Properties of some cement stabilised compressed

Prévia do material em texto

Materials and Structures/Mat4riaux et Constructions, Vol. 30, November 1997, pp 545-551 
Properties of some cement stabilised compressed 
earth blocks and mortars 
Peter Walker and Trevor Stace 
Dept. of Resource Engineering, University of New England, Armidale, NSW 2351, Australia 
Paper received: May 21, 1996; Paper accepted:July 17, 1996 
A B S T R A C T R I~ S U M I ~ 
Findings from an on-going investigation into the 
effects of soil properties and cement content on physical 
characteristics of compressed earth blocks and soil mortars 
are presented. A series of test blocks were fabricated using a 
range of composite soils, stabilised with 5% and 10% 
cement, and compacted with a manual press. Results for 
saturated compressive strength, drying shrinkage, wet- 
ting/drying durability, and water absorption testing are 
presented in the paper. In conjunction with the block tests, 
workability and compressive strength characteristics of 
suitable soil:cement and cement:lime:sand mortars were 
also studied. Mortar consistency was assessed using cone 
penetrometer and slump tests. Water retention properties 
of the mortars were also measured. For a given compactive 
effort, the strength, drying shrinkage, and durability char- 
acteristics of the compressed earth blocks improved with 
increasing cement and reducing clay content. Slump test- 
ing proved the most reliable means of assessing soil:cement 
mortar consistency. Both the flow table and cone pen- 
etrometer tests were found to be unsuitable. Water reten- 
tion properties of soil:cement mortars appear well-suited 
to typical unit water absorption characteristics. Mortar 
strengths were closely related to cement and clay contents, 
but as expected were less than the average unit strengths. 
On pr&ente les r&ultats de recherches en cours sur les effets 
des propri{t& des sob et des taux de ciment sur les caract&istiques 
physiques de blocs de terre comprim& et de mortiers de sol. On a 
fabriqu{ une s&ie de blocs d'essai en utilisant une gamme de sob 
compose's, trait& au ciment h 5% et h 10% et comprim& h 
l'aide d'une presse manueUe. On pre'sente ici les r&ultats pour la 
r&istance h la compression satur&, le retrait de se'chage, la dura- 
bilit~ mouill& et s&he, et l'absorption d'eau. Coujointement 
avec les essais sur blocs, on a &udi{ les propri&e's d'ouvrabilit8 et 
de r&istance h la compression de certains mortiers sol:ciment et 
ciment:chaux:sable. On a &alu8 la consistance des mort#rs au 
, i I \ A 
moyen dun penetrometre ~ cone et des essais d' affaissement, et 
mesur6 les propri(t& de retention d'eau. Pour une pression de 
compactage donne'e, les caract&istiques de re'sistance, de retrait de 
s&hage et de durabilit~ se sont am~lior&s auec un taux croissant 
de ciment et un taux d&roissant d'argile. Les essais d'affaisse- 
ment out ~t~ les plus fiables pour &aluer la consistance du mor- 
tier sol:ciment ; la table a secousses et I'essai au p&{trom~tre h 
c3ne ne conviennent pas. Les propri&& de r~tention d'eau des 
mortiers en sohciment semblent bien assorties aux camct&is- 
tiques d'absorptiou d'eau des blocs typiques. II existe un rapport 
~troit entre les r&istances des mortiers et les taux de ciment et 
d' argile, mais, comme pr&u, ces r&istances sont moindres que la 
re'sistance moyenne des blocs. 
~!iiiiiiiii 
1. INTRODUCTION 
Over the past 40 to 50 years, there has been an increas- 
ing interest in the use of stabilised compressed earth 
blocks for residential construction [1-8]. They maximise 
the use of locally available materials, require relatively sim- 
ple construction methods, whilst offering favourable ther- 
mal and acoustic insulation properties. Environmental 
benefits include reduced energy consumption in produc- 
tion and a lessening demand for non-renewable resources 
[7]. Despite these advantages, however, the use of corn - 
pressed earth blocks is restricted by limited understanding 
of some basic material properties and a lack of appropriate 
building standards. 
Past investigations have concentrated on establishing 
properties of individual compressed earth blocks [8-11]. 
Whilst guidelines broadly outlining soil suitability and sta- 
biliser usage are available [1-3, 8, 10], there has been 
remarkably little work regarding the suitability of mortars. 
