Effect of the Cement–Rice Husk Ash on the Plasticity and Compaction of Soil   Emhammed. A. Basha, Roslan Hashim Department of Civil Engineering, University of Malaya, Kuala Lumpur, Malaysia and Agus S. Muntohar Department of Civil Engineering, Muhammadiyah University of Yogyakarta, Yogyakarta, Indonesia e-mail: muntohar@telkom.net

ABSTRACT

The chemical stabilisation of soils was studied by using cement and rice husk ash. Three types of soils, residual soils, kaolinite and bentonite, were used in the study. The experimental study included the evaluation of the main index properties of the soil and compaction, along with a characterisation of the materials through X-Ray diffraction. Test results show that both cement and rice husk ash reduces the plasticity of soils. In term of compactability, addition of rice husk ash and cement decreases the maximum dry density and increases the optimum moisture content. From the viewpoint of plasticity, compaction characteristics, and economy, addition of 6 – 8 percent cement and 10 – 15 percent rice husk ash are recommended as an optimum amount.

Keywords: chemical stabilisation, cement, rice husk ash, consistency limits, and compaction.

INTRODUCTION

As a result of the increase in the amount of solid waste all over the globe, engineers and researchers carry out many investigations to find use for such wastes. The transportation, construction, and environmental industries have the greatest potential for re-use because they use vast quantities of earthen materials annually. Replacement of natural soils, aggregates, and cement with solid industrial by-product is highly desirable. In some cases, a by-product in inferior to traditional earthen materials, but its lower cost makes it an attractive alternative if adequate performance can be obtained. In other cases, a by-product may have attributes superior to those of traditional earthen materials. Often select materials are added to industrial by-products to generate a material with well-controlled and superior properties.

Mehta (1985) studied the possibility of using fly ash in cement and concrete works. Investigators such as Gidley and Sack (1984) proposed several methods for utilizing some industrial wastes in engineering construction. Tay (1987) studied the use of sludge ash in portland cement concrete works. Other studies examined the possibility of improving soil properties such as increasing shear strength, reducing settlement, and minimizing swelling problems by using solid waste. Kamon and Nontananandh (1991) combine industrial waste with lime to stabilize soil. Atom and Al-Sharif (1998) evaluated burned olive waste for use as soil stabilizer, which a partial solution to the problems associated with the increase of olive waste in Jordan.

In the agriculture countries, such as Indonesia, Malaysia, Philippines, Thailand, and many others, the problems were encountered with abundance of that agriculture waste. Those plants obtain various minerals and silicates from earth in their bodies during growth process. Inorganic materials, especially silicates, are found in higher proportions in annually grown plants than in the long-lived trees. Rice, wheat, sunflower, and tobacco plants therefore contain higher amounts of silica in their cuticle parts. Inorganic materials are found in the forms of free salts and particles of cationic groups combined with the anionic groups of the fibres into such plants (Rydhlom, 1965).

Incineration of organic materials, production of new crystalline phases, or crystallization of amorphous material are exothermic processes that lead to ash production and loss in the total weight. The result of burning organic materials is called thermal decomposition. The ash produced in this way is ground to a fine size and mixed with lime in order to obtain a material with a binding characteristic. The quality of this material depends on burning time, temperature, cooling time, and grinding conditions (James and Rao, 1986a; 1986b). The primary objective of this study is to examine the potential of burnt agricultural by-product, rice husk, for using this material for stabilising soils. The effects on the consistency and density of some various soil types are studied.

EXPERIMENTAL STUDY

Materials Used

Soils

Three different soils types were used in this study, i.e. residual soils, and commercial available soils. Residual granite soil is typically Malaysia residual soil that used in this study. Table 1 and 2 show the properties of the three soils types. Kaolin and bentonite, locally commercial product, were used. Figure 2 shows the diffractograph of the soils.

Kaolinite clay mineral was identified in the residual soil by its strong diffraction line at 3.57 Å, which disappeared when heated to 550o C. Kaolinite was also detected in the kaolin by its strong diffraction lines at 7.13 Å and 3.56 Å. The two peaks remain, even though their intensities were significantly reduced, after heating. Wyoming bentonite was predominantly comprised of Na-montmorillonite.

Figure 1. X-Ray Diffractograph of soils used: Residual Soils, Kaolin, and Bentonite

Figure 2. Variation of Consistency Limits with Cement Percentage

Table 1. Properties of the Kaolin and Bentonite.

Properties	Malaysia Kaolin	Wyoming Bentonite
Physical properties:
Moisture Content	< 5%	Max. 12%
Specific Gravity	2.6	2.4 – 2.8
Particles:
< 2 mm	26.0–35.0 %
< 10 mm	85.0– 98.0 %
< 74 mm		65% (dry), 97% (wet)
< 44 mm		95% (wet)
Chemical Composition:
Silica (SiO2)	45.0- 50.0 %	63.02%
Alumina (Al2O3)	33.0- 39.0 %	21.08%
Iron Oxide (Fe2O3)	< 1.0 %	3.25%
Potash (K2O)	< 2.0 %	2.57%
Magnesia (MgO)	< 1.0 %	2.67%
Loss on Ignition	10.5 - 13.0 %	0.72%

Figure 3. Variation of Consistency Limits with RHA Percentage

Table 2. Properties of the Residual Soil.

