed the formation
of medium capillaries (between 0.01 and 0.05 microns) and outer C-S-H gel pores (< 0.02
microns) [68]. However, higher compressive strengths do not guarantee a better
performance, as it can be inferred from the severe cracking and poor adhesion of the
renders with NS.
0
5
10
15
20
25C
aCO
3/C
a(O
H) 2
rati
o
28 days 91 days
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233
Fig. 19. Pore size distribution in samples with 5% dosage of EVA after 91 days.
Fig. 20. Pore size distribution in samples with 10% dosage of EVA after 91 days.
3.5. Water vapor permeability and water absorption
Water vapor permeability was measured (Table 5). From a hygrothermal point of
view, this parameter for the renders should be as high as possible to avoid undesirable
water condensation and associated problems [69]. The renders must be breathable and the
effect of the different admixtures on this property must be assessed. As a requirement, the
permeability should increase outwards and be similar or higher than that of the support
[70] to allow the water vapor flow through the constructive system.
The mere addition of EVA was seen to just slightly reduce the permeability only
when used at the highest dosage (10%) as compared with the plain lime render (C) or
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234
with the C-O render. In comparison, other renders including the pozzolanic agents
showed lower permeability values as a consequence of the filling effect and the
subsequent pore size reduction (C-NS, C-MK, C-O-NS, C-O-MK) [18]. If EVA was
combined with the starch-based admixture, permeability increased due to the changes in
the pore size distribution induced by the viscosity enhancer (see values of the renders O-
S-E5 and O-S-E10).
In line with the good correlation observed between the pore size distribution and
the water vapor permeability, samples with NS, which were seen to dramatically reduce
the porosity and the population of large capillary pores (0.05 to 10 microns), exhibited
the lowest permeability. The filling effect of NS and the formation of densified C-S-H
structures of low pore size explain this finding.
Table 5. Water Vapor Permeability
Sample Permeance
(kg / m2 · s · Pa) x10-10
Permeability
(kg / m · s · Pa) x10-12
C 1.32 2.64
C-NS 1.12 2.24
C-MK 1.18 2.36
C-O 1.34 2.68
C-O-NS 1.02 2.04
C-O-MK 1.04 2.08
O-E5 1.49 2.97
O-E10 1.26 2.52
O-S-E5 2.32 4.64
O-S-E10 1.62 3.24
O-NS-E5 1.10 2.20
O-NS-E10 0.71 1.43
O-NS-S-E5 1.07 2.14
O-NS-S-E10 0.94 1.90
O-MK-E5 1.01 2.02
O-MK-E10 1.25 2.51
O-MK-S-E5 1.41 2.82
O-MK-S-E10 1.39 2.77
Capítulo III
235
Water absorption is another important property for the assessment of the
applicability of the renders. As outside renders are usually exposed to environmental
phenomena – such as rain – or in contact with wet elements, high water absorption might
jeopardize the durability of the material. High rates of water absorption mean water
movement inside the building structure as well as an increased risk of efflorescence
phenomena and damages in the renders, stones and bedding mortars [67].
Fig. 21 shows the results of water absorption through capillarity of different
renders. Results evidenced that the adhesion enhancer admixture increased the capillary
coefficient with respect to all the set of control samples. Pore size distribution accounts
for this fact, since the population of the large capillary pores (at ca. 0.6-0.8 m) and the
diameter of these capillaries underwent an increase. Due to the same reasons, the use of
the viscosity enhancer produced an even sharper increase. Besides the changes in the pore
size distribution, the hydrophilicity of the functional groups of the admixtures may also
facilitate the water absorption. Some combinations with pozzolanic agents kept the
capillary coefficient values similar to that of the control renders (C, C-NS, C-MK, C-O-
NS and C-O-MK). Nevertheless, the real influence of these water transportation
properties on the durability of the renders will be assessed below.
Fig. 21. Water absorption through capillarity
0.000
0.010
0.020
0.030
0.040
0.050
0.060
0.070
Cap
illa
ry c
oef
icie
nt (
kg m
-2m
in-½
)
Capítulo III
236
3.6. Durability Experiments
Fig. 22 depicts both numerically and in a color scale the different damages
observed in the tested samples during continuous freezing-thawing F-T cycles. The total
decay of the specimen is displayed in red in the graph. Beyond this value, the specimen
was totally destroyed and thus no longer tested. Y-axis indicates the number of withstood
F-T cycles.
The control render (air lime without admixtures/additives) subjected to frost
resistance test underwent serious decay leading to the total destruction of the sample after
just 6 cycles (Fig. 22), in agreement with the poor frost resistance of pure air lime mortars
[17]. The addition of the sodium oleate, due to its water repellency, clearly enhanced the
F-T durability of the renders [10,24], in sample C-O as well as in the renders with
combination of multiple admixtures. Control renders with either MK or NS also increased
the frost resistance due to the densification of the matrix and to the discussed increase in
strength. It can be seen that the addition of EVA, starch derivative and pozzolanic agents
promoted the frost resistance of the renders in comparison with the plain lime sample (C)
or allowed the preservation of a good resistance (beyond 15 cycles) as compared to the
other control samples (C-NS, C-MK, C-O, C-O-NS and C-O-MK). However, samples
with the combination oleate, MK, EVA and S yielded moderate increase of resistance (up
to 9 F-T cycles) in comparison with C render, in line with their high water absorption
rates caused by the large population and diameter of large capillary pores.
Fig. 22. Alteration degrees of grouts after freeze-thaw cycles.
0
5
10
15
20
25
30
Fre
eze
-th
aw c
ycle
s
None Scarce Moderate Large Very Large Total
Capítulo III
237
With similar criteria, Fig. 23 presents the assessment of the resistance of the
renders in the face of MgSO4 sulphate attack. In control samples the addition of NS, oleate
or a combination of both admixtures increased the sulfate attack resistances in comparison
with plain lime render (C). However, the presence of MK resulted in a lower number of
cycles endurance (as observed in samples C-MK and C-O-MK). For samples with
multiple admixtures, the addition of NS, starch and EVA produced the highest tolerance
to sulfate attack compared with the control samples containing MK, being the samples O-
NS-S-E5, O-E10, O-NS-E10 and O-NS-S-E10 the ones which lasted 28 cycles of the test.
The poorer resistance of some of the MK-bearing samples (except O-MK-E5, in which
the positive effect of oleate and EVA increased its sulfate attack resistance) is due to the
chemical composition of the MK, which in reaction with the hydrated lime leads to the
formation of aluminate phases, C-S-A-H and C-A-H, besides the C-S-H phases as
common products of the pozzolanic reaction of both NS and MK [17,37].
Two main mechanisms explain the decay caused by the sulfate attack. On one
hand, sulfate ions react with portlandite and C-A-H, giving rise to the formation of
voluminous and thus expansive gypsum and ettringite. The expansion of these
crystallized compounds results in cracking and disruption of the hardened matrix [71].
On the other hand, the leaching of the calcium from the C-S-H phases leads to the loss of
the mechanical resistance of the paste. The literature has shown that the presence of Mg2+
ions (in case of attack by magnesium sulfate) cause decalcification of C-S-H (by ion
substitution), increasing the degree of alteration [72].
Therefore, the increase in aluminate compounds due to the MK addition favors
the formation of expansive salts, explaining the strongest damage observed, for example,
in the O-MK-S-E5 sample.
The higher MgSO4-attack resistance of the renders with EVA can be correlated
with the inhibition of the carbonation reported in Fig. 18. The presence of non-carbonated
portlandite prevents the formation of expansive sulfates, possibly due to the precipitation
of magnesium hydroxide (brucite) and avoiding the Ca-leaching of the C-S-H gel. As a
consequence, the lasting of the samples increased [71–73].
