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Geotechnical and Geological Engineering, 1992, 10, 117-134
Geotechnical properties of a chemically stabilized soil from Malaysia with rice husk ash as an additive
F. H A J I ALI , A. A D N A N and C H E W K A M C H O Y Department of Civil Engineering, Universiti Malaya, 59100 Kuala Lumpur, Malaysia
Received 2 December 1991
Summary
The stabilization of Malaysian soil by mixing with rice husk ash, a locally available waste material, to improve its engineering properties is described. Stabilizing agents, i.e. cement and lime, were added to produce the reaction products which are responsible for the enhancement of the engineering properties. Based on the strength development, it seems that lime is the more effective stabilizing agent. However, the cheap waste material can be used as partial replacement for the more expensive cement in the cement-treatment of the soil. A durability study was carried out to evaluate the effectiveness of this stabilization method.
Keywords: Soil stabilization, Atterberg limits, compaction test, unconfined compression test, X-ray diffraction.
Introduction
Several methods of soil improvement using hydraulic cementitious materials have been developed and used successfully in practice. Chemical improvement of soil is applied in a variety of engineering works, e.g. in the construction of cheap roads; in providing bases for pavements where good materials are not economically available; for reducing the permeability and compressibility of soils in hydraulic and foundation works; for stabilization of slopes, embankments and excavations. A considerable amount of research concerning stabilization of soil with agents such as cement, lime, lime plus additives such as fly ash and salts, bitumen and polymers is available in the literature. But soil stabilization with lime/cement and rice husk ash (RHA) is a relatively new method.
In recent years the use of various waste products in civil engineering construction has gained considerable attention in view of the shortage and high costs of suitable conventional aggregates, increasing costs of waste disposal and environmental constraints. Rice husk is a major agricultural by-product obtained from the foodcrop of paddy. For every 4 tons of rice
0960-3182/92 © 1992 Chapman & Hall
118 Haji Ali et al.
produced, 1 ton is rice husk. Rice husk has a chemical composition which typically corresponds to the following: cellulose (40-45%), lignin (25-30%), ash (15-20%) and moisture (8-15%). The ash is mainly derived from the opaline which is present in the cellular structure of husk and about 90% of which is silica. The silica content in the rice husk depends on the following: (a) the variety of the rice, (b) soil and climate conditions, (c) prevailing temperature, and (d) agricultural practices ranging from application of fertilizers and insecticides etc. The normal method of conversion from husk to ash is by incineration. Houstin (1972) reported that the properties of rice husk ash depend greatly on whether the husks have undergone complete destructive combustion or have only been partially burnt. Houstin classified rice husk ash into (1) high-carbon char, (2) low carbon (grey) ash and (3) carbon-free (pink or white) ash.
The silica content of the husk can be enriched by converting the husks into ash. A number of researchers (Bartha and Huppertz, 1974; Singh, 1977; Shah et al., 1979; Yeoh et al., 1979; Ibrahim and Helmy, 1981) have studied the physical and chemical properties of the ash. Chemically, rice husk consists of 82-87% silica which exceeds that of fly-ash. The high percentage of siliceous material indicates that rice husk ash can be an excellent material for stabilization, as previous research on fly-ash shows that the stabilized strength depends on the percentage of silicon and aluminium oxides in the fly-ash (Goeker and Handy, 1963; Vincent et al., 1961; Mateos and Davidson, 1962).
Materials used
Rice husk ash (RHA)
Rice husk is generally considered a worthless by-product of rice milling. At the mills, disposal of the husk is achieved by burning them in heaps near the mills. Before use the ash was oven- dried at 60 ° C. The properties of the ash are shown in Table 1. X-ray analysis of the ash is given in Fig. 1. The pattern of the diffractograph shows much scattering with only two peaks, 0.334 nm of quartz and 0.425 nm of tridymite, both of weak intensities.
