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Effect of burning temperature onalkaline reactivity of rice husk ash withlimeLeonardo Behaka & Washington Peres Núñezb

a Geotechnical Department, Faculty of Engineering, University ofthe Republic of Uruguay, Montevideo, Uruguayb Post-Graduation Program in Civil Engineering, Federal Universityof Rio Grande do Sul, Porto Alegre, BrazilPublished online: 02 Apr 2013.

To cite this article: Leonardo Behak & Washington Peres Núñez (2013) Effect of burningtemperature on alkaline reactivity of rice husk ash with lime, Road Materials and Pavement Design,14:3, 570-585, DOI: 10.1080/14680629.2013.779305

To link to this article: http://dx.doi.org/10.1080/14680629.2013.779305

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Road Materials and Pavement Design, 2013Vol. 14, No. 3, 570–585, http://dx.doi.org/10.1080/14680629.2013.779305

Effect of burning temperature on alkaline reactivity of rice husk ashwith lime

Leonardo Behaka* and Washington Peres Núñezb

aGeotechnical Department, Faculty of Engineering, University of the Republic of Uruguay, Montevideo,Uruguay; bPost-Graduation Program in Civil Engineering, Federal University of Rio Grande do Sul, PortoAlegre, Brazil

(Received 4 March 2012; final version received 10 February 2013)

The rice husk ash (RHA) is a by-product of rice milling, being used as a soil stabiliser to buildroads, an economical alternative with environmental benefits. A research of the influence of kindand temperature of burning on the reactivity of RHA and mixtures with sandy soil and lime wasmade. A no controlled temperature RHA and RHAs done with different controlled temperatureswere used. X-ray diffraction analyses and loss on ignition tests were carried out on RHAs. X-raydiffraction analyses, unconfined compressive strength, and splitting tensile strength tests wereconducted on mixtures of sandy soil with different RHAs and lime. The results showed that theoptimal reactivity of the RHA is reached for a range of controlled temperature of 650–800◦C,providing a significant increase on the strength and stiffness of mixtures.

Keywords: road material; soil stabilisation; rice husk ash; burning temperature

1. IntroductionThe rice husk is a by-product of the rice milling. About 108 tonnes of rice husk is generatedannually in the world (Alhassan & Mustapha, 2007). In Uruguay, 1.5 × 106 tonnes of rice areproduced annually while in Brazil 107 tonnes are produced (Behak & Nuñez, 2008). Accordingto Haji Ali, Adnan, and Choy (1992), every 4 tonnes of rice produced, 1 tonne is rice huskwhich means that in Uruguay and Brazil approximately 375,000 tonnes and 2.5 × 106 tonnes aregenerated annually, respectively. The final disposition of such quantities of rice husk is a seriousproblem around the world.

The rice husk is burned in order to reduce the volume to be deposited. To give an economicalbenefit to this burning, the rice husk is used as fuel for furnaces to dry the rice, Portland cementproduction, power generation, etc. The rice husk contains about 80% volatile organic compoundsand water, and the balance 20% of the weight of this husk is converted into ash during the burningprocess, which is known as rice husk ash (RHA) (Juliano, 1985). The RHA is a new residue andits final disposition is also a serious problem.

According to Malhotra and Metha (1996), the pozzolanas are defined as siliceous or siliceousand aluminous materials, which in themselves possess little or no cementing property, but chem-ically reacts with calcium hydroxide, in the presence of water at ordinary temperature, to formcompounds possessing cementitious properties. The RHA contains the highest concentration ofsilica of all plant residues (Boateng & Skeete, 1990), being around 90% amorphous silica (Juliano,1985). The calcium hydroxide required for chemical reactions can be provided by the lime. Soilstabilisation is obtained by the addition of RHA and lime for road pavements and is particularly

*Corresponding author. Email: lbehak@fing.edu.uy

© 2013 Taylor & Francis

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attractive in countries where rice husk is abundant because it leads to cheaper construction andlesser disposal costs, reduces environmental damage and preserves the most highly qualifiedmaterials for priority use (Haji Ali et al., 1992). The stabilisation of sandy-silty soils with RHAand lime reduces building costs, particularly in rural counties of developing countries (Basha,Hashim, Mahmud, & Muntohar, 2005).