It must be noticed that the durability assays were carried out with pure render
samples. However, further works should be conducted to study the durability of the joint
stone-render systems.
Capítulo III
238
Fig. 23. Alteration degrees of grouts after sulfate attack cycles.
3.7. Overall assessment
To provide a comprehensive understanding of the performance of the different
renders and considering six of the most relevant characteristics measured in this study, a
comparison is presented (Fig. 24). For this assessment, a scale from 1 to 3 for each
parameter has been assigned, as described below:
• Adherence: according to Table 4, rate 1 was given to samples in which it was only
possible to quantitatively measure this value in just one type of employed
substrates; a rating of 2 was ascribed to samples in which this property could be
measured in two of employed substrates; and 3 to samples in which this property
was able to be measured in at least 3 of the substrates.
• Cracking: the rating was assigned by comparison with the control sample C,
establishing an average value with all substrates. The renders that showed more
cracking than that of the control sample were scored with 1; a score of 2 for the
samples that showed a similar behavior to the control sample and 3 for renders
showing a better visual appearance and less cracking in comparison with the
control render.
• Compressive strength: the results at 91 days were taken into account. It was
established a grade of 1 for samples with lower resistance than that of the control
sample C; 2 for samples with similar values of strength and 3 for the samples that
had a resistance greater than that of the control sample.
0
5
10
15
20
25
30M
agn
esi
um
Su
lph
ate
att
ack
cycl
es
None Scarce Moderate Large Very Large Total
Capítulo III
239
• Permeability: the assigned rating of 1 was attributed to samples that had lower
permeability than the control sample C, 2 to samples that had a value less or equal
to 1.5 times than the control sample, and 3 to the samples that had a value greater
than 1.5 times of the control sample.
• Frost resistance: A rating of 1 was set for samples that have endured less than 5
more cycles than the control sample C, 2 for samples that have endured 6 more
cycles than the control sample, but less than 16 cycles of this test, and a score of
3 for samples that have endured more than 16 cycles of this test.
• Durability to sulfate attack: The scale is the same as that applied for the resistance
to freeze-thaw cycles.
It is clearly observed that the compressive strength increased in all renders with
nanosilica. Furthermore, the use of the admixtures S and EVA improved adherence and
visual appearance by reducing cracking, except in samples such as O-NS-E5 and O-NS-
S-E5.
There was a clear improvement in resistance to freeze-thaw cycles and sulfate
attack, being optimal in the O-NS-S-E5 mixture. Samples O-MK-S-E5 and O-MK-S-E10
exhibited frost resistance similar to that of the control sample.
With regard to water vapor permeability, the renders without NS showed a higher
permeability. Inasmuch as the renders formulation includes oleate, starch and EVA, it
may be concluded that, whilst the NS addition sharply enhanced strength and durability,
the addition of MK enhanced adherence and cracking. These conclusions are useful to
design tailored renders, which may be prepared according to the specific requirements of
different repair works.
Capítulo III
240
Fig. 24. Overall assessment of the different renders
Capítulo III
241
4. CONCLUSIONS
New air lime-based renders were prepared combining different admixtures: a
water repellent agent (sodium oleate), an adhesion improver (ethylene-vinyl acetate
copolymer, EVA), a rheology modifier (modified starch), and pozzolanic mineral
admixtures (metakaolin or nanosilica) in order to enhance the performance of the renders
when applied on different substrates, particularly adhesion, cracking reduction and
durability.
The results showed that the nature and pore structure of the substrates exerted an
outstanding influence on the performance of the renders: very porous substrates (like
brick) favored the cracking of the renders, whilst the adherence was jeopardized when the
rendering mortars were applied onto substrates with very low porosity and smooth
surfaces (granite and limestone).
The addition of some of the combinations of the admixtures improved some
characteristics of the renderings with respect to a set of control samples including lime
with individual admixtures or combinations of pozzolanic admixture and sodium oleate.
The EVA addition, combined with oleate, MK and starch enhanced the adhesion
on most of substrates and minimized the cracking. For areas in which the renders are not
exposed to aggressive environmental conditions (indoor or protected walls), the use of
these combinations might be useful, for instance O-MK-S-E10.
If the environmental conditions after the application of the renders are expected
to be aggressive – freeze-thawing cycles and/or marine environment -, the presence of
nanosilica improves the mechanical strength and durability in combination with EVA,
oleate and starch (O-NS-S-E10). However, it must be borne in mind that the adhesion and
cracking were not so much enhanced in the combinations including NS studied in the
present work. The study of combinations of starch, EVA and sodium oleate with a lower
percentage of NS could be of interest for further works.
The use of the assayed combinations of admixtures has been proved to be useful
to obtain durable lime-based renders in which the adherence is enhanced and the cracking
formation reduced. The validity of some of these renders for different substrates favors
their application as repair materials of the Built Heritage and also for new works of civil
constructions.
Capítulo III
242
Author Contributions
J.F. González–Sánchez: Main contributor in Investigation, Data curation,
Conceptualization, Formal analysis, Writing– Original draft preparation, Writing–
Reviewing and Editing. J.M. Fernández: Writing– Reviewing and Editing. Í. Navarro–
Blasco: Methodology, Supervision, Visualization, Validation, Project administration. J.I.
Alvarez: Methodology, Supervision, Writing– Reviewing and Editing, Funding
acquisition
Funding
This study was funded by Spanish Ministry of Economy and Competitiveness
(MINECO), grant number MAT2015-70728-P. The first author thanks the Friends of the
University of Navarra, Inc., for a pre-doctoral grant.
Acknowledgments
The authors thank the technical support provided by Cristina Luzuriaga.
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Discusión general
Discusión general
253
Este trabajo de tesis doctoral pretendió analizar el posible efecto conjunto o
incluso sinérgico de las combinaciones más interesantes de dos o más aditivos en el
comportamiento de morteros de cal, con especial enfoque en la obtención de morteros de
restauración del Patrimonio Edificado.
Se han elaborado, en consonancia con los objetivos de la Memoria, tres gamas de
morteros:
• Gama 1: Morteros de inyección o de relleno (grouts), mediante uso de
combinaciones de cal aérea con aditivos puzolánicos, superplastificantes y
agente hidrofugante.
• Gama 2: Morteros con actividad fotocatalítica y propiedad de
autolimpieza mejorada (self–cleaning), mediante combinación de aditivos
fotocatalíticos con aditivos dispersantes o superplastificantes, adición
puzolánica y aditivo hidrofugante.
• Gama 3: Morteros de enlucido o monocapa (renders, one–coat mortars)
incluyendo aditivos para la mejora de adherencia del mortero (copolímero de
etileno–vinil–acetato) aplicado junto con puzolánicos, modificadores de la
consistencia (almidón) e hidrofugante.
Se analizan a continuación las funciones y características aportadas por cada
familia de aditivos en las diversas gamas de morteros.
1. Aditivos puzolánicos
1.1. Microsílice
Este aditivo se ha usado únicamente para la preparación de morteros de inyección.
La adición de microsílice ha mejorado las resistencias de esos morteros, y ha dado lugar
a un aumento de la resistencia de los morteros frente a ciclos hielo deshielo. Sin embargo,
a diferencia de los otros aditivos puzolánicos, la microsílice presentó una fuerte tendencia
a flocular, lo que generó aglomerados grandes y menos reactivos. En la prueba de
inyectabilidad sus partículas actuaron como barrera, debido a sus conocidas fuerzas
cohesivas, impidiendo la inyección de la lechada [1].
