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O R I G I N A L A R T I C L E
Influence of soil grading on the characteristics of cement
stabilised soil compacts
B. V. Venkatarama Reddy M. S. Latha
Received: 6 December 2012 / Accepted: 9 July 2013 / Published online: 19 July 2013
RILEM 2013
Abstract The paper deals with experimental inves-
tigations aiming at specifying optimum soil grading
limits for the production of cement stabilised soil
bricks (CSSB). Wide range of soil grading curves
encompassing both fine and coarse grained soils were
considered. Strength, durability and absorption char-
acteristics of CSSB were examined considering 14
different types of soil grading curves and three cement
contents. The investigations show that there is opti-
mum clay content for the soil mix which yields
maximum compressive strength for CSSB and theoptimum clay content is about 10 and 14 % for fine
grained and coarse grained soils respectively. Void
ratio of the compacted specimens is the lowest at the
optimum clay content and therefore possesses maxi-
mum strength at that point. CSSB using fine grained
soils shows higher strength and better durability
characteristics when compared to the bricks using
coarse grained soils.
Keywords Cement stabilisation Stabilised
soil brick
Soilcement
Compressive strength
Optimum clay content
1 Introduction and scope of the investigation
Stabilised soils find applications in the construc-
tion of roads and buildings. Since the last 67
decades stabilised soils are being exploited for the
construction of structural components of buildings
and other structures. Stabilised soil blocks (SSB)
and stabilised rammed earth represent the two
forms of compacted stabilised soil used for the
structural applications in buildings. Manufacture of
SSB involves compaction of the processed soilmixed with stabiliser (such as cement) at optimum
moisture into a dense block. Cement stabilised soil
brick (CSSB) is energy efficient and low embod-
ied carbon material [13]. CSSB has been used for
load bearing masonry buildings across the world
[410]. The characteristics of CSSB are influenced
by the cement content, brick density, soil grading,
and type and percentage of clay mineral in the soil
[2, 1116]. Cement stabilisation is ideally suited
for coarse grained sandy soils with non-expansive
clay minerals [9, 13, 1719] and hence the bestresults for CSSB are obtained when such soils are
used. Major challenge in the production of CSSB
is in specifying the optimum soil grading limits
which will yield maximum strength and durability
characteristics for the brick. A brief review of the
literature on influence of soil composition/grading
on characteristics of CSSB is as follows.
Investigations by Mitra [17] revealed that soils
containing high silt and clay fractions are not suitable
B. V. Venkatarama Reddy (&) M. S. Latha
Department of Civil Engineering, Indian Institute of
Science, Bangalore 560012, India
e-mail: [email protected]
M. S. Latha
e-mail: [email protected]
Materials and Structures (2014) 47:16331645
DOI 10.1617/s11527-013-0142-1
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for CSSB production. This study emphasised the use
of sandy soils for CSSB in order to achieve satisfac-
tory strength and durability characteristics. Fitzmau-
rice [4] conducted tests on machine pressed CSSB. His
studies revealed that the soils with low clay fraction
and high sand/gravel fraction are best suited for CSSB.
Importance of soils with high sand content wasemphasised in the investigations of Dietz [20] and
Bokhari [21]. Olivier and Ali [11] conducted detailed
investigations in understanding the role of soil grading
on the strength of CSSB and concluded that the CSSB
with 70 % sand and 20 % clay gives maximum
strength.
Reddy and Jagadish [22] examined the influence of
soil composition (using coarse grained soils) on the
strength and durability of CSSB. They concluded that
soils with non-expansive clay minerals having sand
content of 70 5 % and clay content of\15 % yieldmaximum strength for CSSB. Walker and Stace [12]
investigated the properties of CSSB using a number of
reconstituted coarse grained soils with different soil
grading limits. They noticed considerable decrease in
strength and increased mass loss in the durability test
as the clay content of the mix was increased. The
findings showed that for cement contents of 5 and
10 %, soils with 15 and 30 % clay are best suited.
Reddy and Walker [23] examined the strength and
durability characteristics of CSSB and recommend an
optimum clay content of 1012 %.Houben and Guillaud [9] recommend gravely sand
than silty clay soils for CSSB production. Consoli
et al. [14] determined unconfined compressive
strength of soilcement cylindrical specimens consid-
ering a range of density and cement contents. They
concluded that strength is sensitive to the density of
the specimen, and the effectiveness of cement is
greater in high density specimen. Reddy et al. [15]
made a comprehensive study on arriving at optimum
soil grading limits for the manufacture of CSSB
considering both strength and durability characterises.A coarse grained soil was considered and it was
reconstituted by diluting with sand in order to vary the
soil grading. CSSBs prepared using the natural soil
and reconstituted soils were examined for strength,
durability and bond characteristics. The study con-
cludes that 16 % clay fraction in the mix yields
maximum strength with good durability characteris-
tics for CSSB. Reasons for the optimum clay content
of 16 % are not stated.