The reactivity of the RHA significantly depends on the burning process of husk. Houston (1972)proposed to classify the RHA according to the burning conditions in high-carbon char (black),low-carbon ash (grey) and free-carbon ash (pink or white). The colours are associated with theevolution degree of the combustion process and with the structural changes of the silica in theash (Boateng & Skeete, 1990). The white colour indicates the total oxidation of the carbon inthe ash, while very high temperatures and long periods of incineration produce pink ashes typicalof crystalline silica. The RHA quality depends on the temperature, incinerating time, cooling timeand milling conditions (James & Rao, 1986). According to James and Rao (1986), the silica in theash suffers structural transformations with temperature conditions affecting the reactions betweenthe RHA and lime and the properties of the soil-RHA-lime mixtures. The type of ash suitable forthe pozzolanic reactivity is amorphous rather than crystalline (James & Rao, 1986). Rice huskincineration at temperature ranging from 550◦C to 700◦C has been found to produce amorphoussilica while temperatures in excess of 900◦C produce unwanted crystalline structures. However,Smith and Kamwanja (1986) observed formation of crystalline silica in small proportions fortemperatures of about 800◦C maintained for 12 h. Metha (1978) established that a highly reactiveash can be produced by maintaining the combustion temperature below 500◦C under oxidisingconditions for relatively prolonged period or up to 680◦C provided the high temperature exposurewas less than 1 min. Prolonged heating above this temperature may cause the material to convert(at least in part) into crystalline silica. Chopra, Ahluwali, and Laxmi (1981) have reported thatfor incineration temperatures up to 700◦C, the silica was predominant in the amorphous form andthat the crystals present in the ashes grew with burning time. Nehdi, Duquette, and El Darmatty(2003) state that silica in RHA can remain in the amorphous form at combustion temperaturesof up to 900◦C if the combustion time is less than 1 h, whereas crystalline silica is produced at1000◦C with combustion time greater than 5 min. Other reports claim that crystallisation of silicacan take place at temperatures as low as 600◦C, 500◦C, or even at 350◦C with 15 h of exposure(Bui, 2001).

The structural changes at several temperatures affect the reactivity of the RHA since the largerthe specific surface of silica the greater the extent of chemical reactions with lime (Boateng &Skeete, 1990). The technologies of ash production vary from open-heap burning to speciallydesigned incinerators (Metha, 1979). When the rice husk is burned in open heaps or in theconventional oven, crystalline ash with low reactivity index is produced while when is incineratedin an oven with controlled temperatures, the residue is a highly reactive white ash that mixturedwith lime changes into a cement structurally as good as Portland cement (Metha, 1975). Therice husk incinerated in oven at controlled temperature conditions between 800◦C and 900◦Cverified a high reactivity of ash in comparison with the ash resulting from the open-heap burning(Boateng & Skeete, 1990).

Carbon content of RHA influences the stabilisation process, retarding the reactions and produc-ing low increases of strength. The avidity of carbon by calcium ions interfere with the reactionsbetween calcium ions and amorphous silica (Petry & Glazier, 2005). According to these authors,lime stabilisation of soils with 6% of carbon is economically impracticable. Rahman (1987) mea-sured a remaining carbon content less than 3% in an RHA obtained by burning in an oven atcontrolled temperature of 800◦C.

The aim of this paper is to present a research of the effects of both type and temperature ofincineration on the RHA reactivity and stabilisation of a sandy soil of Uruguay with RHA and

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Table 1. Physical characteristics of used material.