Discusión general
254
Figura 1. Tamaño de partícula de la microsílice y del metacaolín
El gran tamaño de partícula de la microsílice (Fig. 1) complicó la inyección y
dificultó el flujo a través de los finos huecos del relleno de partículas de travertino
empleado en las columnas. Debe tenerse en cuenta que en la prueba de inyectabilidad
realizada se ha empleado un relleno de pequeño tamaño de partícula, que generó espacios
libres de sección reducida, complicando la fluidez del mortero fresco. En aplicaciones
reales, cabría esperar un mejor comportamiento incluso en mezclas de menor
inyectabilidad como las generadas por la microsílice.
Comparativamente, la microsílice no fue tan eficaz para aumentar la resistencia a
la compresión como el metacaolín (MK), y este hecho se ha asociado a su menor actividad
puzolánica, consecuencia directa de su mayor tamaño de partícula. El análisis
termodiferencial y termogravimétrico (TG–DTA, en la Fig. 2) apunta a esta reducida
actividad puzolánica tras la evaluación de los porcentajes de Ca(OH)2 después de 182 y
365 días. Estos valores son mayores para las muestras que contienen microsílice (los
valores en promedio están por encima del 4%), mientras que las muestras con metacaolín
exhiben porcentajes por debajo de esa cifra, evidenciando un mayor consumo de
hidróxido de calcio durante la reacción puzolánica para este último aditivo puzolánico.
Figura 2. Porcentajes de portlandita (Ca(OH)2) de las diferentes muestras a 182 y 365
días de curado
Discusión general
255
1.2. Metacaolín
La incorporación de este conocido agente puzolánico perseguía los objetivos de
acortar tiempos de fraguado, mejorar la resistencia a la compresión e incrementar la
durabilidad de los morteros de cal. Se ha incorporado en los morteros de inyección y en
los morteros de adherencia mejorada.
Al emplear metacaolín (MK) en morteros de inyección en una cantidad del 20 %
se ha logrado mejorar con claridad la resistencia a la rotura por compresión y la resistencia
a los ciclos hielo deshielo. Como se puede observar en la Fig. 3 el MK ha incrementado
la resistencia 3.1 veces más en promedio, debido a la buena compatibilidad que existe
entre la cal aérea y este agente puzolánico formando fases C–S–H que causan un
fortalecimiento de la matriz conglomerante y una reducción del tamaño de poro medio y
de la porosidad total de la misma [2,3] (Fig. 4).
El uso combinado de este agente puzolánico con el superplastificante PCE en la
prueba de inyección ha permitido un flujo continuo a través de la columna con material
de relleno, configurando a este aditivo puzolánico como la mejor elección si se quiere
aumentar la resistencia de este tipo de morteros [3] combinada con una adecuada
inyectabilidad.
Debe tenerse en cuenta que en los morteros de inyección se ha incluido un 20%
en peso de cal de este aditivo y que, por su tamaño de partícula y por su reactividad,
aumenta la demanda de agua, pudiendo llegar a complicar la fluidez del mortero de
inyección. Esto obliga a ajustar adecuadamente las relaciones agua/cal y la dosis y
naturaleza química de los superplastificantes a emplear.
Figura 3. Efecto comparativo de la adición de metacaolín en la resistencia a la
compresión de morteros de cal a diferentes tiempos de curado
Discusión general
256
Figura 4. Comparativa de curvas porosimétricas a 365 días entre un mortero de cal vs
un mortero de cal con metacaolín
La interacción de este agente puzolánico con los demás componentes de las
mezclas se ha estudiado principalmente mediante pruebas de adsorción con los
superplastificantes, sin que se hayan constatado grandes cambios, en ausencia o presencia
del puzolánico. La Fig. 5 permite comprobar como, por ejemplo, la adsorción de sulfonato
de naftaleno o de lignosulfonato fue muy parecida en sistemas de cal con o sin metacaolín.
Fig. 5. Adsorción del LS y del PNS en un sistema puro de cal y un sistema cal–MK
Discusión general
257
En morteros con metacaolín, la reducción del tamaño medio de los poros impidió
la absorción de agua líquida, bloqueando su posterior congelación y daño por expansión
y proporcionando así una mejor resistencia a los ciclos de hielo–deshielo. La mejora de
la durabilidad fue más marcada en muestras de MK con algún superplastificante, producto
del refinamiento de la estructura porosa. Sin embargo, la resistencia a ataque por
cristalización de sulfato de magnesio empeoró debido a la composición química del MK
(con presencia de aluminio) y su formación de C–A–H y de C–S–H. Por un lado, en
presencia de iones sulfato, la portlandita y las fases de aluminatos de calcio hidratados
dan lugar a la formación de fases voluminosas y expansivas de yeso y ettringita. Por otro,
se ha reportado en la literatura que los iones Mg2+ provocan la descalcificación de fases
C–S–H, aumentando el grado de alteración [4].
En morteros de adherencia mejorada, al usar metacaolín también aumentó la
demanda de agua, como en los de inyección. Al usar metacaolín en mezcla con el
copolímero de etileno–vinil–acetato, sin importar la dosis de este último, existió una
interferencia en la formación continua de las fases C–S–H lo que ocasionó una
disminución de la resistencia a compresión de las muestras [5]. Sin embargo, el
metacaolín no interfirió con la adherencia al ser combinado con los demás elementos,
sugiriendo su compatibilidad para este tipo de morteros.
1.3. Nanosílice
Este agente puzolánico se ha utilizado en la preparación de morteros de las gamas
2 y 3 a una dosis del 20 %. La selección de esta cantidad se fundamentó en los buenos
resultados advertidos en trabajos anteriores, con muy buenos resultados en cuanto a la
mejora de las características de un mortero de cal, y cambios relevantes en la distribución
de los mesoporos [6–8]. La presencia de NS se ha descrito en la bibliografía como
impulsora del desarrollo de fases C–S–H, dando lugar a una población enriquecida de
poros gel (<10 nm), en el rango microporoso. Se ha señalado una mejora notable de la
resistencia a la compresión de los morteros de cal aérea [6,7,9].
En este trabajo, en los morteros autolimpiantes, se encontró que el efecto de la
adición del aditivo puzolánico depende de la composición del mortero. Para morteros sin
superplastificantes, la adición de NS mejoró la eliminación de NO. El refinamiento de la
estructura de poros explica este hallazgo, relacionado con la prevalencia de poros
capilares entre 10 y 100 nm para muestras con NS (NS–T y O–NS–T) (Fig. 6).
Discusión general
258
Figura 6. Distribución de tamaño de poro de diferentes muestras morteros de cal con
actividad fotocatalítica (muestras sin superplastificante)
Estos poros se han descrito como muy relevantes para la actividad fotocatalítica
en morteros de cemento con TiO2 [10]. Esto se pudo confirmar en este trabajo, ya que la
muestra O–T sin NS mostró porosidad insignificante en ese rango de poros, produciendo
un 28% de remoción de NO. Las muestras con NS (NS–T y O–NS–T), con un notable
aumento de poros capilares en ese rango de poros, incrementaron la degradación del NO
hasta cifras del 34 al 37% (Fig. 7).
En estos morteros la presencia de NS promovió la adsorción y la actividad de los
superplastificantes. Al medirse el potencial zeta en el sistema complejo de Cal–Oleato–
NS–Superplastificante, se detectó que ,en presencia de la nanosílice y por un fenómeno
de sobrecarga en el entorno rico en cationes de Ca2+ , se alcanzaron valores fuertemente
positivos de potencial zeta. Esto favoreció la adsorción de los superplastificantes, cuyos
grupos funcionales al pH alcalino del mortero están ionizados con carga negativa.