Table 1. Composition and properties of open-field burnt ash
SiO 2 90.73% Ca (as an element) 0.022% Fe (as an element) 0.226% Cu (as an element) 0.002% Mn (as an element) 0.042% Moisture content 1.68% Loss on ignition 2.70% Specific gravity 2.12 Passing BS sieve:
BS No. 30 (600 pm) 87% BS No. 200 (75#m) 31%
Chemically stabilized soil with rice husk ash
! I I I
3O
- - 2 0 0.334 nm 0.425 nrn
I I I I 0 35 30 25 20 15 10
Diffraction angle (20, CuK~)
Fig. 1. X-ray diffraction analysis of rice husk ash
119
4O
Soil
A residual granite soil, typical of the Malaysian residual soils, was used in the study. Table 2 shows the properties of the soil. The particle size distribution curve of the soil is shown in Fig. 2. Figure 3 shows the diffractograph of the soil, i.e. soil sample with clay fraction only. Kaolinite clay mineral is identified by its strong diffraction line at 0.719 nm and 0.357 nm which disappeared when heated to 550 ° C. The positions of the two peaks are unaffected by glycerol treatment. A minor amount of mica group minerals is also detected by their diffraction lines at 1.0 nm and 0.33 nm.
Lime
The lime used is hydrated lime commercially called 'White Horse'. The pertinent properties of the lime as supplied by the manufacturer conformed to the requirement of Type 1 Grade A lime as specified in the standard specification for lime for soil stabilization (AASHTO M 216-68). The properties of the lime are given in Table 3. Fig. 4 shows the X-ray diffractograph of the lime. The presence of calcium hydroxide is identified by peaks at 0.490, 0.310, 0.262, 0.192, 0.179 and 0.168 nm. A small peak at 0.304 nm shows the presence of calcium carbonate.
Cement
The cement used was ordinary Portland cement. The physical and chemical properties of the cement are given in Table 4.
Laboratory tests
Compaction and strength tests
Compaction and strength tests were carried out on the residual soil with different mix proportions of RHA, lime and cement (by weight of dry soil). The test programme for the
120 Haft Ali et al.
Table 2. Properties of residual granite soil
Grain size distribution: Gravel 1% Sand 52% Silt 15% Clay 32%
Physical properties: Natural moisture content 27% Specific gravity 2.68 Linear shrinkage 15% Liquid limit 73% Plastic limit 36% Plasticity index 37%
Soil classification: Unified SC Textural Sandy clay AASHTO A-7-5
Engineering properties: Modified AASHTO density 1.71 Mg m- 3 Optimum moisture content 14.7% Unsoaked CBR 32% (density of 1.71 Mg m -a) Soaked CBR 2% (density of 1.71 Mg m -3) Swell 1.55%
I I I 100
90 ~
80 ~-
7o ~-
60
50 ~-
4O
30
20
l o -
10 -4
==
o
I I I I I I I I I I I I I
| | l l i l l l i l t l l i i l l i l i l l l i [ I l i l l I I i I I i l i , i i l l i i 10 -3 10 -2 10 I 10 0 101 10 2
Particle size (rnm)
Clay Fine I MediUms,, I C°arse I Fine I MediUmsand I Coarae Fine I MediUmGravel I Coarse Cobbles
Fig. 2. Grain size distribution for the residual soil
0.994 nm
(a)
0.332 nm
0.357 nm
0.334 nm ...__,__.,AJ (c)
0.357 nm
(b)
L719 nm
Chemically stabilized soil with rice husk ash 121
0.719 nm
L I I 1 I I 1 ! ! ! i ! I ! 3 0 28 26 24 22 20 18 16 14 12 10 8 6 4
Dif f ract ion angle (28, CUE.)
Fig. 3. X-ray diffraction analysis of the residual soil: (a) heated, (b) glycerol treated and (c) untreated
122
Table 3. Composition and properties of hydrated lime
Ca(OH)2 90_+ 1% C a C O 3 6% Mg(OH)2 3% Fe20 3 0.15% A120 3 0.5% SiO 2 0.9% Moisture content 0.5%
Passing BS sieve: BS No. 100 (150 pro) 99% BS No. 200 (75 #m) 95%
Haft All et al.