Considering Unified Soil Classification (USC)
system the soils can be broadly classified into two
groups: (1) Coarse grained soils and (2) Fine grained
soils. Both of these soil types can be used for the
production of CSSB. The gaps found in the literature
for recommending optimum soil grading limits for the
CSSB production are as follows.
(a) There are only limited studies which attempt to
specify the exact soil grading limits for CSSB
manufacture. These studies specify optimum
clay fraction for only coarse grained soils,
(b) The reasons for the optimum clay fraction
yielding maximum strength are not discussed.
(c) There are no attempts to specify optimum
grading limits considering fine grained soils
(especially soils with high silt fraction) for the
production of CSSB.The present investigation is aimed at understanding
the influence of clay and silt size fractions of both
coarse grained and fine grained soils on strength and
durability characteristics of CSSB, and to arrive at
optimum soil grading limits considering wide range of
soil grading limits. Gradation of a natural soil with
31.6 % clay fraction was varied by reconstituting it
with sand and silt size fractions to obtain wide range of
grading limits representing both coarse and fine
grained soils. Strength characteristics were examined
by testing small cylindrical specimen, while thedurability characteristics were examined by testing
CSSB bricks. Cement content used (by the industry)
for the CSSB production is in the range of 610 %.
Therefore, in the present study three cement contents
(4, 7 and 10 % by weight) were considered.
2 Materials used in the study
2.1 Natural soil
Locally available red soil was used in the experimental
studies. Figure1 shows the grain size distribution
curve for the soil and Table2gives various charac-
teristics of the soil. Figure2shows the XRD analysis
for the soil. The soil has 31.6, 18.1 and 50.3 % clay,
silt and sand size fractions respectively. The soil
contains predominantly kaolinite clay mineral. The
liquid limit, plasticity index and shrinkage limit values
for the soil is 40, 21 and 14 %, respectively. The soil is
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a coarse grained soil belonging to class SC in USC
system.
2.2 Sand and silt
Natural river sand was used in the investigations,
whose grain size distribution curve is shown in Fig. 1.Also, the grain size distribution curve for the silt is
shown in Fig. 1. The silt was obtained by sieving
(washing) manufactured sand through 0.075 mm
sieve.
2.3 Reconstituted soils
The natural red soil was reconstituted by mixing
different percentages of sand and silt, and the details of
the mix proportions along with the designation of the
mix are given in Table1. Grain size distributioncurves of reconstituted soils are displayed in Fig.1.
The properties of reconstituted soil mixtures are given
in Table2. Totally 14 different soil variants were
generated representing wide range of grain size
distributions representing both coarse and fine grained
soils. The clay, silt and sand fractions of the 14 soil
samples vary over wide limits (clay: 4.531.6 %; silt:
4.888.3 %; sand: 7.287.3 %). Also, the Atterbergs
limits of the natural soil and its reconstituted variants
vary over a wide range (Liquid limit: 23.740 %;
Plasticity Index: 9.621; shrinkage limit: 1.6214 %).
2.4 Cement
Ordinary Portland cement conforming to IS 8112 [24]
code was used in the manufacture of CSSB and
cylindrical specimens. The cement composition: Alu-
mina iron ratio of 1, Magnesium oxide 1.4 %,
Sulphuric anhydride 1.9 %, Alkalies 0.6 % and Chlo-rides 0.01 %. The initial setting time of the cement
was 46 min and the mean 28 day compressive strength
of 50.7 MPa.
3 Casting test specimens and testing procedure
3.1 Casting specimens
Strength of compacted cement stabilised soils were
examined through the compression tests on compactedcylindrical specimens. The study involves preparation
of large number of specimens considering fourteen
soil grading curves and three cement percentages.
Hence, use of compacted bricks for such a parametric
study necessitates handling huge quantities of mate-
rials (soil, sand, silt and cement). Therefore, smaller
cylindrical specimens of size 76 mm height and
38 mm diameter were considered for examining the
strength. Durability characteristics of compacted
cement stabilised soil as the soil grading was
changed was examined through tests on CSSB of
Fig. 1 Grain size distribution curves for natural soil, sand, silt and reconstituted soils
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size 230 9 108 9 75 mm considering 7 % cement
content.