Current recommendations for compressed earth block con- 
struction suggest using soil:cement mortars in proportions 
similar to those used for block production [2, 4, 8, 11, 12]. 
EditOraii note 
Dr, Peter Walker is a RILEM Senior Member 
0025-5432/97 �9 tLILEM 545 
Materials and Structures/Mat~riaux et Constructions, Vol. 30, November 1997 
Alternatively, weak cement:lime:sand mortars, similar to 
those used with low-grade concrete blocks, have been 
recommended [1]. Contrary to general masonry prac- 
tice, some proposals have suggested using mortars with 
similar strength to that of the blocks [12]. To date, how- 
ever, there has been little published data to support these 
recommendations. 
Most recently, Venu Madhava Rao et al. [13] reported 
on bond strength testing of stabilised soil block masonry, 
built using cement:sand, cement:lime:sand, and cement: 
soil:sand mortars. The bond strength using cement:soil: 
sand mortars was 68% higher than that developed using a 
similar cement:sand mix. This was attributed to an 
improved grading of the soil-based mortar. However, no 
mortar consistency or water retentivity testing was 
reported. 
This paper outlines an experimental study to assess the 
properties and compatibility of both soil:cement and 
cement:lime:sand mortars for cement stabilised com- 
pressed earth blocks. Initially, results for compressive 
strength, drying shrinkage, wetting/drying durability, and 
water absorption testing of a range of compressed earth 
blocks are presented. Workability and compressive 
strength testing of suitable soil:cement and cement: 
lime:sand mortars are then outlined. Mortar consistency 
was assessed using cone penetrometer and slump tests. 
Water retention properties of the mortars were also mea- 
sured. Mortar suitability for a range of blocks is discussed 
and recommendations outlined. 
2. MATERIALS AND TEST METHODS 
2.1 Constituent materials 
Test blocks and mortars were fabricated using a range 
of composite (blended) soils formed from mixing varying 
proportions of a dark-red residual kaolinite clay soil, type 
CH (liquid limit = 69%, plasticity index = 38%) with a 
well graded sand, type SW-SM. Sieve gradings and mix 
proportions are outlined in Tables 1 and 2 respectively. 
The losses on ignition for the clay soil and sand were 6.8% 
and 0.8% respectively. Block and mortar mixtures were 
stabilised using a General Purpose (type GP) ordinary 
Portland cement. 
The physical stabilisation of soils for construction, by 
altering their grading with the addition of clay or sand, is a 
commonly employed technique [3]. In this study, mixing 
together the clay soil and sand provided a range of six soils 
with characteristics suitable for compressed earth block 
production. The advantage of this approach lies in restrict- 
ing the main parametric variation to soil grading, whilst 
avoiding probable variations in soil chemistry when using a 
range of natural deposits. Use of composite soils in this 
manner has proven effective in past work [10]. 
2.2 Preparation 
Initially the clay soil was air-dried, pulverised using a 
vibrating compactor, and passed through a 5-ram aperture 
screen sieve. The sand was also thoroughly air-dried and 
screened before use. The composite soils were formed by 
mechanically dry mixing together, in a concrete pan 
mixer, the prepared clay soil and sand for approximately 
2-3 minutes, after which the cement was added and mix- 
ing continued for a further 60 seconds: Once the dry 
ingredients werethoroughly mixed, water was gradually 
added until the appropriate consistency was attained. The 
blocks were compacted immediately after the mixture had 
been wet mLxed for around 2 minutes. However, in accor- 
dance with Australian Standard 2701 [14], wet mixing of 
all mortars was stopped only 60 seconds after addition of 
the water, at which point they were covered and left to 
stand for 10 minutes, and then mixing recommenced for a 
further 60 seconds prior to use. 