Material

Physical characteristics Rice husk RHAr Soil Lime

Particle size distribution (%)Passing #4 100.0 99.6 98.4 100.0Passing #10 90.5 97.5 87.3 100.0Passing #40 3.9 71.0 28.5 99.5Passing #200 0.8 11.8 6.5 92.9< 2 μm – 0.8 6.0 1.6

Atterberg limits (%)Plastic index No plastic No plastic No plastic No plasticSpecific gravity 1.46 1.81 2.65 2.48

lime. For this scope, RHA made without temperature control into a furnace (named in this paper asresidual RHA) and RHAs made in a laboratory oven at different controlled temperatures (RHATC)were used.

2. Materials2.1. Residual RHA and rice huskThe residual RHA (RHAr) and rice husk for laboratory controlled temperature incineration werecollected from Arrozur, a rice parboiled plant sited in the city of Treinta y Tres, north-easternUruguay. The rice husk is used in Arrozur as a fuel for rice-dried furnaces. The incinerationprocess is done without temperature control and the temperature greatly varies in the furnace dueto its large size. A leaf-shaped and black RHAr results from rice dried which can be classified ashigh-carbon ash according to Houston (1972). Given these characteristics might be expected anash with low pozzolanic reactivity. The values of physical characteristics of used materials aregiven in Table 1.

The particle size distribution of the rice husk and RHAr is shown in Figure 1. The RHAr iscoarse-sized, with 88% of weight retained on #200 sieve and 11% of the fine fraction greater than2 μm. A quite high content of organic of 18.7% was verified by loss on ignition analysis. Theparticle size distribution of RHAr is finer than that of the rice husk because of the volatilisation

Figure 1. Particle size distribution of residual RHA and rice husk.

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Figure 2. X-ray diffractograph of residual RHA. Cr, cristobalite; C, carbon.

of coarse elements of rice husk during the incineration process. Among these, 99% of rice huskis retained on #200 sieve. The specific gravity of rice husk and RHAs grains was, respectively,1.46 and 1.81. During the incineration process of rice husk are volatilised their lighter elementssuch as organic matter, remaining the heaviest in the ash, as is the silica. As a result, the RHArspecific gravity is greater than that of the rice husk.

Characteristic peaks of cristobalite are identified from the X-ray diffractograph of the RHAr(Figure 2) which indicates that part of the silica which is in the crystalline state and, therefore,is not reactive. The peak of carbon confirms the presence of crystallised organic compounds inthe ash.

2.2. SoilA sedimentary sandy soil with low content of fines from the quaternary period was collected froma quarry sited 24 km west of Montevideo. This soil is constituted by 1% of gravel, 92% of sand,1% of silt and 6% of clay. The soil classifies as well-graded silty sand (SW-SM) according to theUnified Soil Classification System and classifies as A-1-b by the American Association of StateHighway and Transportation Officials (AASHTO) classification system. The X-ray diffractographof the sedimentary soil (Figure 3) shows the presence of quartz typical of the sandy fraction andthe main components of the clay fraction are kaolinite and montmorillonite.

2.3. LimeA commercial lime manufactured in Uruguay was used. This lime constituted 66% of calciumoxide, 5% of magnesium oxide and others elements like silica and ferric oxide. The lime was finewith 100% of weight passing the #10 sieve and 93% passing the #200 sieve, whereas 91% wasgreater than 2 μm.

3. Methodology3.1. RHA at controlled temperatureRice husk was incinerated into the oven at controlled temperatures of 500◦C, 650◦C, 800◦C and900◦C (RHA500, RHA650, RHA800 and RHA900, respectively). Due to the low-volume capacity

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Figure 3. X-ray diffractograph of sandy soil. Qz, quartz; K, kaolinite; Mm, montmorillonite.

of the oven and low specific density of the rice husk, 30–40 g of rice husk were placed in aporcelain vessel and then burned for 4 h in turns. After the burning process, the produced ash wasair-cooled. The ash reactivity depends on the burning and cooling time (James & Rao, 1986).Smith and Kamwanja (1986) have reported that temperatures of 800◦C maintained for 12 h givesmall proportions of crystalline silica. The complete burning took 7–9 h in an incinerator speciallybuilt by Boateng and Skeete (1990), depending on the ambient conditions. However, the burningtime was arbitrarily fixed by the authors because of the study scope was only focused in theinfluence of burning temperature on the ash reactivity and properties of soil-RHA-lime mixtures.Rice husk was weighted before each one of the incineration turns and produced ash was weightedafter each turn.