3.1.1 Casting cylindrical specimens
The following procedure was followed for casting the
cylindrical specimens.
(a) Soil was oven dried (at 60 C) and then blended
with requisite quantity of Portland cement in a
small ball mill (for 10 min) to ensure uniform
mixing of the cement.
(b) Requisite quantity of potable water was sprayed
onto the dry soilcement mixture and mixed
thoroughly (manually) in order to ensure uniform
distribution of the moisture in the entire mix.(c) Partially saturated soilcement mixture was fed
(known weight) into an open-ended cylindrical
mould. Then the mould was mounted horizon-
tally in a screw-jack set-up and then compaction
was carried out from both ends using a mechan-
ical screw-jack set-up.
(d) The specimen was extruded from the mould
immediately after the compaction. Compacted
specimens were kept for curing under wet burlap
after 24 h of casting. Curing is continued inside
the laboratory and was ensured that the specimenis always saturated. Temperatures inside the wet
burlap varied in a narrow range of 2427 C.
3.1.2 Casting CSSB specimens
Manually operated machines are employed in the field
for the manufacture of CSSB. One such machine was
used to prepare CSSB of size 230 9 108 9 75 mm.
Oven dried soil (at 60 C) was powdered and then
blended with requisite quantity of sand/silt. Uniform
dry mixture is then blended with 7 % cement (by
weight). Requisite quantity of potable water was
sprayed on to the uniform dry mixture of soil, sand/silt
and cement, and thoroughly mixed to get a uniform
partially saturated mixture. Known weight of the
partially saturated soilcement mixture is fed into themachine mould, compacted, extruded and kept in a
stack for curing. CSSB is cured for 28 days and then
air dried for 7 days. Air dried specimens were
then oven dried at 50 C to attain constant weight
and then used for the testing. Figure3 shows the
cylindrical as well as CSSB specimens.
3.1.3 Moulding water content, specimen density
and compaction energy
Moulding water content and density influence thestrength of CSSB. Hence, moulding water content and
the dry density of the compacted cement stabilised soil
specimens have to be kept constant in order to make a
comparison of strength and other characteristics across
different cement percentages and clay contents of the
mixes. Compressive strength of cement stabilised
compacted specimens increases with the increase in
dry density [13,14,16,25]. In the present investiga-
tion dry density was kept in a narrow range of
Fig. 2 XRD analysis for the natural soil
Table 1 Mix proportions of natural and reconstituted soils
Mix proportion (by weight) Designation
of the mixNatural soil (NS) Sand Silt
0 1 0 Sand
0 0 1 Silt
1 0 0 NS
1 3 0 CG1
1 2 0 CG2
1 1.25 0 CG3
1 0.75 0 CG4
1 0.5 0 CG5
1 0.25 0 CG6
1 0 6 FG1
1 0 3 FG2
1 0 2 FG3
1 0 1.25 FG41 0 0.75 FG5
1 0 0.5 FG6
1 0 0.25 FG7
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Table2
Characteristicsofnaturalsoil,sand,siltandreconstitutedsoils
Typeofsoil
Properties
Sand
Silt
NS
CG1
CG2
CG3
CG4
CG5
CG6
FG1
FG2
FG3
FG4
FG
5
FG6
FG7
Texturalcomposition(%)
Sand(4.750.075mm)
100
0
50.3
87.3
83.2
77.7
71.4
66.7
60.2
7.2
12.6
16.8
22.4
28
.8
33.6
40.2
Silt(0.0750.002mm)
0
100
18.1
4.8
6.3
8.3
10.6
12.3
14.5
88.3
79.5
72.7
63.6
53
.2
45.4
34.5
Clay(\0.002mm)
0
0
31.6
7.9
10.5
14
18
21
25.3
4.5
7.9
10.5
14.0
18
.0
21.0
25.3
Atterbergslimits
Liquidlimit(%)
40.0
23.7
25.6
26.9
29.7
32.0
35.0
24.2
25.8
26.5
28.0
29
.5
31.1
33.5
Plasticlimit(%)
19.0
8.6
9.3
9.4
11.2
12.3
14.8
15.9
16.1
16.4
17.8
18
.1
18.4
18.9
Plasticityindex
21.0
15.1
16.3
17.5
18.5
19.7
20.2
8.24
9.6
10.1
10.2
11
.4
12.7
14.5
Shrinkagelimit(%)
14.0
1.6
2.7
4.6
6.4
7.4
11.0
1.1
2.6
3.5
6.9
8.0
9.4
12.1
USCclassification
SC
SC
SC
SC
SC
SC
SC
CL
CL
CL
CL
CL
CL
CL
Predominantclayminerals
K
K
K
K
K
K
K
K
K
K
K
K
K
K
pH
9.11
8.43
7.75
8.57
8.35
8.07
7.94
7.82
7.79
8.35
8.26
8.21
8.16
8.09
8.03
7.97
