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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

1634 Materials and Structures (2014) 47:16331645


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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

Materials and Structures (2014) 47:16331645 1635


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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)



10/14

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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