2.3 Compressed earth blocks 
Table 1 -Soil grading 
Grading Clay Soil Sand 
Fine Gravel fraction (2-6 mm) 25% 4% 
Sand fraction (0.06 - 2 mm) 27% 89% 
Silt fraction (0.002 - 0.06 mm) 4% 4% 
Clay fraction (<0.002 mm) 44% 3% 
Table 2 - Composite soil properties 
Clay soil Sand conten! Clay mineral Liquid limit Plasticity Linear 
content content index shrinkage 
(mass %) (mass %) (mass %) 
Non-plastic 
3.1 
7.2 
15.7 
19.8 
21.8 
80 
70 
60 
40 
20 
10 
9 
14 
18 
26 
35 
40 
20 
30 
40 
60 
80 
90 
21.9 
23.7 
26.7 
32.7 
39.8 
44.3 
The blocks were compacted at each mixture's stan- 
dard Proctor optimum moisture content [15]. Cement 
was added to each composite soil mixture in proportions 
of 5% and 10% by mass. Blocks were fabricated using a 
manual constant volume press, producing units having 
nominal dimensions 295 mm (length) x 140 mm 
(breadth) x 125 mm (height). Under normal operation 
the single acting ram develops a compaction 
pressure of approximately 2 MN/m 2. A total 
of thirty blocks were fabricated from each 
mix type. Block testing commenced 28 days 
after pressing. 
The compressive strength, drying 
0.4% shrinkage, and initial rate of absorption of 
2.4% each block mix were determined in accor- 
5.6% dance with methods developed for concrete 
8.0% masonry units [16]. In addition, durability 
10.0% was assessed using the ASTM wetting/dry- 
12.8% ing test for soil:cement materials [17]. 
546 
Walker, Staee 
A random sample of five blocks was used to determine 
the saturated compressive strength of each mix. The blocks 
were prepared by immersion in water for 24 hours prior to 
testing. Each block was tested in uniaxial compression 
uncapped between two 4-mm thick sheets of plywood in 
its normal, as pressed, bed face aspect. Using a 3000 kN 
capacity ELE testing machine, the load was applied contin- 
uously at a steady rate of 3.5 MPa/min. up to failure. The 
compressive strength of each block was obtained from its 
failure load and averaged bed face area. The unconfined 
characteristic compressive strength, Cu, of each mix was 
calculated from [4, 16]: 
C u = 0.67 [C- 1.65 s'] 
where: 
C = average compressive strength for test batch 
s' = unbiased compressive strength standard deviation 
0.67 = aspect ratio correction factor (table El, Bulletin 5) [4]. 
The average drying shrinkage was determined in 
accordance with Australian Standard 2733 [16], using three 
blocks from each mix. Shrinkage was measured using a 
150-ram demec gauge, with studs placed along the longi- 
tudinal faces of each block. Studs were attached using an 
epoxy resin found to be stable under test conditions. Total 
water absorption was determined from comparison of the 
oven dry and 24 hour water immersed masses of five 
blocks randomly selected from each mix type. The average 
initial rate of absorption (suction rate) was also determined 
using the same five blocks from each mix [16]. 
The ASTM wetting/drying test was used for acceler- 
ated durability testing of three blocks from each mix [17]. 
The blocks were subjected to twelve 48 hour cycles, com- 
prising 6 hours immersion in water, followed by 42 hours 
oven drying at 70 ~ C. After drying, each surface of the 
sample was subjected to two complete strokes from a stan- 
dard wire scratch brush. If the total reduction in dry mass 
after 12 cycles is less than 10%, the durability performance 
is considered satisfactory for general construction [1]. 
2.4 Mortars 
Mortars were formed by stabilising composite soil 
mixes with 5%, 10%, and 15% general-purpose cement 
by mass. In addition, three conventional cement:lime:sand 
mortars, 1:1:6, 1:2:9 and 1:3:12 (by volume), were also 
included in the investigation. Cement:lime:sand mortars 
were formed from a mix of general purpose cement, 
hydrated lime, and the sand used for block production 
(Table 1). 
Mortar consistency was assessed using both the slump 
and cone penetrometer tests outlined in Australian Standard 
2701 [14]. In addition, water retentivity and compressive 
strength testing was undertaken. Mortar strengths were 
obtained from three 50-mm cubes of each mix tested in 
uniaxial compression. The cubes were tested saturated, after 
curing immersed in water for 28-to-30 days. Loading was 
applied at a steady rate ofl.2 MPa/min. up to failure. Water 
retention properties were assessed by comparing the flow 
table consistency of fresh mortars before and after samples 
had been subjected to a suction pressure of 50 mm Hg [14]. 
In general, fresh mortar samples are prepared to provide a 
flow of between 105% and 115% when subjected to 25 
drops of the table over 15 seconds. The water retention of 
the mortar, 1L, is determined from: 
P,.= F2/F 1 x 100% 
where: 
F 2 = flow after suction 
F 1 = flow immediately after mixing and before suction. 