Specificgravity
2.68
2.67
2.68
2.68
2.68
2.68
2.69
2.50
2.50
2.51
2.53
2.53
2.54
2.54
Compactioncharacteristics
Withoutcement
StandardproctorOMC(%)
15.60
8.50
8.69
9.15
9.58
10.81
11.26
17.10
18.90
19.80
20.60
21
.40
21.90
22.03
MDD(kN/m3)
17.95
19.03
19.15
19.21
18.93
18.74
18.57
19.45
19.73
20.26
20.14
20
.13
20.07
20.02
With7%cement
StandardproctorOMC(%)
13.16
8.00
8.50
8.61
9.37
10.04
10.93
13.52
14.24
16.26
17.02
17
.79
18.31
18.45
MDD(kN/m3)
18.28
19.36
19.42
19.57
19.45
19.36
19.29
20.91
21.19
21.72
21.60
21
.59
21.51
21.43
NSNaturalsoil,SCclayeysand,C
Linorganicclaysoflowandmediumplasticity,Kkaoliniteclaymineral,MD
Dmaximumdrydensity
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16.917.1 kN/m3 for specimens using coarse grained
soils and 16.616.9 kN/m3 for specimens using fine
grained soils.
Moulding moisture content to be used need not
depend upon the standard Proctor OMC. This is
because static compaction process was employed in
the preparation of cylindrical and CSSB specimens,
whereas the Proctor test is a dynamic one. Also, thecompaction energy inputs are different in these two
types of compaction processes. For example to
compact a soil block of size 230 9 190 9 76 mm to
a dry density of about 17.5 kN/m3 the energy required
is 0.3 MJ/m3 when static compaction process was
employed. The energy input in Standard Proctor
compaction test is 0.60 MJ/m3. Energy required in
the compaction process decreases with increase in
moulding moisture content of the soil mix. Soil
grading also influences the energy required to achieve
a particular density. More information on the staticcompaction of soils can be found in the studies of
Reddy and Jagadish [26]. In the commercial opera-
tions, CSSB are produced either using a manually
operated or mechanised machine. In both the types of
machines static compaction process is used. Such
machines produce constant volume bricks/blocks and
hence to achieve a specified dry density for the brick/
block the machines have adequate capacity to supply
the required energy.
It has been shown that compaction on the wet side
of standard Proctor OMC yields better results for
cement stabilised soil compacts [25]. After making
few trial mixes moulding moisture content was fixed at
12.5 and 18 % by weight for the coarse and fine
grained soils respectively. Fine grained soils need
more water to get the required consistency to compactthe specimens using static compaction process in the
screw-jack set-up employed for casting the cylindrical
specimens. For both the cases the moulding water
content chosen is on the wet side of Standard Proctor
OMC, except for FG6 and FG7 soils where the
moulding water content is close to Standard proctor
OMC. It should be noted here that moulding water
content for a particular soil type (either coarse grained
or fine grained soil series) is kept constant so that the
influence of moulding water content on strength
characteristics is minimised.
3.2 Testing
3.2.1 Testing cylindrical specimens
Compressive strength tests were performed in both
saturated and dry condition. The wet compressive
strength (i.e. strength in saturated state), was deter-
mined by testing the cured and oven dried specimen
soaked in water for 48 h prior to testing, The drycompressive strength was obtained by testing the
cured and oven dried specimen. The testing procedure
is as follows.
(a) The dimensions of the cylindrical specimens
were measured using a callipers and the mass of
the specimen noted (either wet or dry) at the time
of testing.
(b) The specimens were subjected to compression in
a loading frame through the application of
uniform concentric load at a piston displacementrate of 1.25 mm/min. The load at failure was
recorded and the compressive strength assessed.
(c) After the compression test, moisture content of
the failed specimen (especially the saturated
ones) was ascertained by drying at 110 C in an
oven for 24 h.