Properties	Value
Physical properties:
Natural moisture content	26%
Liquid Limit	36.77%
Plastic Limit	22.95%
Plasticity Index	13.82%
Linear Shrinkage	6.71%
Specific Gravity	2.37
Particles:
Sand	46 %
Silt	44 %
Clay	10 %
Chemical Composition:
Silica (SiO2)	71.16%
Alumina (Al2O3)	16.15%
Iron Oxide (Fe2O3)	4.98%
Potash (K2O)	1.46 %
Magnesia (MgO)	0.25%
Loss on Ignition	5.61%

Rice husk ash

Rice husk was considered a valueless by-product of rice milling. At the mills, disposal of the hulls was achieved by burning them in heaps near the mills. Even though, the ashes have been potential pozzolanic materials suitable for use in lime - pozzolana mixes and for portland cements replacement (Payá, et al, 2001). The ashes used in this study were obtained from burning of rice husk in the incinerator. The properties of the ashes are presented in Table 3.

Table 3. Physical and Chemical Properties of the Cement and RHA.

Properties	Cement	RHA
Physical properties:
Moisture Content		3.68%
Specific Gravity	3.12	2.08
Fineness	2975 cm2/g	12.5% (Retained 45 mm sieving)
Chemical Composition:
Silica (SiO2)	20.44 %	93.15%
Alumina (Al2O3)	5.50%	0.21%
Iron Oxide (Fe2O3)		0.21%
Calcium Oxide (CaO)	64.86%	0.41%
Potash (K2O)		22.31%
Magnesia (MgO)	1.59%	0.45%
Loss on Ignition	1.51%	2.36%
PH	12.06	9.83
3CaO.SiO2	66.48%
2CaO.SiO2	10.12%
3CaO.Al2O3	8.06%
4CaO.Al2O3.Fe2O3	9.43%
Free Lime	1.65%

Cement

The cement used was general portland cement. The physical and chemical properties of the cement are given in Table 3.

Laboratory Tests

Atterberg limits tests

The Atterberg consistency limits testing and reporting was carried out in accordance with British Standard methods - BS 1377: Part 2 (1990). The residual soil was sieved with 425 mm. Materials that retained on that sieve were rejected for this test. Since the kaolin and bentonite used were either fine, no sieving was carried out. The soils, then, were oven dried for at least 2 hours before the test. The tests were carried out on the soils with different proportion of cement and rice husk ash.

Compaction tests

Proctor standard compaction test (BS 1377 – 1990: Part 4) was used to determine the maximum dry density (MDD) and the optimum moisture content (OMC) of the soils. The soil mixtures, with and without additives, were thoroughly mixed with various moisture contents and allowed to equilibrate for 24 hours prior to compaction. The first series of compaction tests was aimed at determining the compaction properties of the unstabilised soils. Secondly, tests were carried out to determine the proctor compaction properties of the clay upon stabilisation with varying amounts of cement and rice husk ash.

RESULTS AND DISCUSSION

Effect on the consistency limits

The effect of cement and rice husk ash (RHA) stabilised soils on the liquid limit (LL) and plasticity index (PI) on the different soils are shown in Figure 2 and 3. In this context, it was illustrated a different characteristics of each soils. Reduce in plasticity of cement/RHA stabilised - residual soils as result of increase in liquid limits and plastics limits, in contrast with cement stabilised bentonite. For cement/RHA treated kaolin, liquid limits decreases while the plastic limits increase corresponds to cement/RHA increases.

It has been observed that cement and RHA reduces the plasticity of all soils. Cement treated bentonite reduced significantly the PI of bentonite as shown in Figure 4(a), while blend of RHA with cement indicated no significant improvement. Cement – RHA mixture exhibited further improvement for kaolinite soils as illustrated in Figure 4(b). It proved that cement works effectively with bentonite or expansive clays. Grim (1968) mentioned that Ca2+ cation will easily replace Na+, this term possible occurred in the cement stabilised bentonite. In general, 6 – 8 % of cement and 10 – 15 % RHA shows optimum amount to reduce plasticity of soils. Reduce in plasticity index is an indicator of improvement.

Figure 4. Effectiveness of Additives on PI of Kaolin and Bentonite

Effect on the compactability

Table 4 present the maximum dry density (MDD) and the optimum moisture content (OMC) for various cement and RHA percentage on the three soils type. Figure 5, 6, and 7 shows respectively the effect of the addition of cement, RHA, and cement – RHA mixtures on the compaction characteristics of the soils tested.

Table 4. MDD and OMC of Cement/RHA - Stabilised Soils.