Figura 7. Abatimiento de NO y NOx de muestras con NS sin superplastificantes
(morteros fotocatalíticos)
Discusión general
259
En la prueba de hidrofilicidad fotoinducida, al agregar la nanosílice disminuyó el
valor de ángulo de contacto, por tanto, aumentando la humectabilidad de la superficie:
por ejemplo, en la muestra O–NS–T, la irradiación continua durante 30 minutos provocó
una caída moderada del ángulo de contacto de 51º a 37º (es decir, un 27% de reducción
del ángulo): este hecho se relacionó con el efecto de dilución de la nanosílice (20% en
peso respecto a la cal) sobre el agente hidrofugante (oleato sódico) (Fig. 8). Para la
actividad de autolimpieza, el mantenimiento de hidrofilicidad fotoinducida favorece la
eficacia de los morteros al permitir la creación de una capa acuosa de arrastre de suciedad
sobre la superficie de los morteros.
En enlucidos y monocapas de adherencia mejorada, la resistencia a la compresión
aumentó en todos los morteros preparados con nanosílice, sin importar la dosis de aditivo
para mejorar la adherencia (Fig. 9). La adición de nanosílice a la mezcla aumentó la
demanda de agua para alcanzar el valor de flujo establecido incluso más que el
metacaolín. Esta modificación era previsible por la mayor área superficial de NS (500
m2g–1) y su reactividad [11,12]. La mejora en la resistencia mecánica, parámetro de
interés relativo para monocapas, sucedió de forma paralela al aumento de viscosidad de
los monocapas, lo que dificultó la penetración del revoco fresco en poros de reducido
tamaño de los sustratos utilizados, provocando un contacto defectuoso y por tanto una
mala adherencia. La excepción se observó en el ladrillo empleado como sustrato, ya que
su superficie estriada permitió una mejora en la adherencia (Fig. 10).
Figura 8. Ángulo de contacto durante la prueba hidrofilicidad fotoinducida de muestras
sin superplastificante, con y sin nanosílice
Discusión general
260
Figura 9.Resultados de la resistencia a la compresión de monocapas, con dosis de
0.50% y 1.00% de aditivo (E) para mejorar la adherencia, a 28 y 91 días
Figura10. Adherencia del mortero con NS (muestra O–NS–E), sobre dos sustratos:
a) arenisco y b)ladrillo
Discusión general
261
La adición de NS, almidón y EVA ha permitido obtener monocapas con notable
resistencia al ataque por sulfatos y a los ciclos hielo deshielo en comparación con las
muestras que contienen MK, siendo las muestras O–NS–S–E5, O–E10, O–NS–E10 y O–
NS–S–E10 las de mayor durabilidad (con el total de 28 ciclos de la prueba) (Fig. 11 y
12).
Figura 11. Resistencia al ataque de sulfatos de monocapas con adherencia mejorada
Figura 12. Resistencia a los ciclos hielo deshielo de monocapas con adherencia
mejorada
Discusión general
262
2. Superplastificantes
2.1. Lignosulfonato
En morteros de inyección con lignosulfonato se incrementó el aire incorporado
durante el proceso de mezcla como resultado de sus características tensioactivas. Este
aditivo ralentizó el tiempo de fraguado de morteros de cal por la fácil complejación de
iones Ca2+ y la consiguiente interferencia con la carbonatación [13,14]. El anclaje del
polímero sobre las partículas activas se realiza mediante una interacción electrostática
favorable de acuerdo al modelo de doble capa: en los sistemas de cal el apantallamiento
fuerte por iones calcio positivos explica que, cuanto mayor carga aniónica, más intensa
es la adsorción del polímero, en este caso el LS.
Sin embargo, la formación de complejos LS–Ca2+ impidió que algunas moléculas
de LS se adsorbieran sobre algunas de las partículas presentes en los sistemas en fresco:
portlandita, C–S–H, C–S–A–H o C–A–H. Por ello este aditivo tuvo menor eficacia como
agente dispersante en comparación con otros aditivos utilizados.
La Fig. 13 representa la resistencia a compresión de los morteros de inyección con
diversos tipos de superplastificantes. En el caso del LS la dosis de 0.5% presentó un ligero
incremento respecto a la muestra control.
Figura 13. Resistencia a la compresión de los diferentes muestras que contienen
metacaolín y los diferentes superplastificantes
Discusión general
263
Figura 14. Distribución de tamaño de poro de las muestras de cal–metacaolín con 0.5%
y 1.0% de LS y PNS
En cuanto al efecto en la durabilidad, este aditivo, por su efecto tensioactivo,
generó un incremento en el tamaño de poro (Fig. 14), en comparación con el PNS. La
tasa de absorción de agua se incrementó y por ello los morteros con LS en la prueba de
durabilidad solo soportaron 12 ciclos.
2.2. Éteres de policarboxilato
En morteros de inyección se obtuvo un incremento de la resistencia mecánica
atribuido al refinamiento de la estructura de los poros provocado por este
superplastificante (PCE–1). La adición de PCE provocó una fuerte caída en el número de
poros, de aproximadamente 1 µm. Además, el tamaño del poro principal se desplazó hacia
diámetros inferiores (entre 0.5 y 0.8 μm) como se puede observar en la Fig. 15.
Figura 15. Distribución de tamaño poros de muestras con y sin PCE de los morteros de
inyección
Discusión general
264
Figura 16. Valores de fluidez (diámetro de dispersión medido en la mesa de sacudidas)
de los diferentes grouts
La adición de PCE dio como resultado un fuerte incremento de fluidez, con
valores de diámetro de dispersión superiores a 300 mm (mesa de sacudidas),
independientemente de la composición de la mezcla y de si se incluía en la mezcla un
agente puzolánico (Fig. 16). Por lo tanto, se confirmó la destacada eficacia de los
derivados de éter de policarboxilato tanto en sistemas a base de cal como de cemento
[8,9,15], siendo la mejor alternativa de los cuatro aditivos probados para incrementar el
valor de esta propiedad. La arquitectura molecular ramificada de este polímero, junto con
la reconocida actividad de las moléculas del mismo no adsorbidas, mejoró claramente la
inyectabilidad. La eficacia de la acción dispersante del PCE en sistemas a base de cal se
ha observado en trabajos anteriores [8,9,16,17] y se confirmó en esta investigación. Por
tanto, las mezclas con este aditivo presentaron los mejores valores inyectabilidad sin
importar si estaba mezclado con aditivo. La mezcla O–MK–PCE mostró la mejor
inyectabilidad con un valor de 0.08 s−1, que es mayor que los resultados reportados por
otros autores [18–20].
En el estudio de potencial zeta de este superplastificante se comprobó que el PCE–
1 no modificó dramáticamente la carga superficial de las partículas, lo que confirma la
débil influencia de la carga aniónica de este superplastificante, apuntando a que el efecto
predominante fue el impedimento estérico de las cadenas laterales de este polímero que
resulta más eficaz que las repulsiones electrostáticas de los grupos carboxilatos cargados
negativamente [21,22]. La hidro–repelencia no se vio afectada debido a los valores de
Discusión general
265
potencial zeta que muestran poca interacción y mayor compatibilidad con el oleato
sódico.
En el diseño de morteros autolimpiantes y fotocatalíticamente activos se han
utilizado 4 tipos de éteres de policarboxilato: PCE–1, 23APEG, 45PC6 y 52IPEG. En la
prueba de potencial zeta se demostró que todos ellos ayudan a la dispersión de las
partículas de TiO2, no por su carga electrostática sino, fundamentalmente, por su efecto
estérico. Se advirtió este mecanismo en la no negativización de valores de potencial de
superficie en comparación con la curva resultante de la adición de PNS (Fig. 17).