3.2. Mixture specificationsThe unconfined compression strength (UCS) of mixtures of soil with RHA and lime generallyincrease with the RHA content up to a maximum value beyond which increases of RHA contentcause a decrease of UCS (Haji Ali et al., 1992). The authors observed an increase of strength inmixtures of sandy clay soil with lime and 6% and 12% of RHA. A research of Basha et al. (2005)showed that the UCS of mixtures of silty sandy soil with cement can be increased by adding15% and 20% of RHA. This latest range of RHA content was taken for the research since theRHAr to be used was supposed to be of low reactivity. Lime is a commercial product so it is ofinterest to use a lower content of lime to obtain an economic benefit in the use of soil-RHA-limemixtures. A lime content of 5% was adopted for research because it is generally assumed thatlime content between 3% and 5% is economically admissible. Another lime content sample of10% was adopted in order to compare results, although it is understood that such lime content isnot economical.

The three mixtures of sandy soil with RHAr and lime adopted for the research are summarisedin Table 2. Only one mixture of soil with 15% of RHA at controlled temperature (RHACT) and 5%of lime was evaluated due to the difficulties to produce high volumes of RHACT in the used oven.Initially, air-dried soil was manually mixed with the corresponding content of RHA and lime ina dry state. Both the RHAr and RHACT were not milled even though the increase of the specificsurface of ash benefits their reactivity. The ash volumes to be used in road materials are higherthan to building concrete materials. In addition, the construction of roads in developing countries

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Table 2. Combination and denomination of used mixtures.

Combination

RHAr (%) RHACT (%) Lime (%) Denomination

0 0 0 Control soil15 0 5 15RHAr-5L20 0 5 20RHAr-5L20 0 10 20RHAr-10L0 15 5 15RHACT -5L

is made of practically craft form with a lot of manpower and little machinery. As conclusion, theash milling for road materials would be impractical and uneconomical, so that was not taken intoaccount in the research.

Thereafter, water was added at optimum moisture content (OMC) of standard Proctor test andthen manually mixed. Specimens were compacted in moulds immediately after water additionto reach a maximum dry density (MDD) of the standard Proctor test. After compaction, thespecimens were put into plastic bags and cured in a humidity room at ambient temperature.

3.3. Testing proceduresX-ray diffraction tests were performed on samples of soil, RHAr, RHACT and mixtures of soilwith RHA and lime cured for 28 days. A dusty diffractometer with Cu K-alpha radiation andwavelength of 1.5418 Å was used. The used samples were obtained by milling the specimensof the UCS test immediately after tests. The milling was made in a mortar to reach a grain sizeless than 0.075 mm. Loss on ignition tests were carried out on RHAr and RHACT to determinethe organic content. A temperature of 1000◦C was adopted for loss on ignition tests due to themaximum burning controlled temperature of 900◦C.

The MDD and OMC for soil and soil-RHA-lime mixtures were determined by standard Proc-tor compaction tests in accordance with AASHTO T99 (1986). UCS tests were conducted byAASHTO T208 (1986) on soil, soil-RHAr-lime mixtures for 7, 14, 28 and 56 days and mixturesof soil with RHACT to 600◦C and 800◦C and lime for 28 days. Specimens were statically com-pacted in a tri-split metallic mould with an internal diameter of 3.72 cm and a height of 7.65 cm.Three specimens of each mixture were tested to reach the peak strength.