3. RESULTS AND DISCUSSION 
3.1 Block tests 
3.1.1 Compressive strength 
Failure in uniaxial compression was similar to that 
observed in standard concrete and mortar cube testing. 
Results for both average and unconfined characteristic 
block compressive strengths are given in Table 3. For 
consistent compactive effort, compressive strength is pri- 
marily a funct ion of cement and clay contents. 
Compressive strengths reduced steadily with increasing 
clay mineral content, Fig. 1. However, although clay is 
clearly detrimental to block performance, a small quan- 
tity is necessary during initial production. Experimental 
results are in general agreement with previously reported 
work [10, 13]. 
Cement undergoes a three-phase stabilising reaction 
with the clay minerals during hydration [3]. The reduc- 
tion in compressive strength with increasing plasticity 
can primarily be attributed to the weakening effect of 
clay minerals on bonding between the cement paste and 
inert soil matrix. As clay content increases, the sand and 
fine gravel content decreases and block strengths are 
reduced. As clay content increases, the effectiveness of 
cement can also be impaired by the presence of small 
pockets of unstabilised cohesive soil which may form 
during wet mixing [10]. 
Cement stabilised soil blocks are ideal for low-rise 
residential construction, where min imum strength 
r) 
4 
0 
0 
~ Average - 5% cement 
Average - 10% Cement 
�9 - ,~ - - Character i s t i c - 5% 
- - �9 - . Character i s t i c - 10% 
� 9 -# , " � 9 . . . . . . . 
t b i . . . . . "~ 
10 20 30 40 
C lay content (mass %) 
Fig. 1 - Influence of cement content on block compressive strength. 
547 
Materials and Structures/Mat~riaux et Constructions, Vol. 30, November 1997 
Cement 
content 
(mass %) 
5% 
10% 
Clay 
content 
(mass %) 
9 
14 
18 
26 
35 
40 
9 
14 
18 
26 
35 
40 
Average 
dry 
density 
(kg/m 3) 
(c.v.) 
1733 
(0.3%) 
1702 
(0.3%) 
1659 
(0.7%) 
1584 
(0.6%) 
1503 
(o. 7%) 
1408 
(0.6%) 
1775 
(0.3%) 
1757 
(0.5%) 
1671 
(0.3%) 
1620 
(1.o%) 
1520 
(1.4%) 
1452 
(0.9%) 
Table 3 - Block properties 
Average 
compressive 
strength 
(MPa) 
(c.v.) 
3.67 
(5.1%) 
2.95 
(3.8%) 
2.34 
(2.7%) 
1.45 
(9.1%) 
0.39 
(14.3%) 
0.30 
(44.9%) 
7.11 
(6.5%) 
5.78 
(5.9%) 
4.71 
(3.4%) 
3.86 
(13.6%) 
2.91 
(8.6%)2.13 
(8.2%) 
Unconfined Average 
:haracteristk drying 
compressive shrinkage 
strength 
(MPa) (C.V.) 
2.18 0.011% 
(14.9%) 
1.80 0.020% 
(9.1%) 
1.45 0.033% 
(2.1%) 
0.80 0.067% 
(7.8%) 
0.19 0.167% 
(4.2%) 
0.05 0.244% 
(3.7%) 
4.13 0.031% 
(4.6%) 
3.39 0.026% 
(7.8%) 
2.89 0.042% 
(17. 7%) 
1.95 0.063% 
(9.9%) 
1.62 0.093% 
(5.2%) 
1.20 0.121% 
(2.8%) 
Average 
wet/dry test 
(% reduction 
in dry mass) 
(C.V.) 
4.9 
(16.2%) 
9.9 
(22, 7%) 
15.5 
(16.1%) 
38,2 
(10.9%) 
56.2 
(7.2%) 
75.7 
(16.7%) 
0.7 
(28.6%) 
1.2 
(30.0%) 
3.1 
(26.4%) 
5.5 
(50.6%) 
14.2 
(4.0%) 
25.7 
(2.8%) 
Average 
total water 
absorption 
(c.v.) 