Los morteros con esta familia de aditivos dieron como resultado valores promedio
de degradación de NO entre 43% y 50%, mientras que el porcentaje promedio para PNS
fue un 37%. El 45PC6 degradó en promedio el 50% del NO, lo que resultó ser la tasa más
alta de eliminación de NO. El PCE–1 y el 23APEG exhibieron los mismos porcentajes
de abatimiento de NO (valores promedio de 45%), mientras que el porcentaje para
52IPEG fue levemente menor (43%) (Fig. 18). El fundamento de esta mejora se
circunscribe a la mejor dispersión del TiO2 y a la disminución de la velocidad de
recombinación de los pares hueco positivo–electrón.
Figura 17. Potencial zeta de las diferentes superplastificantes en los morteros
autolimpiantes en el sistema Cal–Nanosílice–Oleato–TiO2–Superplastificante
Discusión general
266
Figura 18. Abatimiento de NO y NOx promedio de los diferentes superplastificantes
Los morteros con PCE–1, a pesar de arrojar resultados aceptables de abatimiento
de NO, no fueron muy eficaces desde el punto de vista de la autolimpieza (degradación
de depósito de tinta de rodamina B). La cinética de degradación de estas muestras fue la
más lenta (en promedio, solo el 15% de degradación durante los primeros 20 minutos de
irradiación). Los otros tipos de policarboxilatos, empero, fueron mucho mejores en el
desarrollo de esta propiedad. Se puede atribuir a que el PCE interfiere en el mecanismo
de fotosensibilización para la autolimpieza (dependiente de luz visible) utilizando solo el
mecanismo de degradación fotocatalítica (UV), más lento.
Además, la presencia de superplastificantes en los morteros indujo cambios
notables en los valores del ángulo de contacto en la prueba de hidrofilicidad fotoinducida
debido a la dispersión de los sitios activos de TiO2. Se obtuvieron valores inferiores a 10º
para algunas muestras después de 30 min de irradiación. Los valores más bajos se
alcanzaron en morteros con 45PC6 (9º) y 52IPEG (8º). En promedio, los porcentajes de
reducción de ángulo de contacto para las muestras antes y después de 30 minutos de
irradiación de luz UV–Vis fueron de aprox. 44% para PCE–1 y para el 23APEG, 52%
para 45PC6 y 64% para 52IPEG.
2.3. Sulfonato de naftaleno
Con carácter previo al diseño de los morteros de inyección, este aditivo fue
probado primero en conjunto con varias dosis de metacaolín, mostrando mejores
propiedades que el lignosulfonato, tanto en resistencia a la compresión como durabilidad
y compatibilidad con el metacaolín. Al obtener esos resultados se optó por incluirlo en la
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mezclas de cal–oleato–MK/NS–superplastificante, donde su inclusión también mostró
mejoras en el mortero de cal. Cabe mencionar que el oleato sódico jugo un papel
importante en la adsorción de los superplastificantes, ya que por la carga electrostática
que contiene compite por la adsorción sobre las partículas de cal con los
superplastificantes que actúan por repulsiones electrostáticas. Esto implicó una limitación
de la actividad del aditivo PNS, de acción esencialmente electrostática. La adsorción de
este superplastificante no varió cuando se añadió el oleato sódico (Fig. 19). La adsorción
provocó una disminución del valor del potencial zeta, resultando una inversión de carga
en valores negativos, lo que prueba que su mecanismo de acción está ligado a las
repulsiones electrostáticas, particularmente en condiciones alcalinas que favorecieron la
ionización de los grupos sulfónicos [14,21].
En la prueba de inyectabilidad el PNS dio resultados deficientes, debido al
fenómeno anterior y a la demanda de agua por parte de los agentes puzolánicos. En el
corte transversal de las columnas de travertino en donde se trató de hacer fluir al mortero
con PNS se pudo observar que el grout preparado con este aditivo no fue capaz de llenar
por completo los huecos y existieron áreas heterogéneas causadas por la limitada
inyectabilidad y la mala adherencia a las partículas de travertino.
En la prueba de resistencia al hielo–deshielo este aditivo mantuvo una buen
comportamiento siempre y cuando estuviera presente simultáneamente el metacaolín. El
papel positivo de ambos aditivos se contrastó por el hecho de que no hubo gran variación
al agregar oleato al sistema, ya que el comportamiento fue similar con y sin oleato,
soportando 18 y 19 ciclos respectivamente.
Figura 19. Adsorción de los superplastificantes en las diferentes muestras
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En morteros autolimpiantes, por la naturaleza de este aditivo PNS y su mecanismo
de acción por repulsión electrostática [6,23], fue fuertemente adsorbido por las partículas
de TiO2. En los ensayos de potencial zeta se comprobó que el patrón de la curva es
totalmente diferente al de los éteres de policarboxilato (Fig 17).
Los morteros con PNS también produjeron una degradación de rodamina B en el
rango del 60–70%. Las cinéticas fueron ligeramente más lentas que en morteros con
PCEs: después de 20 minutos de irradiación, se degradó entre un 15% y un 30% de la
tinta, valor inferior al de morteros con PCEs, más eficaces en la dispersión estérica de
TiO2 en matrices de cal.
2.4. Condensado de melamina–formaldehído sulfonato (SMF)
Este superplastificante tiene un mecanismo de acción análogo al del PNS, por lo
que la adsorción es similar como se puede observar en la Fig. 18 y de igual forma el valor
del potencial zeta comienza a negativizarse cuando es añadido al sistema (Fig. 20).
Figura 20. Valores de potencial zeta de los sistemas: a) cal + oleato, b) cal + oleato +
MS y c) cal + oleato + MK titulados con SMF y PNS al 1%
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Figura 21. Diferencias de porosimetría entre muestras con PNS y SMF
También el comportamiento del mortero es similar al de los morteros con PNS,
aunque la estructura porosa de las diferentes mezclas presentó una reducción del tamaño
de poro (Fig.21), lo que explica que durante las pruebas de durabilidad las mezclas con
este aditivo presentaran mejor resistencia a los ciclos hielo–deshielo.
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3. Aditivo hidrofugante: oleato de sodio
En ensayos previos de adsorción se comprobó que la adsorción de oleato sobre
partículas de agente puzolánico es mínima. En contraste, en morteros de inyección en
estado fresco, se evaluó cómo la adsorción de las partículas de oleato se lleva a cabo
mediada por la influencia de los iones calcio Ca2+, logrando de esta forma una saturación
sobre las partículas de cal creando una primera capa adsorbida [24]. Debido a este
fenómeno, la adsorción de los superplastificantes se verificó en una segunda capa, lo que
disminuyó la eficacia de aquéllos que actúan con repulsión electrostática como el SMF y
el PNS, con valores bajos de inyectabilidad para estos dos superplastificantes. Sin
embargo, el oleato presentó mayor compatibilidad con el PCE, debido a que la dispersión
que éste provoca se debe a su efecto estérico y a su menor tasa de adsorción.