0.262 nm
0,179 nm 0.192 nrn 01310 n m
I I I I I I 55 50 45 40 35 30
Diffraction angle (20, CuKq)
Fig. 4. X-ray diffraction analysis of lime
0.490 nm
I I l 25 20 15 10
Table 4. Properties of ordinary Portland cement
Chemical properties (%): CaO 64.86 SiO 2 20.44 A120 3 5.50 MgO 1.59 SO a 1.96 Fe20 3 2.18 Loss of ignition 1.51 Insoluble residue 0.31 Free lime 1.65
Physical properties: Fineness (cm 2 g- l ) 2975 Specific gravity 3.12
Chemically stabilized soil with rice husk ash 123
invest igat ion is shown in Table 5. In the compac t ion tests, the soil was thoroughly mixed
with various mois ture contents and allowed to equil ibriate for 24 h prior to compact ion . The
dry dens i ty -mois tu re re la t ionship was determined using the s tandard compac t ion method
A A S H T O T 180-74 (Method A).
Table 5. Compaction and unconfined compression strength tests (UCS) carried out on stabilized residual soil
UCS tests UCS tests curing temperature curing temperature of 30°C of 60°C
Compaction Curing period (days) Curing period Mix description tests 7 28 56 28 days
(% lime+ % RHA) 3 + 0 * * * 3+6 * * * 3+12 * * * 3+18 * * * 6 + 0 * * * 6+6 * * * 6+12 * * * 6+18 * * * 9 + 0 * * * 9 + 6 * * * 9+12 * * * 9+18 * * *
(% cement + % RHA) 3 + 0 * * * 3+6 * * * 3+12 * * * 3+18 * * * 6 + 0 * * * 6+6 * * * 6+12 * * * 6+18 * * * 9 + 0 * * * 9 + 6 * * * 9+12 * * * 9+18 * * *
The unconf ined compressive strength is the most useful method of evaluat ing the effectiveness of s tabi l izat ion (Herr in and Mitchell, 1961; Lambe, 1962; Mateos, 1964; Kat t i et al., 1966; Ingles and Metcalf, 1972; Hossain , 1986; Rahman , 1987). Each specimen used in the unconf ined compress ion tests was statically compacted in the 50 m m by 100 m m mould at the relevant o p t i m u m mois ture conten t and to the modified A A S H T O m a x i m u m dry
124 Haft Ali et al.
density determined earlier. Specimens were cured in groups of three, in a thermostatically controlled incubator set at 30 ° C _+ 2 ° C before being loaded in compression. Curing times adopted were 7, 28 and 56 days, for lime-RHA-soil mixtures, and 7 and 28 days for cement- RHA-soil mixtures. At least three specimens were tested for each case. To study the influence of curing temperature on unconfined compressive strength, the mixtures containing 6% and 9% lime were moist cured in an oven for 28 days where forced air circulation was maintained at 60 ° C.
Durability tests
The wet-dry test was used to evaluate the durability of the specimens. The procedure suggested by Hoover et al. (1958), which were adopted for this study, are as follows:
(a) Specimens were prepared at the designated density and optimum moisture content, then moist cured for a specific period.
(b) Specimens were air dried for 24 h at room temperature and then completely immersed in distilled water for 24 h. This completed one cycle of drying and wetting.
(c) After the designated cycles of drying and wetting the height and weight of the specimens were measured before testing for unconfined compressive strength.
Results and discussion
Compaction and strength properties
In the compaction test results (Fig. 5), addition of RHA alone (i.e. for 0% lime) decreases the maximum dry density but increases the optimum moisture content. RHA cannot be used alone in the soil stabilization because of its lack of cementitious properties. The maximum dry density of RHA-soil mixture is reduced by the presence of the RHA owing to its relatively low specific gravity. The increase in the optimum moisture content may be caused by the absorption of water by the RHA.