14.6% 
(0.8%) 
16.4% 
(0.9%) 
18.1% 
(1.1%) 
21.1% 
(0.4%) 
24.8% 
(1.5%) 
27.3% 
(6.8%) 
13.1% 
(0.9%) 
14.4% 
(1.1%) 
17.2% 
(1.2%) 
20.7% 
(2.4%) 
23.1% 
(3.9%) 
25.9% 
(1.4%) 
Average 
initial rate of 
absorption 
(kg/m2/min) 
(c.v.) 
11.5 
(3.6%) 
11.4 
(2.0%) 
11.5 
(3,4%) 
12.2 
(2.2%) 
14.2 
(4.8%) 
13.8 
(4.6%) 
8.2 
(4.7%) 
9.4 
(8.6%) 
10.8 
(3.6%) 
9.9 
(5.9%) 
11.7 
(7.6%) 
13.4 
(3.2%) 
0.25 
0.2 
0.15 
0.1 
0.0, 
+,,: .... t I / 
10 20 30 40 
Clay content (mass %) 
Fig. 2 - Influence of clay content on drying shrinkage. 
50 
100 
80 
60 
40 
20 
7 
5% cement / 
/ 
10 20 30 40 
Clay content (mass %) 
50 
Fig. 3 - Influence of clay content on wetting/drying durability. 
requirements are often dictated by handling rather than 
load carrying requirements. For this purpose a minimum 
unconfined characteristic saturated compressive strength 
of 1.0 MPa may be considered satisfactory. Whilst all 
blocks in this study stabilised with 10% cement satisfied 
this requirement, blocks made using soils with greater 
than 20% to 25% clay mineral content proved unsuit- 
able at 5% cement stabilisation. 
3.1.2 Drying shrinkage 
Results for block drying shrinkage are outlined in 
Table 3 and Fig. 2. Drying shrinkage increased as clay 
mineral content increased. For low clay contents, the rate 
of increase in moisture movement was steady, but beyond 
25% to 30% there was a marked gain in block drying 
shrinkage, especially at 5% cement stabilisation. At low 
clay mineral content, shrinkage was greatest for blocks 
with the higher cement content, which might be expected 
for cementitious materials. For clay mineral contents 
above 25%, higher cement usage proved more effective at 
controlling shrinkage. Commonly used drying shrinkage 
limits between 0.08% and 0.10% [1, 16] limit block mix 
suitability in this study to those with clay contents less 
than between 25% and 35%, depending on cement usage. 
3.1.3 Durabil ity 
Results for durability assessment of the blocks in 
accordance with the ASTM wetting/drying test [17] are 
given in Table 3 and summarised in Fig. 3. The results 
were in-line with expectations and similar to those 
observed in compression, durability improving with 
increased cement usage and lower clay content. Durability 
548 
Walker, Stace 
performance deteriorated more rapidly once the clay con- 
tent exceeded 20% to 30%. In accordance with the 
ASTM requirement, test block durability was assured 
using soils with clay contents less than 15% and 30% for 
cement stabilisation at 5% and 10% respectively. 
3.1.4 Water absorption properties 
Total water absorption values and initial rates of 
absorption are also outlined in Table 3. Water absorption 
increases with clay content, as a greater proportion of water 
is adsorbed by the clay minerals, and reducing cement con- 
tent, as the stabiliser becomes less effective at stabilising the 
clay minerals. Porosity increased as clay content increased. 
Water absorption is unlikely to be a significant problem for 
earth block construction, as roof protection for durability 
will limit moisture ingress. However, such highly 
absorbent blocks are, in general, unlikely to prove suitable 
in applications such as damp proof coursing. 
Initial rates of absorption values show much less corre- 
lation with block mix details, Table 3. However, suction 
rates tend to increase with soil plasticity, which may in 
part be attributed to the moisture-attracting characteristics 
of an increasing clay mineral content. Past work with clay 
unit masonry has indicated that a suction rate in excess of 
1.5 kg/m2/min, may prove detrimental to mortar worka- 
bility and resultant bond strengths [18]. Although material 
characteristics obviously differ, experimental values sug- 
gest that compressed soil blocks may require the use of 
highly water-retentive mortars and/or pre-wetting during 
construction to reduce suction effects. 
for construction. This behaviour was exacerbated by 
increasing clay content. Thereafter, the water content nec- 
essary to achieve a suitable consistency was established on 
the basis of 'feel' and then quantified using the slump and 
cone impression tests. As a result, mortars with initial flow 
values as low as 75% were found to be acceptable, Table 5. 