La principal característica debida a este aditivo y prevista en estas formulaciones
fue la hidro–repelencia. Esta fue provocada por el carácter tensioactivo del oleato de
sodio. Durante el proceso de mezclado en una dispersión acuosa, la parte hidrofóbica (no
polar) de la molécula se orienta hacia la fase aérea, mientras que el segmento polar está
en el sistema acuoso. El efecto combinado de la distribución del tamaño de los poros (con
poros pequeños) y el agente hidrófugo activo (repartido a lo largo de la superficie de
contacto del material) condujo a una hidro–repelencia adecuada. La muestra O–MK–
PCE1 presentó la mejor repelencia al agua, gracias a su baja porosidad total y a la mayor
disponibilidad de moléculas del agente hidrofugante (incluso asumiendo que la mayoría
de las moléculas de oleato se adsorberán en partículas de cal), al no presentar tanto
superplastificante adsorbido.
En cuanto a la resistencia a los ciclos hielo–deshielo, la adición de oleato de sodio
mejoró rotundamente la durabilidad: como ejemplos, la muestra Cal–Oleato soportó 18
ciclos mientras que un mortero de cal sin este aditivo soportó solo un ciclo.
En morteros autolimpiantes la hidrofobicidad al agregar este aditivo fue evidente,
ya que en las mezclas que no lo contienen fue imposible determinar el ángulo de contacto
por la instantánea absorción de las gotas de agua.
En estos morteros, aunque se agregó este agente hidrofugante a la mezcla, la
humectabilidad de las muestras sometidas a iluminación (hidrofilicidad fotoinducida) no
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se vio afectada, por lo que este aditivo no interfiere en el proceso de autolimpieza pero
mejora la durabilidad de esta gama de morteros.
Debido a la inclusión del oleato sódico en revocos de adherencia mejorada, se
mejoró la durabilidad sin afectar a los valores de permeabilidad al vapor de agua, ni a la
adherencia o a la formación de fisuras que tuvo el mortero al aplicarse sobre los diferentes
sustratos, lo que demuestra su compatibilidad.
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4. Fotocatalizador: TiO2
Este aditivo fue agregado en la Gama 2 con el fin de aprovechar las propiedades
fotocatalíticas del TiO2 que ayudan a la autolimpieza de los morteros, así como la
eliminación de bacterias o microalgas que se pueden formar en la superficie de las
edificaciones.
4.1. Estudio biocida
Durante el desarrollo de este trabajo se trató de establecer la metodología para
conocer el efecto biocida del TiO2 en masa. Los ensayos presentaron serias
complicaciones y sólo se lograron resultados cualitativos que han carecido de
repetibilidad. Sólo se constataba cierto crecimiento de Pseudomonas fluorescens en la
muestra control y disminuían (menos unidades formadoras de colonias, UFC) −sin
resultados cuantificables− las colonias en donde se encontraba otro tipo de aditivo,
incluso si no era TiO2 (nanosílice y oleato).
Tabla 1. Crecimiento de Pseudomonas fluorescens en las diferentes muestras de mortero
Muestra Fotografía de las colonias formadas siguiendo el
diseño experimental expuesto Observaciones
Cal
Aparecen colonias
Cal–Oleato
La disminución de
colonias es drástica
comparadas con las
del mortero puro de
cal
Cal–
Oleato–NS
Se observa un
efecto similar a la
muestra sin
nanosílice
Cal–
Oleato–
NS–TiO2
El comportamiento
fue el mismo en
todas las muestras
con TiO2: el
crecimiento de
Pseudomonas
fluorescens se
inhibió por
completo
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273
Se ha postulado que, posiblemente, la hidrofobicidad de la superficie dificultara
la colonización microbiológica. En las muestras que contenían TiO2 −sin importar el uso
superplastificante− no se observó aparición de ninguna colonia bacteriana, por lo que
tampoco fue posible analizar el efecto dispersante en la mejora biocida: el
comportamiento fue el mismo en todas las muestras obtenidas (Tabla 1). Además, el pH
fuertemente alcalino del mortero de cal genera condiciones agresivas para el crecimiento
de las bacterias (naturaleza biocida de la cal). Se sugiere el uso en futuros estudios de
algún otro microorganismo como algas u hongos, menos sensibles al valor de pH del
medio de crecimiento [25].
4.2. Abatimiento de NO y autolimpieza
Este estudio ha demostrado la compatibilidad de los diversos superplastificantes
con el aditivo fotocatalítico, incrementando −como se ha mencionado− en un 33% la
degradación de NO. El TiO2 presenta mayor y mejor compatibilidad con los
superplastificantes policarboxilados que con el PNS.
La capacidad de autolimpieza de los morteros también fue mejorada
evidentemente con la adición de superplastificantes que permitieron una mejor
distribución del TiO2. El estudio de la degradación del colorante rodamina B depositado
sobre la superficie de los morteros mostró aprox. un 70% de decoloración después de 310
minutos de irradiación UV–Vis con respecto a los morteros sin TiO2.
En resumen, el TiO2 ha mostrado buenos resultados tanto en abatimiento como en
autolimpieza demostrando que es compatible en la matriz de cal con superplastificantes
e hidrofugante, con un pH alcalino y la presencia de iones alcalinotérreos.
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5. Modificador de la viscosidad: almidón de patata modificado
El efecto del almidón modificado sobre la trabajabilidad de las muestras de
revocos de adherencia mejorada fue pronunciado, debido a la acción de retención de agua
de las moléculas del derivado del almidón y al aumento de la viscosidad de la fase líquida
[26]. Esto puede atribuirse al elevado número de grupos funcionales –OH como se
observa en su espectro FTIR (Fig. 22).
Estos grupos mejoran la retención de agua y el fenómeno de reticulación (y por
tanto la viscosidad) entre las cadenas de polímero mediante enlaces de hidrógeno. Al
retrasarse el secado de los revoques, se dificultó el acceso de CO2 a la parte interior de
los mismos, lo que provocó un tiempo de endurecimiento prolongado. Es importante
recalcar que la capacidad de retención de agua de los morteros influye en el secado. Una
mayor retención de agua conduce a tasas de secado más bajas, tanto por evaporación
como por succión del sustrato. Este efecto promueve naturalmente una mejor hidratación,
en el caso de los ligantes de cal con fase hidráulica, y también puede producir buenas
condiciones para la carbonatación, asegurando un contenido de humedad suficiente,
aunque moderado, durante un largo período, incluso en condiciones externas de baja
humedad relativa [27,28].
Figura 22. Espectro FTIR obtenido para el almidón (Casaplast KO09)
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275
Figura 23. Comparación entre dos muestras: a) Muestra sin almidón (O–E5) y muestra
con almidón (O–S–E5)
Sin embargo, el aumento de viscosidad presentado por las muestras que contienen
a este aditivo dificulta la penetración del revoco fresco en poros de reducido tamaño,
provocando defectos de contacto y, por tanto, mala adherencia (Fig. 23).
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276
6. Modificador de la adherencia: copolímero de etileno– acetato
de vinilo (EVA)
Al añadir el aditivo EVA al mortero se midió un incremento en su fluidez,
comparado con las muestras control. Esto se puede explicar debido a que este aditivo
genera el efecto ”ball bearing” que facilita la fluidez de la mezcla, y al efecto simultáneo
dispersante de los compuestos tensoactivos utilizados en la formulación del aditivo.
Se ha indicado que la adición de EVA al hormigón y al mortero aumenta la
resistencia a la flexión porque los grupos activos en sus moléculas reaccionan con los
cationes de los productos de hidratación del cemento y mejoran la estructura física del
mortero [29–31]. Sin embargo, cuando se agrega metacaolín al sistema existe una
formación continua de fases C–S–H donde, de acuerdo con la literatura, el EVA interfiere
[5], causando que las resistencias a compresión sean menores. Además, con el análisis
termogravimétrico, se observó una disminución en la carbonatación del mortero,
confirmándose la interferencia de este aditivo. A diferencia de otros aditivos de carácter
iónico, esta interferencia no puede atribuirse a la complejación de los cationes de calcio
que aporta la cal [23,26], sino que se debe a los cambios en la porosidad y la retención de
agua, por parte del EVA y del almidón modificado que modificaron el acceso del CO2 y
la disponibilidad de agua dentro de los poros requerida para la carbonatación [5,26,32].