Based on the experimental data generated, wet and
dry compressive strength, dry density and water
absorption values of the specimen were calculated.
Fig. 3 Compacted cylindrical specimens and CSSB bricks
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3.2.2 Testing CSSB specimens
3.2.2.1 Compressive strength of CSSB (wet and
dry) Cured and oven dried CSSB bricks were soaked
in water for 48 h prior to testing for determining the wet
compressive strength, whereas dry compressive
strength was obtained by testing the oven driedspecimen. IS 3495-Part I [27] code guidelines were
followed for determining the compressive strength.
3.2.2.2 Rate of water absorption and saturated water
content Cured and air dried CSSB was oven dried at
50 C to constant weight. The weight of the dry CSSB
was recorded and then soaked in water. Weight of the
CSSB was noted at the end of different time intervals
(i.e. 1, 2, 4, 6, 8, 10, 15, 20, 30, 60, 240, 360, 1,440 and
2,880 min). The percentage of moisture absorbed by
the CSSB was determined with respect to the dryweight. A plot of moisture content with the soaking
duration was obtained.
Water absorption (saturated water content) of
CSSB was determined using 24-h immersion cold
water test as per the guidelines of IS 3495-Part II [28]
code.
3.2.2.3 Weight loss after cyclic wetting and drying
test The cyclic wetting and drying test was carried
out by following the ASTM D559 [29] code
guidelines.
3.2.2.4 Linear expansion on saturation A length
comparator was used to measure the linear expansion
on saturation. The procedure outlined in IS 17252011
code [30] was followed to measure the linear
expansion value for CSSB.
4 Results and discussion
4.1 Compressive strength and soil grading
Apart from sand and silt fractions, clay mineral type
and its percentage controls the characteristics of a soil
mix. Therefore, compressive strength of cement
stabilised soil compacts was obtained by varying the
soil gradation over wide limits. Figures4and5 show
the compressive strengths of cylindrical specimen
plotted against clay contents of the mix for wet and dry
compressive strength cases respectively, whereas
Fig.6 shows a similar plot for CSSB. The results
shown in these plots represent the mean of six
specimens, and the density as well as moulding
moisture contents have been controlled while gener-
ating the strength results. Following points emerge
from the results shown in these Figures.
The strength increases with the increase in claycontent of the soil mix, reaches a peak value and then
decreases for further increase in clay content. This
behaviour is noticed for both the types of soil groups
(coarse and fine grained) in dry as well as saturated
condition irrespective of cement content and the
specimen type. The optimum clay content corresponds
to about 10.5 % for the fine grained soils and 14.0 %
for the coarse grained soils. Investigations of Reddy
et al. [15] show optimum clay content of 1416 % for
coarse grained soils. There is a considerable difference
between the maximum strength at optimum claycontent and the lowest strength for any given case.
For 410 % cement content the difference between
peak strength (at optimum clay content) and lowest
strength vary in the range of 2050 % for the dry
strength case and 50130 % for the wet strength case.
The grain size curves of CG3 and FG3 soils (Fig.1)
represent the optimum grading curves corresponding
to the optimum clay content for coarse and fine grained
soils respectively. The optimum grading curve for
coarse grained soils (i.e. CG3 curve in Fig.1)
indicates 14, 8 and 78 % of clay, silt and sand sizefractions respectively. Similarly for fine grained soil
0
1
2
3
4
5
6
7
0 4 8 12 16 20 24 28 32
Clay content (%)
Wetcompressiv
estrength(MPa)
Coarse grain soil 4%CCoarse grain soil 7%CCoarse grain soil 10%CFine grain soil 4%CFine grain soil 7%CFine grain soil 10%C
Fig. 4 Wet compressive strength versus clay content of the mix
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the optimum grading curve (i.e. FG3 curve in Fig. 1)
shows 10.5, 72.7 and 16.8 % clay, silt and sand size
fractions respectively. From these results it is possible
to specify a narrow range for the grain size curves
leading to maximum strength for CSSB. For example
2 % from the optimum clay content (Figs. 4, 5, 6)
will give a band close to optimum value, with marginaldeviation from the maximum strength.
For fine grained soils the maximum wet compres-
sive strength (of cylindrical specimen) at optimum
clay content varies between 2.25 and 6.0 MPa for the
cement content in the range of 410 %. The corre-
sponding values for dry compressive strength are in
the range of 4.710 MPa. The wet strength to dry
compressive strength ratio is in the range of 0.480.60.