Initial flow values tended to decrease with increasing clay 
content as the clay minerals increasingly contributed to 
workability. 
Slump testing provided the easiest and most consis- 
tent means of quantifying the consistency of soil:cement 
mortars. In general, slump values ranged between 5 and 
15 mm, with little variation as a result of changes in clay 
or cement content. Initial problems with the soil:cement 
mortars adhering to the inside of the slump cone were 
overcome by lightly coating it with mould oil. The cone 
impression test proved less reliable for measuring consis- 
tency, with values varying between 30 and 65 mm, Table 
5. Below an impression of 40 mm, the test is considered 
unreliable for conventional building mortars [14]. 
Water retention results, based on initial flow values 
shown, are summarised in Tables 4 and 5. For the mix 
ratios considered, the water retention values for the 
cement:lime:sand mortars were largely as expected. 
Water retentivity generally improved with increasing 
lime content. Except for the lowest clay content mix, the 
soil:cement mortars showed improved water retention 
properties compared with the conventional building 
mortars, Table 5. Retention tended to improve with clay 
content, which may in part be ascribed to the increasing 
3.2 Mortar tests 
3.2.1 Consistency and water 
retention 
Results for slump, cone 
impression, and initial flow are 
presented for the cement:lime: 
sand and soil:cement mortars 
in Tables 4 and 5 respectively. 
Consistency values for the 
cement:lime:sand mortars 
were in-line with expectations, 
with slumps between 15 mm 
and 20 mm, and cone impres- 
sion values between 50 mm 
and 55 mm, at an initial flow 
value of 110%. 
Consistency testing of the 
soil:cement mortars proceeded 
in a similar manner, initially 
establishing a suitable moisture 
content to give an initial flow 
of 110% + 5% [14]. However, 
this approach soon proved 
inappropriate for all but the 
lowest clay content mortars. 
To achieve an initial flow of 
110% _+ 5%, the soil:cement 
mortars were much too liquid 
Table 4 - Cement:lime:sand mortar properties 
Cement:lime:sand Cement Water/ Slump Cone Initial Water Average 
mix proportions content cement impression flow retention compressive 
ratio strength 
(volume %) (mass %) (by mass) (mm) (mm) (MPa) (C.V.) 
1:1:6 13.6 1.3 20 55 110% 66% 10.10(1.7%, 
1:2:9 9.3 1.9 15 50 110% 79% 4.06 (2.0%) 
1:3:12 7.1 2.6 15 55 110% 77% 1.93 (3.0%) 
Table 5 - Soil:cement mortar properties 
Cement Clay Water/ Slump Cone Initial Water Average 
content content cement impression flow retention compressive 
ratio strength 
(mass%) (mass%) (bymass) (mm)(mm) (MPa)(C.V.) 
9 3.9 15 55 101% 76% 1.47 (3.9%) 
18 4.6 5 30 83% 82% 1.60 (0.0%) 
5% 26 5.2 10 35 82% 91% 1.00 (0.0%) 
35 6.1 5 35 75% 86% 0.38 (2.6%) 
40 6.9 15 45 77% 98% 0.20 (6.4%) 
9 2.1 25 65 110% 72% 4.80 (2.1%) 
18 2.4 5 45 87% 87% 4.53 (2.5%) 
10% 26 2.9 10 40 88% 87% 3.47 (4.4%) 
35 3.1 10 40 84% 90% 2.13(2.7%) 
40 3.4 5 40 87% 87% 1.90 (9.1%) 
15% 
9 
18 
26 
35 
40 
1.4 
1.6 
2.0 
2.2 
2.5 
10 
10 
5 
10 
10 
50 
50 
40 
40 
45 
98% 
98% 
80% 
79% 
77% 
76% 
82% 
90% 
89% 
92% 
9.30 (6.5%) 
8.23 (1.4%) 
5.47 (2.8%) 
4.60 (2.2%) 
4.73 (2.4%) 
549 
Materials and Structures/Mat6riaux et Constructions, Vol. 30, November 1997 
10 
�9 5% cement 
�9 "-.....\ X ~ ..... Block-~0# 
10 20 30 40 
Clay content (mass %) 
Fig. 4 - In f luence o f clay content on mortar cube strength. 