En cementos y hormigones, el EVA también mejoró la adhesión entre los
agregados y la matriz del material cementoso, redujo el módulo de elasticidad del
hormigón y mejoró su capacidad para absorber tensiones en condiciones de temperatura
variable [33–35]. En los revocos de cal preparados en este trabajo, el uso de EVA aumentó
la fuerza de unión después de la aplicación sobre arenisca, granito y ladrillo. Esta mezcla
fue incluso más eficaz en combinación con MK, aunque en este caso el EVA tuvo una
dosis del 5%, siendo la mezcla O–MK–E5 la que presentó mejor adherencia sobre ladrillo
y granito. La presencia del almidón modificado empeoró ligeramente los valores de
resistencia de la unión, por las razones antes mencionadas de interferencia con la
carbonatación. Sin embargo, la combinación de MK, oleato, EVA (5%) y almidón puede
seleccionarse como la más adecuada considerando la atenuación del agrietamiento y la
buena adherencia.
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7. Resumen de resultados y recomendaciones de
combinaciones de aditivos
7.1. Morteros de inyección
Los resultados mostraron que el superplastificante PCE ha mejorado la
inyectabilidad y la fluidez más que SMF y PNS. Este estudio confirmó que el mecanismo
de acción estérico funciona mejor que el mecanismo por repulsión electrostática: la
arquitectura molecular de estos polímeros fue fundamental para explicar su desempeño.
En cuanto a los aditivos puzolánicos, el metacaolín impartió mejores
características en las muestras que la microsílice, particularmente cuando se combinó con
algún superplastificante, proporcionando mayor inyectabilidad, mejor adherencia y
envoltura a las partículas de travertino durante la inyección, así como mayores
resistencias mecánicas, debido a la reacción puzolánica del metacaolín. La durabilidad,
frente a los ciclos de hielo–deshielo, también se incrementó notablemente debido a la
presencia de MK debido a la disminución del tamaño medio de los poros. Como se ha
mencionado debido a que la microsílice mostró una marcada tendencia a aglomerarse en
dispersiones acuosas, su empleo como agente puzolánico limitó notablemente la
inyectabilidad de las muestras. Además, al utilizar la microsílice se observaron bajas
resistencias mecánicas y poca durabilidad frente los ciclos hielo–deshielo.
La inclusión en las mezclas del oleato de sodio provocó una reducción en la
eficacia de todos los superplastificantes, más acusada en los superplastificantes cuya
acción se fundamenta en repulsiones electrostáticas (SMF y PNS). Al mismo tiempo, la
gran adsorción de SMF y PNS en la capa de oleato redujo la hidro–repelencia de las
lechadas tratadas. Por tanto, se recomienda el uso de PCE que imparte máxima
inyectabilidad e hidro–repelencia.
La mezcla que tuvo mejores características, según los resultados, fue la mezcla de
cal, metacaolín, oleato de sodio y PCE (dosis de 1% en peso).
7.2. Morteros autolimpiantes
Los resultados han demostrado que la adición de superplastificantes aumentó
claramente la acción descontaminante de los morteros de cal (degradación de NO)
respecto a los morteros libres de superplastificantes. En las mezclas en las que se
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utilizaron como superplastificantes éteres de policarboxilato la estructura porosa se vio
afectada, generando macroporos mayores a 1 m (en especial al emplear 45PC6) y a su
vez este fenómeno ha beneficiado la actividad fotocatalítica y por tanto se ha obtenido el
mejor abatimiento.
El efecto positivo de los superplastificantes, las características de la matriz de cal,
el pH alcalino que proporcionan y los iones alcalinotérreos obstaculizan la formación de
NO2 como producto intermedio muy tóxico, proporcionando excelentes valores de
selectividad.
La capacidad de autolimpieza de los morteros también se mejoró mediante la
adición de los superplastificantes. Se observó que el bajo rendimiento de PCE con
respecto a la actividad de autolimpieza está relacionado con la interferencia con el
mecanismo de fotosensibilización de la degradación del colorante.
También se demostró que la presencia de superplastificantes potencia la
hidrofilicidad fotoinducida, mecanismo que favorece la acción autolimpiante. Todas las
muestras que contuvieron superplastificante, excepto las de PCE, favorecieron valores de
ángulo de contacto bajos (aprox. 10º) durante su irradiación.
7.3. Morteros de adherencia mejorada
Al combinar la cal aérea con diferentes aditivos: minerales puzolánicos
(nanosílice o metacaolín), un agente repelente al agua (oleato de sodio), un mejorador de
la adherencia (copolímero de etileno–vinil–acetato, EVA) y un modificador de reología
(almidón modificado) se obtuvieron dos resultados principales: en primera instancia las
mezclas con nanosílice, EVA, oleato y almidón, aumentaron la resistencia a la
compresión y la durabilidad siendo óptima la mezcla cal–oleato–nanosilice–almidón y
EVA al 5% aunque la adhesión y el agrietamiento no presentaron mejora. Y el segundo
hallazgo importante fue que la adición de MK con los demás aditivos mejoró la
adherencia en la mayoría de los sustratos y minimizó el agrietamiento. Desde el punto de
vista de la mejora de la durabilidad, estos revoques no fueron tan efectivos. Son adecuadas
las mezclas cal–oleato–metacaolín–almidón y EVA al 5% y al 10%.
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Conclusiones
Conclusiones
287
La evaluación de los resultados obtenidos y su posterior discusión en este trabajo de
investigación ha permitido extraer las siguientes conclusiones:
1. Se han preparado tres gamas de morteros de cal aérea con propiedades mejoradas
enfocados a la restauración de edificaciones del Patrimonio Cultural, mediante la
incorporación combinada de diferentes aditivos.
2. En la primera gama se diseñaron mezclas cuaternarias de cal aérea, superplastificantes
poliméricos de distintos tipos, un agente hidrofugante y aditivos puzolánicos,
obteniéndose morteros de inyección de elevada resistencia y durabilidad para ser
utilizados como materiales de reparación para Patrimonio Edificado.
2.1. Los resultados de esta gama mostraron que el éter de policarboxilato, PCE, fue
mucho más eficaz para aumentar tanto la inyectabilidad como la fluidez de las
lechadas que los polímeros condensados de sulfonato de melamina–
formaldehido (SMF) y sulfonato de naftaleno–formaldehido (PNS).
2.2. Se confirmó que, en estas mezclas, el mecanismo de acción de este
superplastificante polimérico PCE es principalmente estérico, mientras que SMF
y PNS actúan a través de un mecanismo de repulsión electrostática.
2.3. La adsorción de oleato de sodio, añadido como hidrofugante, sobre las partículas
de cal fue evidente y provocó una reducción en la eficacia superplastificante de
los aditivos, particularmente de SMF y PNS.
2.4. La gran adsorción de SMFC y PNS sobre capas pre–adsorbidas de oleato en
partículas de cal redujo la hidro–repelencia de los morteros, como lo confirma
el ángulo de contacto estático del agua.
2.5. El PCE se adsorbió bastante menos y dio lugar a mejoras claras en la
inyectabilidad, que alcanzó el máximo valor, e hidro–repelencia.