t . 8 - 2 5
1.7
E
E
1.4
e ' ' ' ~ ' - e ' ' " ~ " " " t ~ e6% RHA
~
~ ~ ' ~ 1 2 % RHA-
~ ~ " ~ 18 "/, RHA-
23
c
Zt o o~
E
E K o
t 7
I I I I
~ . ~ ! 8 % RHA
. . ~ 1 2 % RH/ I
6% RHA
w i I i 15 I I I ! 0 3 6 9 12 0 3 6 9
Lime content (%) Lime content (%) t2
Fig. 5. Variation of compaction characteristics of the residual soil with lime-RHA content
Chemically stabilized soil with rice husk ash 125
Figure 5 also shows that for a given RHA content addition of lime decreases the maximum dry density of the stabilized soil, but increases the optimum moisture content. As noted by earlier workers in lime-treatment of soil (Lu et al., 1957; Remus and Davidson, 1961; Mateos and Davidson, 1962; Wang et al., 1962; Mateos, 1964; Compendium-8, 1979), addition of lime leads to an immediate decrease in the maximum dry density of the soil and an increase in the optimum moisture content, for the same compactive effort. The decrease in the maximum dry density of the treated soil is reflective of the increased resistance offered by the flocculant soil structure to the compactive effort (Nagaraj, 1964). The increase in optimum moisture content is probably a consequence of the additional water held within the flocculant soil structure resulting from lime interaction. Fig. 5 illustrates that by increasing the RHA content, the maximum dry density decreases further and at the same time the optimum moisture content increases. The presence of RHA in excess of the amount required for reaction with the lime, may have reduced the dry density. This increase in moisture content is probably caused by additional water absorbed by the excess RHA.
Figure 6 shows the effects of adding cement and RHA on the compaction characteristics of the soil. It can be seen that the maximum dry density increases slightly, except for a RHA content of 18%, and the optimum moisture content decreases when the cement content is increased. However, addition of RHA has the opposite effect.
I.'1
E
._~ 1.6
E t.~
1.8 2 5
1.4
t . 3
I I I
~ 6 % R H A
...~....-~t2% RHA
• • ~_._._..._~18% RHA
23
E E 21 oo 0~
E t9
E
o
17
I I I I
~ ! 2 % RHA
• ~ ~--"'=6% RHA
I I I I t 5 ,, I I I I 0 3 6 9 t2 0 3 6 9
Cement content (%) Cement content (%) 12
Fig. 6. Variation of compaction characteristics of the residual soil with cement-RHA content
The effects of adding RHA on the unconfined compressive strength of soil-lime mixtures after curing for 7, 28 and 56 days are shown in Fig. 7. A general pattern is observed in which the strength develops rapidly with addition of RHA until an optimum is reached, beyond which the strength begins to decrease. The maximum strength significantly varies with curing time.
Addition of lime alone can bring about several beneficial changes in the engineering properties of fine-grained soils. Treatment with lime is observed to improve the strength
126 Haji Ali et al.
t.B
1.6
E
1.4 .¢
• ~ 1.2
0 . 8
0 . 6
I
(a) I I I
9% l ime
~ , , ~ 6% Ilml
0 6 12 18 24
I I I I
(b)
9%1/IT,
%l!me
l ime
I
(el
I i I
ii'iii' I I I I I I I I 0 6 t2 t8 24 0 6 12 t8
Rice husk ash content (%)
Fig. 7. Variation of unconfined compressive strength of the residual soil with lime-RHA content (a) 7 days, (b) 28 days and (c) 56 days
2 4
characteristics of the soil (Laguros et al., 1956; Lu et al., 1957; Herrin and Mitchell, 1961; Lambe, 1958; Mateos, 1964; Katti et al., 1966; Ingles and Metcalf, 1972; Compendium-8, 1979). The unconfined strength test results obtained in this investigation show that the improvement in strength in the lime-stabilized soil can be enhanced by adding a certain amount, say between 6 to 12%, of rice husk ash. It may also be inferred from the test results that, in a lime-RHA-residual soil mixture, a lesser amount of lime is required to achieve a given strength as compared with a lime-residual soil mixture. Since lime is much more expansive than RHA this will result in cheaper construction costs. In tropical countries where rice husks are abundant and considered to be waste materials, use of RHA in the construction of roads, airfields and other earthworks is particularly attractive because this would generally lend itself to low-cost applications, help alleviate disposal costs and environmental damage and conserve high-grade construction materials for higher priority u s e s .
The gain of strength of lime-stabilized soil is regarded primarily as a result of pozzolanic reaction between amorphous silica and/or alumina from the soil and lime to form various types of cementing agents. By introducing rice husk ash to the soil, additional amounts of amorphous silica are available for reaction with lime resulting in further increases in strength.