For coarse grained soils the maximum wet com-
pressive strength (of cylindrical specimen) at optimum
clay content varies between 1.0 and 4.7 MPa for the
cement content in the range of 410 %. The corre-
sponding values for dry compressive strength are in
the range of 2.46.5 MPa. The wet to dry strength ratio
is in the range of 0.420.70. The wet to dry compres-sive strength ratio increases as the cement content
increases.
Maximum brick (CSSB) compressive strength (at
optimum clay content) is 6.3 and 10.8 MPa in wet and
dry state respectively for fine grained soil (Fig.6)
using 7 % cement. The corresponding values using
coarse grained soil are 5 and 9 MPa in wet and dry
cases respectively.
The compressive strength of specimens using fine
grained soils is much higher than those using coarse
grained soils irrespective of cement content. Consid-ering 410 % cement content range, the compressive
strength of specimens (at optimum clay content) using
0
2
4
6
8
10
0 4 8 12 16 20 24 28 32
Clay content (%)
Drycompressivestrength(MPa)
Coarse grain soil 4%CCoarse grain soil 7%CCoarse grain soil 10%CFine grain soil 4%CFine grain soil 7%CFine grain soil 10%C
Fig. 5 Dry compressive strength versus clay content of the mix
1
3
5
7
9
11
4 8 12 16 20 24 28 32
Clay content (%)
Compressivestr
ength(MPa)
Coarse grain soil CSSB (Wet)Coarse grain soil CSSB (Dry)
Fine grain soil CSSB (Wet)Fine grain soil CSSB (Dry)
Fig. 6 Strength versus clay content for CSSB with 7 % cement
0
1
2
3
4
5
4 8 12 16 20 24 28 32
Clay content (%)
Compressivestrength(MPa)
0.53
0.54
0.55
0.56
Void-ratio
Wet strength
Dry strength
Void-ratio
Fig. 7 Strength, clay content and void ratio relationships for
coarse grained soil with 7 % cement
0
1
2
3
4
5
6
7
8
4 8 12 16 20 24 28 32
Clay content (%)
Compressivestren
gth(MPa)
0.46
0.47
0.48
0.49
0.50
0.51
0.52
Void-ratio
Wet sterngthDry strengthVoid - ratio
Fig. 8 Strength, clay content and void ratio relationships for
fine grained soil with 7 % cement
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fine grained soil is 30110 % higher when compared
with the strength of specimens using coarse grained
soils. The reasons for higher compressive strength at
optimum clay content and higher strengths obtained
for specimens using fine grained soils are discussed in
the following sections.
4.2 Strengthclay contentvoid ratio relationships
The question arises as to why there is an optimum clay
content yielding maximum strength, even though the
densities of the specimens are nearly equal. This can
be explained through an analysis of void ratio of the
specimens. Void ratio of the compacted cylindrical
specimens was estimated based on the density of the
specimen and specific gravity of the mix. Figures 7
and8show typical relationships between strength and
clay content, and void ratio and clay content forcylindrical specimens (with 7 % cement) using coarse
grained and fine grained soils respectively. These
results represent the mean of six specimens. These
relationships clearly indicate that the void ratio is the
lowest at the optimum clay content of the specimen for
both the types of soils. Lower void ratio at optimum
clay content indicates better packing density and
contacts among the particles leading to maximum
strength.The specimens with fine grained soils possess lower
void ratio than those using coarse grained soils. At
optimum clay content (for 7 % cement specimen) the
void ratio values are 0.465 and 0.532 for fine and
coarse grained soil specimens respectively. There-
fore, for a given combination of density and cement
content the specimens with fine grained soil show
lower void ratio and hence result in higher strength.
Strength and void ratio are linearly related as
illustrated in Fig.9for CSSB using different cement
contents. The strength decreases with the increase invoid ratio of the specimen. For 4 % cement CSSB, the
wet compressive strength increases by 175 % as the
void ratio reduces from 0.63 to 0.38, whereas for 10 %
CSSB the strength increase is about 50 %. Strength of
CSSB at lower cement contents (4 %) is more
sensitive to the void ratio of the brick.
4.3 Surface porosity and void ratio
The pore structure of the broken surface of the cured
and dried specimen was examined through SEMimaging. Figures10and11show typical SEM images
of the surface pore structure of the specimen having
different clay contents with 7 % cement for coarse
grained and fine grained soils respectively. The
surface porosity was determined from these images
using image processing and analysis software.