The development of tensile strength is indicative of pozzolanic products formation that cementsthe soil grains due to the alkaline reactions between the silica of ash and calcium ions of lime(Seddom & Bhindi, 1983). Also, the tensile strength development enables one to evaluate theefficiency of the stabilisation in the strength gain. For these reasons, splitting tensile strength(STS) tests were conducted on mixtures of soil with different contents of RHAr and lime with14, 28 and 56 days in accordance with ASTM D6931 (1989). Cylindrical specimens 10.14 cm indiameter and a 6.39 cm in height were compacted by the standard Proctor compaction test method.Each mixture was tested in triplicate and tests were carried out to break the specimens.

4. Test results and discussion4.1. Characteristics of controlled temperature RHAsThe loss on ignition of rice husk, expressed by the ash weight to husk weight ratio, as a functionof ignition temperature of husk is shown in Figure 4. The ash weight – husk weight ratio is in the

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Figure 4. RHA weight to rice husk weight ratio as a function of incineration temperature.

range of 15–20%, which is consistent with the results of Juliano (1985) and Haji Ali et al. (1992).The loss on ignition decreases when the incineration temperature increases up to 650◦C fromwhich the ash weight – husk weight ratio tends to be constant. The elements of rice husk, such ascellulose, lignin and water can be volatilised at temperatures up to 650◦C so that for temperaturesbeyond 650◦C the weight loss is similar.

The change of ash colour with incineration temperature, as was established by Houston (1972),was confirmed during the rice husk burning. The RHA500 was grey with black points and theRHA650, RHA800 and RHA900 were whitish grey with a pinkish tone. The pinkish tones of RHA800and RHA900 were more intense than in the RHA650. According to Houston (1972), the RHA500and RHA650 may be classified as low-carbon ashes while the RHA800 and the RHA900 are free-carbon ashes. The greater the incineration temperature, the produced ashes were thinner and morepointed and more brittle, as can be observed in Figure 5.

X-ray diffractographs of RHACT are shown in Figure 6 where it is also included the X-raydiffractograph of the RHAr for comparison purposes. The X-ray diffractographs of RHA500,RHA650, and RHA800 are similar and did not show well-defined peaks, which indicates thatthe produced ashes are amorphous rather than crystalline. The RHA900 presented a diffrac-tograph similar to RHAr diffractograph with well-defined peaks for diffraction angles (2θ )of 22◦ and 36.35◦, characteristic of cristobalite. According to these results, silica crystallisa-tion would begin at controlled temperatures between 800◦C and 900◦C when the incinerationtime is 4 h.

The organic content of RHACT, expressed as ash weight loss on ignition at 1000◦C, linearlydecreases with the increase of rice husk incineration temperature (Figure 7). A remarkable dif-ference in organic content is observed between the RHAr and RHACT. Both the control oftemperature and method of incineration are of great importance when the objective is to pro-duce RHA with high pozzolanic reactivity. The organic content of RHA500 (7.8%) and RHA650(4.1%) are relatively high for stabilisation purposes. On the other hand, the organic content ofthe RHA800 (2.3%) may be acceptable. Therefore, it might be recommendable to burn RHA attemperatures above 650◦C in order to prevent the effects of the organic compounds on pozzolanicreactions.

For studied ashes, the optimum range of incineration temperature for soil stabilisation would beof the range 650–800◦C if the incineration time is of 4 h. Ashes of optimum pozzolanic reactivityare produced with amorphous structures and negligible contents of organic compounds in thiscontrolled temperature range.

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Figure 5. Appearance of RHACT as a function of incineration temperature.

Figure 6. X-ray diffractographs of RHAr and RHACT.

4.2. Compaction characteristics of mixturesThe dry density-moisture content relation curves from standard Proctor compaction tests carriedout on control soil and mixture of soil with 20% of RHAr and 10% lime (20RHAr-10L) is shownin Figure 8. The Proctor compaction curve of control soil is oddly shaped without an MDD andOMC defined. This shape of the compaction curve is typical of sandy soils and it is due to theimpact effort used in the Proctor test it is not efficient to compact sandy soils. The compactioncurve of the tested mixture is similar in shape to that of the control soil because the compactionwas performed immediately after the mixing without enough time to the development of alkaline

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Figure 7. Organic content of RHA as a function of incineration temperature.