The drop in strength due to a further increase in RHA after the optimum amount may be attributed to the decrease in the maximum dry density as a result of the presence of RHA in excess of the amount required for reaction with the available lime.
Figure 8 shows the variation of unconfined compressive strength with curing period. It can be seen that addition of RHA produces not only higher strength but also higher initial rate of strength development, 6% RHA content gives an average increase in strength of 35 % as the curing time is increased from 7 to 28 days at 30 ° C, while 9% RHA content gives an increase of 49% for the same increase in curing time. Fig. 8 also shows that the rate of strength
Chemically stabilized soil with rice husk ash 127
1 . 8
t . 6
E z a::
~ 1.2
~ 1.0
0 . 8
0.6
I I I
1 ~ 6 % RHA t2% RHA I
IB °/o RHA
OV, RHIa i
7 28 56
f 6% RHA
/ j........-,~2% RHA - ~ / ~ |8% RHA
I 1%RHA(b )
7 28 56 Curing period (days) Curing period (days)
Fig. 8. Variation of unconfined compressive strength of lime-RHA-soil mixture with curing period, mixtures with (a) 6% and (b) 9% lime
development reduces at the later stages. The dependence of strength development on curing provides a considerable factor of safety for designs based on say 7-, 28- or 56-day strength.
To study the influence of curing temperature on unconfined compressive strength, the mixture containing 6% and 9% lime were moist cured in an oven for 28 days where forced air circulation was maintained at 60 ° C. The results are illustrated in Fig. 9. Samples cured at 60 ° C showed a significantly higher rate of strength development. It is interesting to note that an increase in temperature not only results in development of higher strength in lime treatment of soil but also enhancement of this strength when RHA is added. Since temperature is relatively high in tropical countries like Malaysia, the use of RHA is very suitable in this type of soil stabilization.
The effect of adding cement and RHA on the unconfined compressive strength of the soil is shown in Fig. 10. It seems that there is an optimum value of rice husk ash content (about 6 %) for each cement content. By hydrating Portland cement, calcium hydroxide (lime) is liberated which reacts with the rice husk ash to produce additional cementitious compounds. When the RHA content is in excess of the amount required for the reaction, the strength begins to drop. There is also a significant increase in the maximum unconfined strength with the curing period.
Figure 11 compares the strength development in lime-RHA mixtures and cement-RHA mixtures for a RHA content of 12%. It can be seen that higher strength is developed in the lime-RHA mixtures at all stages of the curing period. It should be noted that lime is the more effective stabilizing agent.
X-ray diffractographs of the lime-RHA-soil mixtures cured for 7, 28 and 90 days are shown in Fig. 12. After curing for 7 days new peaks appear at 1.263, 1.077, 0.818, 0.628, 0.419, 0.304, 0.288 and 0.229 nm with smaller peaks at 0.540, 0.512, 0.280, 0.262, 0.249, 0.237 and
128 Haft Ali et al.
2 . 6
2 . 4 g-. I
E z 2.2
c
~ 2.c
0 o
e -
0
c
1 . 4
Ik
9% lime
m
6 % lime
t . 2 r I I ! 0 6 ~2 18 2,1.
Rice husk ash content (%)
Fig. 9. Variation of unconfined compressive strength of lime-RHA-soil mixture cured at 60°C with RHA content
0.180 nm. The peaks for untreated residual soil, as observed in Fig. 3, are considerably reduced in intensity.
Extending the curing time to 28 days resulted in the disappearance of the peaks at 0.262 nm (i.e. for lime). New peaks appeared at 1.263, 0.759, 0.628, 0.419, 0.385, 0.304, 0.249, 0.228 and 0.191 nm, with additional smaller peaks at 0.288, 0.280, 0.237 and 0.180 nm. By further extending the curing time to 90 days, more peaks disappeared in the diffractograph. Peaks observed at 1.236, 0.759, 0.628, 0.419 and 0.288 nm reveal the presence of dicalcium aluminate monosilicate-8-hydrate, C2ASH s (Stratling's compound). In a study by Jambor (1963) the same compound was reported to result from reactions between lime and activated kaolin. Croft (1964a,b) identified Stratling's compound as one of the products produced in the lime-kaolinite reactions at 40 ° C. The rest of the peaks are attributable to the tetracalcium aluminate-13-hydrate [C4AH13 ] and calcium silicate hydrate 1 [CSH (1)].