9% clay )P 26% clay / 
8 --.o- 40% clay / 
/ 6 
4 ~ 
2 
0 I ! 
5 10 15 20 
Cementcon~nt(rnass%) 
Fig. 5 - Influence o f cement content on mortar cube strength. 
proportion of adsorbed water. There was little correla- 
tion between water retentivity and cement content. For 
workabil ity the water retentivity properties of 
soil:cement mortars seem better suited to the blocks 
than similar cement:lime:sand mixes. 
3.2.2 Compressive strength 
Compressive strengths for the cement stabilised soil 
mortars are outlined in Table 5 and Fig. 4. Mortar 
exhibited similar strength behaviour to that reported for 
the compressed earth blocks. For reasons outlined previ- 
ously, compressive strengths were closely related to both 
cement and clay Contents. Compressive strengths of the 
three cement:lime:sand mortars tested satisfied expecta- 
tions, Table 4. 
The soil:cement mortar strengths were generally less 
than the corresponding average block compressive 
strengths, varying between 40% and 100% of the aver- 
age block values. This is to be expected since the mortar 
mixes were not subjected to the same compactive effort 
as the blocks. For workability requirements, the initial 
moisture contents of the mortars were also significantly 
higher than the optimum Proctor values. This disparity 
between block and mortar strength was greatest for the 
low day content mixes. 
The influence of cement content on mortar com- 
pressive strength is outlined in Fig. 5. For clarity, only 
three of the five day contents considered are shown. For 
each soil mix tested, the relationship between strength 
and cement content is strongly linear over the experi- 
mental range (linear regression correlation coefficients 
varied between 0.981 and 0.996). 
Recommendations for compressed earth block con- 
struction commonly, and contrary to general masonry 
practice, suggest that mortars should have the same com- 
pressive strength as the masonry units [12]. The tradi- 
tional practice of using weaker mortars generally ensures 
that any cracking of the masonry occurs along the joints 
rather than through the units. On this basis, therefore, 
the range of soil:cement mortars tested would seem 
directly compatible with the corresponding block mixes 
without the need for additional cement stabilisation. 
Indeed, a reduction in cement usage may be acceptable 
in some cases. Weaker cement:lime:sand mixes would 
also appear strength compatible with the range of com- 
pressed earth blocks. 
A full assessment of material compatibility also 
requires flexural bond strength and durability testing of 
masonry panels. The work outlined in this paper repre- 
sents the initial findings of an on-going study, currently 
being undertaken by the authors, which includes bond 
strength testing of compressed earth block masonry. 
4. CONCLUSIONS 
This paper outlines experimental work undertaken 
to assess the physical characteristics of a range of com- 
pressed soil:cement blocks and compatible mortars. The 
main conclusions from this study may be summarised as 
follows: 
�9 Within the experimental range, block compressive 
strength, drying shrinkage, and durability are improved 
by increasing cement content and impaired by increasing 
clay content; 
�9 For cement stabilisation between 5% and 10%, test 
soil suitability was governed by the ASTM wetting/dry- 
ing durability test criteria. Depending on cement usage, 
soils with clay mineral contents less than 15% to 30% 
were most suitable; 
�9 Total water absorption and initial rates of absorp- 
tion increased with the clay mineral content of the test 
blocks. Block water absorption characteristics are indica- 
tive of the need to use highly water-retentive mortars; 
�9 Slump testing provided the most reliable means of 
quantifying soi l :cement mortar consistency. 
Conventional flow table and cone impression test crite- 
ria proved inappropriate for the soil:cement mortars. As 
expected, water retentivity of the soil:cement mortars 
improved with increasing clay mineral content. The 
retentivity of soil:cement mixes was generally superior to 
that of similar cement:lime:sand mortars; 
�9 Mortar compressive strength was enhanced with 
increasing cement and reducing clay contents. 
550 
Walker, Stace 
Compressive strength increased linearly with cement 
content over the experimental range. Due to reduced 
compaction and increased initial moisture contents, 
soil:cement mortar strengths were generally less than 
corresponding block values. On the basis of these lim- 
ited test data, soil:cement mortars would seem directly 
suited to their corresponding blocks without the need 
for additional cement stabilisation. 
ACKNOWLEDGEMENTS 
The authors wish to acknowledge the support of staff 
in the Department of Resource Engineering, University 
of New England, and Mr. Ken Dickins for his transla- 
tion of the abstract. This work was funded through the 
Australian Research Council's small grants scheme. 
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