Figure 8. Dry density–compaction moisture relation curves of control soil and 20RHAr-10L mixture.

reactions. The addition of RHA and lime does not produce textural changes to the original soilwhich results in a similar trend compaction curve for the mixture.

The optimum compaction parameters adopted for control soil and soil-RHA-lime mixtures arepresented in Table 3. The MDD of mixture is lesser than that of control soil meanwhile the OMCis greater. The results are in accordance with other researchers and can be explained as follows.The specific gravity of RHA and lime are lower than the specific gravity of the soil so when addedto soil, the specific gravity of the mixture is reduced (Haji Ali et al., 1992). One portion of thewater added to the mixture is absorbed by the RHA due to its porous characteristic, as reportedby Zhang, Lastra, and Malhotra (1996). Other portion of the added water is consumed by the limehydration which is required for alkaline reactions. As a result, the soil stabilisation with RHA iswater consuming and it is needed to add a greater moisture content to reduce the suction effect inthe pores and to reach the greatest compaction efficiency.

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Table 3. Optimum compaction parameters of soiland soil-RHA-lime mixtures.

Material MDD (kN/m3) OMC (%)

Control soil 17.4 5.520RHAr-10L 13.6 8.5

Figure 9. X-ray diffractographs of mixtures of soil with RHACT and lime. Qz, quartz; Al, albite.

4.3. X-ray diffraction of mixturesThe X-ray diffractographs of the mixtures of soil with RHA650 and lime and RHA800 and limeafter 28 days are similar and is shown in Figure 9. The observed peaks of quartz correspond tothe soil and remain in the mixtures because the quartz is crystalline and cannot chemically react.New peaks of albite, a calcium aluminium silicate, are observed and its presence proves thatthe pozzolanic reactions between the amorphous silica of the RHA and the calcium ions of thelime have occurred after 28 days. The similarity among the X-ray diffractographs shows that thepozzolanic reactivity of the RHA is quite independent of controlled incineration temperatures inthe range of 650–800◦C.

The X-ray diffractographs of mixtures of soil with RHAr and lime are presented in Figure 10.The mixtures of soil-RHAr-lime present new peaks of a pozzolanic product, named as antigonite.The presence of lime in the 20RHAr-10L mixture is due to the excess lime that has not reacted up to28 days. It is to be noted that was difficult to identify the pozzolanic products in the soil-RHA-limemixtures which would indicate that the pozzolanic reactivity of the RHACT is higher than that ofthe RHAr, especially at controlled temperatures between 650◦C and 800◦C. The RHACT contentneeded to obtain similar reactions is lower than the RHAr content. The use of RHA incinerated atcontrolled temperatures between 650◦C and 800◦C is more efficient and economical to stabilisesandy soils.

4.4. Unconfined compression strengthThe results of unconfined compression tests on sandy soil with different types of RHA, vari-ous percentages of RHA and lime, and curing times are summarised in Table 4. The UCS of

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Figure 10. X-ray diffractographs of mixtures of soil with RHA and lime. Qz, quartz; Ill, illite; Cr,cristobalite; An, antigonite; L, lime.

Table 4. Unconfined compressive strength values of soil-RHA-lime mixtures.

control SoilMaterialTime (days) UCS (kPa) UCSAvge (kPa) SD

8.20 13.9 13.7 54

19.1

15RHAr-5L 20RHAr-5L 20RHAr-10LMaterialTime UCS UCSAvge UCS UCSAvge UCS UCSAvge(days) (kPa) (kPa) SD (kPa) (kPa) SD (kPa) (kPa) SD

88.1 77.3 176.57 86.1 90.3 5.6 112.5 95.3 17.6 104.5 173.5 4.3

96.7 96.1 170.450.0 141.7 207.3

14 139.8 126.4 19.0 107.5 128.2 18.2 186.2 209.6 24.6113.0 135.4 235.2135.3 186.3 265.8