Durability
A stabilized soil should be durable, i.e. it should have the ability to retain its integrity and strength under 'in service' environmental conditions. The determination of the durability
Chemically stabilized soil with rice husk ash 129
1.6
t.4
E Z
12
"~ 1.¢
E a
.-~ 0 . 8
0 .6
I
7 d a y s
I I I
28
: 3 % c e m e n 1
(a) t I
18 24 0 Rice husk ash content (%)
~ ~ ) ° / . ce men
6% c e m e n t - -
I I I 0 6 12
I I I I
6% c e m e n t
f
3%cement-
(b) I I
t2 t8 24
Fig. 10. Variation of unconfined compressive strength of the residual soil with cement- RHA content. (a) 7 days and (b) 28 days
properties of a stabilized soil is a problem because it is difficult to simulate exactly in the laboratory the detrimental effect acting on the soil in the field. Many different test conditions have been used for this purpose, e.g. freezing and thawing, heating and cooling, and wetting and drying. Of these only the last two conditions are relevant to tropical areas.
The results of the durability tests are shown in Table 6 and Fig. 13. Table 6 shows the reduction in the unconfined compressive strength of the specimens subjected to 12 cycles of wetting and drying, expressed as a proportion of the strength of specimens moist cured for the same period but not subjected to the wet-dry process. It can be seen that the strengths of specimens with three different compositions (i.e. 9% lime and 0% RHA; 9% lime and 12% RHA; and 9% lime and 18% RHA) and moist cured for 28 days, drop to 32, 60 and 57% of the original strengths, respectively, after being exposed to 12 cycles of wetting and drying. The sample with 12% RHA content retains the highest strength after the wetting and drying.
Comparing the strength ratios, it seems that addition of RHA significantly enhances the durability of the stabilized soil.
Comparing the strengths of the 'moist-cured only' samples given in Table 6 and the strengths of the samples subjected to wetting and drying shown in Fig. 13, a general pattern is observed in the figure in which the strength is drastically reduced after the first cycle and an improvement in strength for subsequent cycles. This may be caused by the combination of deterioration induced during wet-dry cycles and the gain in strength owing to the curing effect.
It may be inferred from the figure that addition of RHA produces not only stronger, but also more durable samples as compared with those samples treated with lime only. Comparing the detrimental effect of saturation on samples with RHA contentsof 12% and 18%, the former is less affected because it is nearer to the optimum RHA content.
130 Haji Ali et al.
1.8
1.6
C E
v . = 1.4
>= ~ .2
8 '
~ 1.0
¢ -
:3
0.8
I I I
Ash: t 2 % by weight of dry soil
-- y 9 % lime -
/
0 . 6 f I 7 2 8 5 6
~ ~ % cement
Curing period (days)
Fig. 11. Comparison between unconfined compressive strength development for lime- RHA and cement-RHA stabilized soil samples
E = E c E
( c )
~ E
= m • ~ c! d o d ~ r~ ~, ~ ~, ~ o = o b c5 ~ g
E E E E E E E E ~ o ~ E e
o ~ ~ ~ ~ ~ o ~o ~ ~ a d o c~ c~ c5 u2 ~ ~q c5
| I I I I I I I 50 45 40 35 30 25 20 15 10
Di f f rac t ion a n g l e (20, CuK~)
Fig. 12. X-ray diffraction analysis of lime-RHA-soil mixtures. (a) 7 days, (b) 28 days, (c) 90 days
Chemically stabilized soil with rice husk ash
Table 6. Redaction in the unconfined compression strength of soil-lime-RHA mixture after being subjected to cycles of wet-dry
131
Mix proportion A B A C D C (%lime+%RHA) (MNm -2) (MNm -1) B (MNm -2) (MNm -2)
9+0 0.255 0.816 0.31 0.430 1.343 0.32 9 + 12 0.810 1.250 0.65 0.850 1.411 0.60 9 + 18 0.748 1.173 0.64 0.769 1.343 0.57
A: Unconfined compressive strength (UCS) of specimen moist cured for 14 days and subjected to 12 wet-dry cycles B: UCS of specimen moist cured for 14 days C: UCS of specimen moist cured for 28 days and subjected to 12 wet-dry cycles D: UCS of specimen moist cured for 28 days.