0
1
2
3
4
5
6
7
8
9
0.35 0.40 0.45 0.50 0.55 0.60 0.65
Void ratio
Wetcompressivestrength(MPa)
4% cement
7% cement
10% cement
Fig. 9 Strength versus void-ratio for CSSB
C1 C2 C3 C4
Fig. 10 SEM images of compacted cylindrical specimen using coarse grained soil with 7 % cement (C17.9 % clay, 22.7 % surface
porosity;C2 14.0 % clay, 19.6 % surface porosity; C3 21 % clay, 25.3 % surface porosity; C4 31.6 % clay, 27 % surface porosity)
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Figure12shows the surface porosity plotted against
strength for the specimens using both coarse and fine
grained soils with 7 % cement. These results again
confirm that the surface porosity is the lowest for
specimens having optimum clay content (10.5 % for
fine grained soil and 14.0 % for coarse grained soil).
The surface porosity at optimum clay content is 14.5
and 20 % for the specimens using fine grained and
coarse grained soils respectively. The specimens using
fine grained soils show a lower value of porosity when
compared with those using coarse grained soils. Thisexplains the reason for higher compressive strength
values for specimen with fine grained soils when
compared with those of coarse grained soil specimens.
4.4 Strength and Atterberg limits
Atterbergs limits of a soil indicate the plasticity and
shrinkage characteristics of the soil mix. Liquid limit,
plastic limit, shrinkage limit and plasticity index of all
the soil mixtures used in the experimental investiga-
tions are given in Table2. Typical relationship
between plasticity index (PI) and the compressive
strength of cement stabilised compacted cylindrical
specimen is shown in Fig. 13. Plots have been made
for both wet and dry compressive strengths and using
both coarse and fine grained soils using 7 % cement.
Each point in the plot represents mean of six
specimens. The trend lines indicate decrease in
strength as the PI increases. The PI value for theoptimum clay contents yielding maximum strength are
17.5 and 10.1 for coarse grained and fine grained soils
respectively.
4.5 Durability characteristics of CSSB
Satisfactory durability or long term performance of
CSSB is an important issue to be addressed apart from
examining the strength. Structures using CSSB are
F4F1 F2 F3
Fig. 11 SEM images of compacted cylindrical specimen using fine grained soil with 7 % cement (F1 7.9 % clay, 18.44 % surface
porosity; F2 10.5 % clay, 14.5 % surface porosity; F3 14 % clay, 15.1 % surface porosity; F4 25.3.6 % clay, 19.32 % surface porosity)
12
14
16
18
20
22
24
26
28
4 8 12 16 20 24 28 32
Clay content (%)
Surfaceporosity
(%)
Coarse grained soil-7%Cement
Fine grained soil-7%Cement
Fig. 12 Relationship between surface porosity and clay content
0
1
2
3
4
5
6
7
8
6 8 10 12 14 16 18 20 22
Plasticity index
Compressivestreng
th(MPa)
Wet compressive strength
Dry compressive strength
Fig. 13 Relationships between compressive strength and plas-
ticity index
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prone for the cyclic exposure to rain or hygroscopic
environment and drying due to the changes in the
surrounding climate. Since the CSSB contains certain
amount of clay minerals even after cement stabilisa-
tion [31], hence there is a possibility for cyclic
expansion and shrinkage phenomenon taking place in
CSSB due to the variations in the surrounding climaticconditions. Therefore, evaluation of the durability of
CSSB should address the issue of cyclic wetting and
drying and dimensional stability. ASTM D559 [29]
code suggests monitoring the weight loss of the brick
after subjecting to 12 cycles of cyclic wetting and
drying including scratching the surface with metal
wire brush. Linear expansion on saturation gives an
idea of the dimensional stability of CSSB [15]. IS1725
[30] has adopted this test and specifies limits for
weight loss (\3 %) and linear expansion on saturation
(\0.10 %) for satisfactory performance of CSSBmasonry walls.
Weight loss after the cyclic wetting and drying test,
and linear expansion on saturation were monitored for
the CSSB using both coarse grained and fine grained
soils with 7 % cement. Figure14 shows a typical
variation in weight loss after the cyclic wetting and
drying test with the clay content of CSSB. The lowest
value of the weight loss for the CSSB occurs at the
optimum clay content yielding maximum strength.
The optimum value of clay content giving least weight
loss is 10.5 and 14 % for the CSSB using fine grainedand coarse grained soils respectively.