2.6. Se estudió la compatibilidad del LS y del PNS, aditivos de acción repulsiva
fundamentalmente electrostática, mezclados con MK. Se observó que el PNS, al
tener una mayor carga aniónica y una disposición molecular lineal, se adsorbió
en mayor cantidad que el LS. Además, la presencia de PNS favoreció la reacción
puzolánica, dando como resultado resistencias mecánicas más altas con un valor
de 4.8 MPa después de 182 días en muestras con 20% MK y 0.5% PNS.
Conclusiones
288
2.7. En durabilidad frente al ataque por cristalización de sulfatos, se observó que los
morteros con PNS, con mayor formación de C–S–H, C–S–A–H y C–A–H, se
alteraron por descalcificación de las fases hidratadas y por formación de fases
expansivas como hexahidrita, yeso y ettringita. En cambio, la menor reactividad
puzolánica en presencia de LS permitió que los morteros mantuvieran una
cantidad significativa de Ca(OH)2 que proporcionó mejor resistencia de las
mezclas al ataque por sulfatos.
2.8. En cuanto a los aditivos puzolánicos, el metacaolín imprimió mejores
características que la microsílice, particularmente en combinación con SP
obteniendo: mayor inyectabilidad, mejor adherencia y envoltura de las partículas
durante la inyección, así como mayores resistencias mecánicas.
2.9. La durabilidad frente a los ciclos de hielo–deshielo también se incrementó
notablemente debido a la presencia de MK.
2.10. La microsílice mostró una marcada tendencia a aglomerarse en dispersiones
acuosas, lo que perjudicó notablemente la inyectabilidad de las mezclas
preparadas con este aditivo puzolánico.
2.11. Según los resultados, la mezcla compuesta de cal, metacaolín, oleato de sodio
y PCE (éste al 1% en peso), resultó ser la composición más efectiva, mejorando
la resistencia mecánica, la inyectabilidad y la hidrofobicidad.
3. Se diseñó una gama de morteros de cal aérea con capacidad fotocatalítica y
autolimpiante, mediante el uso de un aditivo fotocatalizador nanoestructurado (TiO2)
y la incorporación de agentes dispersantes (superplastificantes). Se mejoraron las
prestaciones mecánicas y la durabilidad de estos morteros a través de una adición
puzolánica (nanosílice) y un agente hidrofugante (oleato sódico) que redujo la
penetración de agua.
3.1. La adición de superplastificantes mejoró en un 33% de media la acción
descontaminante de los morteros de cal, con cifras de degradación de óxido
nítrico, NO, del 44%.
3.2. Se comprobó que la separación efectiva de los pares hueco positivo–electrón
formados por acción de la luz en el semiconductor, mejora la fotoactividad.
Esta dispersión fue particularmente eficaz en morteros con 45PC6 como
superplastificante.
Conclusiones
289
3.3. Los derivados de policarboxilato eterificados, PCEs, aumentaron más la
fotoactividad que el PNS debido a su mejor eficacia como dispersantes en
medios de cal asociada a su arquitectura molecular, densidad de carga aniónica
y longitud de sus cadenas laterales.
3.4. La formación de NO2 (intermedio de alta toxicidad) se vio apreciablemente
reducida debido al uso de aditivos dispersantes, con valores de selectividad
(relación porcentual NO/NOx) tan altos como 87%.
3.5. La capacidad de autolimpieza de los morteros se mejoró mediante la adición
de superplastificantes derivados de policarboxilatos, 23APEG, 45PC6 y
52IPEG: en el estudio de la degradación del colorante rodamina B depositado
sobre la superficie de los morteros la eficiencia de decoloración aumentó 1,43
veces en promedio después de 310 minutos de irradiación UV–Vis (con
lámpara de simulación de luz solar).
3.6. Se asoció el bajo rendimiento de autolimpieza de morteros con polímero PCE–
1 con una interferencia con el mecanismo de fotosensibilización de la
degradación del colorante. Este mecanismo, en cambio, fue fomentado por los
otros derivados de policarboxilatos, que mostraron una cinética de degradación
más rápida. Este mecanismo es estrictamente dependiente de luz visible, lo que
explica la aparente disparidad de que, a pesar del bajo rendimiento de
autolimpieza, la reducción de NO de las muestras de PCE–1 fuera elevada (al
ser proceso dependiente de luz UV).
3.7. La presencia de los superplastificantes mejoró el efecto de hidrofilicidad
fotoinducida, mecanismo que favorece la acción autolimpiante.
3.8. La presencia de oleato de sodio como hidrofugante fue compatible con la
hidrofilicidad fotoinducida.
3.9. La eficacia de estas mezclas podría ser mejorada mediante más estudios para
ajustar las dosis y la relación agua/cal de estos morteros en función de su
aplicación final como morteros de revoque (monocapa, multicapa) o de
rejuntado.
Conclusiones
290
3.10. Las condiciones fuertemente alcalinas de los morteros de cal junto con la
actividad fotocatalítica del TiO2 impidieron el crecimiento adecuado de cepas
de la bacteria Pseudomonas fluorescens en el estudio biocida para determinar
la influencia del efecto dispersante de los superplastificantes. Se recomienda
en futuras investigaciones la realización del ensayo con algún otro
microorganismo resistente a pH alcalino como algas u hongos.
4. Se desarrollaron morteros de revoco a base de cal aérea combinando diferentes
aditivos: un agente repelente al agua (oleato de sodio), un mejorador de la adherencia
(copolímero de etileno–vinil–acetato, EVA), un modificador de reología (almidón
modificado) y aditivos minerales puzolánicos (metacaolín o nanosílice) con el fin de
mejorar el rendimiento de los revoques cuando se aplican sobre diferentes sustratos,
en particular la adherencia, la reducción del agrietamiento y la durabilidad.
4.1. Se demostró que la adición de EVA, combinada con oleato, metacaolín y
almidón mejoró la adherencia en la mayoría de los sustratos y se minimizó el
agrietamiento. En cambio, desde el punto de vista de la mejora de la durabilidad
(ciclos hielo–deshielo y resistencia a sulfatos), estos revoques no fueron tan
efectivos.
4.2. La presencia de nanosílice mejoró la resistencia mecánica y la durabilidad en
combinación con EVA, oleato y almidón, aunque la adherencia y el
agrietamiento no mejoraron tanto.
4.3. La aplicación esta gama de morteros sobre diferentes sustratos mostró el efecto
determinante de la porosidad del sustrato, responsable de originar
agrietamientos en la monocapa aplicada, debido a la absorción de agua. Mayor
porosidad conduce a mayor absorción de agua y fisuras superficiales más
intensas en número y dimensión. Debido a ello, los morteros aplicados sobre
granito (1.69% de porosidad total) tienen mejor aspecto visual que aquellos
sobre ladrillo (35.8% de porosidad total) que presentan un notable grado de
fisuras superficiales.
4.4. La distribución de tamaño de poros del sustrato también condiciona la
fisuración superficial de los morteros: una población de poros pequeños (entre
0.01 y 1 micras) genera una importante fuerza de succión capilar, que conduce
a la formación de microfisuras, como se comprobó en los casos de aplicación
sobre ladrillo y piedra caliza.
Conclusiones
291
4.5. Se observó que la adición de EVA a una dosis del 10% redujo ligeramente la
permeabilidad en comparación con la muestra control. La combinación de
EVA con el almidón modificado ocasionó un incremento en la permeabilidad
debido a los cambios en la distribución del tamaño de poro inducidos por el
potenciador de viscosidad.
4.6. Se comprobó que el uso de las combinaciones ensayadas de aditivos es útil para
obtener revoques de cal duraderos en los que se mejora la adherencia y se
reduce la formación de grietas