Conclusions
From the results of the study the following conclusions can be derived:
(1) In a lime-RHA-residual soil mixture, addition of lime and RHA increases the optimum moisture content and reduces the maximum dry density for the same compactive effort. However, the strength gain in the soil will more than compensate for changes in compaction optima.
(2) In a cement-RHA-residual soil mixture, addition of cement slightly increases the maximum dry density for RHA content less than 18 %, and decreases the optimum moisture content. However, by increasing the RHA content, the maximum dry density is reduced and the optimum moisture content is increased.
(3) The developments of the unconfined compressive strength of lime-stabilized and cement-stabilized residual soils are enhanced by adding RHA. However, for a given lime or cement content there is an optimum value of RHA content which corresponds to the maximum unconfined compressive strength. The optimum rice husk ash content is about 6%. Comparing the strength developments, lime is the more effective stabilizing agent.
(4) The reaction products responsible for the strength development in lime-RHA-residual mixture are calcium silicate hydrate [CSH] gel which after prolonged curing transforms into a more crystallized calcium silicate hydrate 1 [CSH(1)], tetracalcium aluminate-13-hydrate [C4AH13] and Stratling's compound [C/ASHs].
(5) The enhancement of unconfined compressive strength development by adding RHA is influenced by the curing period and temperature. As the curing time and temperature increases, the rate of strength development is intensified by the addition of RHA. The results are regarded as advantageous especially for applications in tropical countries where the temperature is relatively high.
(6) Addition of RHA enhances not only the strength development but also the durability of lime-stabilized residual soil. A stabilized soil with the optimum RHA content suffers the least detrimental effects of saturation. Therefore, it can be inferred that the use of RHA in the chemical-treatment of residual soil for construction of roads, airfields, etc. would require reduced annual maintenance costs.
132
0 . 8 ¸
.>
0 6
e -
0.2
14 day moist cured
/ 2 7 5 A
Haft All e t a l .
28 day moist cured I j i
~ A
T f [ r
"~ o "~
10
5
0
I 1 l
! I ! !
1 t 1 i
I r Y !
1.1
~- 0.9 o .E
~.! 0..5
oO
~- 0..3 X LI.I
0.1
I 1 i I
C I T , T 0 4 8 12
Cycles of wet dry
1 i l 1
~ A
I I l I 0 4 8 ' !2
Cycles of w e t dry
Fig. 13. Effect of wetting and drying on strength, moisture absorption and expansion of lime-RHA stabilized soil. A--9% lime; B--9% lime + 12% RHA; C- -9% lime + 18% RHA
Chemically stabilized soil with rice husk ash 133
(7) In tropical countries where rice husks are abundant and considered as waste materials, use of RHA in the construction of roads, airfields and other earthworks is particularly attractive, because this would generally lead to cheaper construction costs, help alleviate disposal costs and environmental damage and conserve high-grade construction materials for higher priority uses.
References
Bartha, P. and Huppertz, E.A. (1974) Structure and crystallization of silica in rice husk, in Proceedings of the Rice Byproducts Utilization international Conference, Valencia, Spain.
Compendium-8 (1979) State-of-the-Art: Lime stabilisation, Transportation Research Circular, Transportation Research Board, Natl. Research Council, Washington, DC, pp. 45-75.
Croft, J.B. (1964a) The processes involved in the lime stabilization of clay soils, in Australia Road Research Board Proceedings, 2(2), pp. 1169-203.
Croft, J.B. (1964b) The pozzolanic reactivities of some New South Wales fly ashes and their application to soil stabilization, in Australia Road Research Board Proceedings, 2(2), pp. 1144-67.
Geoeker, G.R. and Handy, R.L. (1963) Lime-clay mineral reaction products, Highway Research Board Bulletin, 129, 63-80.
Herrin, M. and Mitchell, H. (1961) Lime-soil mixtures, Bulletin 304, Highway Research, National Research Council, Washington, DC.
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