Linear expansion on saturation for the CSSB using
7 % cement and with coarse and fine grained soils is
shown in Fig.15. Linear expansion increases with
increase in clay content irrespective of the soil type.
The linear expansion varies in the range of
0.030.20 % as the clay content changes from 8 to
31.6 %. At the optimum clay content the linear
expansion values are 0.04 and 0.045 % for the bricks
using fine and coarse grained soils respectively. These
values are well within the accepted value of 0.1 %
[30].Based on the strength and durability tests it can be
concluded that the optimum clay content of the soil
mix yielding best results for CSSB is about 10 and
14 % for the fine grained and coarse grained soils
respectively. It may be difficult to adjust the clay
fraction of the soil mix precisely in the field/factory
while producing the CSSB. Therefore, a narrow band
for optimum clay fractions can be defined for field
applications. Hence, the optimum clay content can be
10 2 and 14 2 % for fine grained soils and coarse
grained soils (containing non-expansive clay miner-als) respectively for the cement content in the range of
410 %.
4.6 Absorption characteristics of CSSB
The rate of water absorption and saturated water
content (designated as water absorption) were deter-
mined for the CSSB manufactured using both coarse
and fine grained soils. The rate of water absorption
with the soaking duration in water for CSSB is
displayed in Fig.16. Relationship between waterabsorption and clay content of CSSB is shown in
Fig.15. The following points emerge from the results
of Figs. 15and 16.
(a) The dry CSSB absorbs water at a faster rate
initially up to 60 min of soaking duration and
0
2
4
6
8
10
12
4 8 12 16 20 24 28 32
Clay content (%)
Weightloss
(%)
Coarse grained soil CSSB
Fine grained soil CSSB
Fig. 14 Weight loss versus clay content of the mix for CSSB
using 7 % cement
0
5
10
15
20
25
4 8 12 16 20 24 28 32
Clay content (%)
Waterabsorptio
n(%)
0
0.05
0.1
0.15
0.2
0.25
Linearexpansio
n(%)
Water absorption of CSSB - Coarse soilWater absorption of CSSB - Fine soilLinear expansion of CSSB - Coarse soilLinear expansion of CSSB - Fine soil
Fig. 15 Variation of water absorption and linear expansion
with clay content for CSSB using 7 % cement
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later on the absorption rate reduces. CSSB using
fine grained and coarse grained soils attain 85
and 90 % saturation respectively in about 60 min
of soaking in water. The bricks saturate com-
pletely when soaked in water for 24 h.
(b) The water absorption increases with increase in
clay content. This is on the expected lines
because the clay has more affinity for water,
more clay means more water absorption.
(c) Water absorption values range between 18 and
20 % for CSSB using fine grained soil as the clay
content changes from 8 to 31.6 %. For bricks
with coarse grained soils the range is 1316 %.
(d) At optimum clay content the water absorption is14 and 18 % for CSSB using coarse grained and
fine grained soils respectively.
These results clearly indicate that CSSB absorbs
water and attains 8090 % saturation in about an hour
after soaking in water. The bricks attain complete
saturation in 24 h of immersion in cold water.
5 Conclusions
Influence of soil grading especially the clay sizefraction of the soil in controlling the strength, dura-
bility and absorption characteristics of cement stabi-
lised soil compacts and bricks was examined in great
detail considering both coarse and fine grained soils.
The investigations show that clay fraction of the
soil mixture and the void ratio (density) of the
compacted specimen play crucial role in influencing
the characteristics of cement stabilised soil compacts.
There is optimum clay content leading to maximum
strength and lowest mass loss (in the durability test)
for CSSB. The optimum clay content is about 10 and
14 % for fine grained soils and coarse grained soils
respectively for cement contents in the range of
410 %. Large deviations from the optimum clay
content value results in considerable loss in strength
for cement stabilised soil compacts. Void ratio of thecompacted specimen is the lowest at the optimum clay
content and therefore possesses maximum strength at
optimum clay content. Void ratio of specimens using
fine grained soils is lower than those using coarse
grained soils. Hence, the CSSB using fine grained soils
possess higher strength than those with coarse grained
soils. In order to achieve optimum clay fraction for a
particular soil, the soil grading can be adjusted by
reconstitution with sand.
The mass loss after the durability test is lowest for
the bricks having optimum clay content. CSSB absorbwater rapidly in the initial 1 h of soaking in water and
attain complete saturation in 24 h. Bricks with fine
grained soils show higher value of water absorption
when compared to the bricks using coarse grained soils.
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