		Residual Soil			Kaolinite			Bentonite
Additives	Percentage	OMC	MDD		OMC	MDD		OMC	MDD
		%	Mg/m3		%	Mg/m3		%	Mg/m3
Cement	0	15.26	1.76		32.60	1.30		31.20	1.26
	2	19.4	1.65		29.00	1.33		36.80	1.23
	4	21.80	1.59		30.00	1.33		37.40	1.20
	8	22.90	1.56		35.20	1.28		38.70	1.23
	12	24.65	1.54		34.17	1.27		37.45	1.24
RHA	0	15.26	1.76		32.60	1.30		31.20	1.26
	5	17.10	1.66		30.90	1.34		28.50	1.23
	10	19.90	1.58		32.00	1.29		27.70	1.18
	15	23.70	1.49		32.45	1.26		35.25	1.17
	20	24.50	1.44		32.75	1.27		35.70	1.16
4% Cement	0	21.80	1.59		30.00	1.32		37.40	1.20
+ RHA	5	20.00	1.52		35.10	1.24		39.00	1.15
	10	25.40	1.45		38.00	1.18		43.50	1.09
	15	27.40	1.36		40.70	1.12		46.65	1.05
	20	31.50	1.35		42.10	1.10		49.25	1.04
8% Cement	0	22.90	1.56
+ RHA	10	27.70	1.27
	20	32.20	1.25
	25	36.50	1.21

Figure 5 shows that adding cement slightly increased the OMC and diminish a small amount of the MDD correspond to increasing of cement percentage. Except for bentonite, a little increment of the MDD was observed associated with increases in cement content. In contrast with RHA stabilised soil, the MDD decreases further and at the same time the OMC increases as shown in Figure 6. The same characteristics can be observed also when 4% cement was blended with RHA as illustrated in Figure 7. The increase in optimum moisture content is probably a consequence of two reason: (1) the additional waster held with the flocculant soil structure resulting from cement interaction, and (2) exceeding water absorption by RHA as a result of its porous properties, as reported in Zhang et al (1996). Principally, increase in dry density is an indicator of improvement. But unfortunately, both cement and RHA, instead, reduces the dry density. Rahman (1987) revealed an opinion that the change-down in dry density occurs because of both the particles size and specific gravity of the soil and stabiliser.

Figure 5. Variation of Compaction Characteristics
of the Soils with Cement Content

Figure 6. Variation of Compaction Characteristics
of the Soils with RHA Content

Figure 7. Variation of Compaction Characteristics
of the Soils with 4% Cement – RHA mixture

Decreasing of dry density indicates that it need low compactive energy (CE) to attain its maximum dry density, as the result the cost of compaction will be economical (Muntohar and Hantoro, 2000).

CONCLUSIONS

The following conclusions can be drawn on the basis of the test results obtained from cement – RHA stabilised soils.

Cement and rice husk ash reduced the plasticity of residual soil, kaolin, and bentonite. A considerable reducing was attained by cement-stabilised soils. Each soil type exhibited a specific characteristic of reduction.

In general, 6 – 8 % of cement and 10 – 15 % RHA shows optimum amount to reduce plasticity of soils. Reduce in plasticity index is an indicator of improvement.

The maximum dry density of cement – stabilised residual soil and kaolin slightly decreased with increases in cement content, in contrast with cement-stabilised bentonite. Adding rice husk ash and cement increase the optimum moisture content of all soils.

Rice husk ash can potentially stabilise the expansive and non-expansive soils solely or mixed with cement and or lime. The utilising is an alternative to reduce construction cost, particularly in the rural area of developing countries.

REFERENCES

British Standard Institution, 1990, Methods of test for soils for civil engineering purposes, BS 1377 – 1990: Part 2, and 4.

Gidley, J.S. and Sack, W.S., 1984, Environmental aspects of waste utilization in construction, Journal of Environmental Engineering, ASCE Vol. 110 (6), pp. 1117 – 1133.

Grim, R.E., 1968, Clay Mineralogy, 2nd Ed., Mc. Graw Hill, New York, USA, ch. 7, p. 185 – 233.

James, J., and S.M. Rao, 1986a, Silica from rice husk through thermal decomposition, Thermochimica Acta, Vol. 97, pp. 329 – 336.

James, J., and S.M. Rao, 1986b, Reactivity of rice husks ash. Cement and Concrete Research, Vol. 16, pp. 296 – 302.

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Payá, J., Monzó, J., Borrachero, M.V., Mellado, A., and Ordoñez, L.M., 2001, Determination of amorphous silica in rice husk ash by rapid analytical method, Cement and Concrete Research, Vol. 31, pp. 212 – 231.

Rahman, M.A., 1987, Effect of cement-rice husk ash mixtures on geotechnical properties of lateritic soils, Journal of Soils and Foundations, Vol. 27 (2), pp. 61-65.

Rydholm, S.A., 1965, Pulping Processes, Interscience Publishers, New York, p. 1049 – 1053.

Tay, J.H., 1987, Sludge ash as filler for portland cement, Journal of Environmental Engineering, ASCE Vol. 113, pp. 345 – 351.

Zhang, M.H., Lastra, R., and Malhotra, V.M., 1996, Rice husk ash paste and concrete: Some aspects of hydration and the microstructure of the interfacial zone between the aggregate and paste, Cement and Concrete Research, Vol. 26 (6): 963 – 977.

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