28 77.3 117.7 25.0 178.3 181.0 4.6 369.0 248.8 24.1100.0 178.4 231.7107.7 139.1 202.7

56 126.6 132.9 28.9 211.8 225.6 19.4 254.1 275.1 98.7164.5 239.3 368.4

15RHA650-5L 15RHA800-5LMaterialTime UCS UCSAvge UCS UCSAvge(days) (kPa) (kPa) SD (kPa) (kPa) SD

670.0 531.728 638.9 654.4 22.0 437.1 504.3 58.5

400.0 543.9

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Figure 11. UCS of soil-RHA-lime mixtures as a function of RHA type, RHA and lime content, and curingtime.

mixtures with RHAr increases with the RHAr and lime content as can be seen in Figure 11. Themaximum UCS value corresponds to the 20RHAr-10L mixture and it was at 28 and 56 days,respectively, 18 and 20 times greater than the UCS of control soil. The increase in ratio of UCSwas greater with the increase of lime content from 5% to 10% rather than with the increase ofRHAr content from 15% to 20%. Thereby, when the RHAr content was increased maintainingthe lime content constant, the resulting UCS were 1.1, 1.3, 1.5, and 1.5 times greater at 7, 14,28, and 56 days, respectively. Meanwhile, when the lime content was increased from 5% to 10%maintaining the RHAr content constant, the UCS was 1.6 times greater at 7 and 14 days and1.4 times greater at 28 and 56 days. A high lime content is required to reach high strengths dueto the relatively low reactivity of the studied RHAr, however, such lime content is not advis-able because the resultant material would be very expensive. The little change of the UCS whenthe RHAr content was varied from 15% to 20% indicates that a 20% of RHAr is close to theoptimal content.

An increase of the UCS with the curing time is observed for all mixtures with RHAr being theincrease ratio greater for earlier edges. With 7-curing days, the UCS values were 6.6, 6.9, and10.9 times greater for the15RHAr-5L, 20RHAr-5L, and 20RHAr-10L mixture, respectively. For14-curing days, the increase was, respectively, 1.1, 1.3, and 1.4 times greater when compared withthe UCS at 7 days. The strength gain at 56 days with respect to control soil was significant, 10,14, and 20 times for 15RHAr-5L, 20RHAr-5L, and 20RHAr-10L mixtures, respectively, despitethe relatively low reactivity of this ash type. As can be seen in Figure 11, an increase of 5% inlime content accelerates the pozzolanic reactions at an early age while these reactions are slowerwith the increasing of the ash content.

A notable increase of UCS resulted when the RHATC were added to mixtures. The UCS valueswere greater than the UCS of all mixtures with RHAr, regardless the RHAr and lime content andcuring time. The UCS values of 15RHA650-5L and 15RHA800-5L mixtures after 28 days were,respectively, 41.6 and 36.8 times greater than that of the control soil. For a constant curing time,the UCS of mixtures with RHA650 and RHA800 were, respectively, 3.1 and 2.0 times greater thanthe UCS of 20RHAr-5 L mixture and were 2.3 and 2 times greater than that of the 20RHAr-10L mixture. When compared with the highest UCS value, corresponding to the 20RHAr-10Lmixture with 56 days, the UCS values of the mixtures with RHA650 and RHA800 after 28 dayswere, respectively, 2.1 and 1.8 times greater.

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The highest UCS values obtained for mixtures with RHACT in the temperature range of 650–800◦C demonstrates the higher pozzolanic reactivity of the RHACT with respect to the RHAr. Itwas necessary that lower contents of RHA and lime to reach greater strengths when RHACT wasused. Therefore, the use of RHA burned at controlled temperature between 650◦C and 800◦Cwould be a more economic alternative with respect to the use of RHAr. The UCS values of testedmixtures with RHACT were quite similar which evidences a similar pozzolanic reactivity for bothincineration temperatures. The UCS of the mixtures of soil with RHACT and lime cured for 28days would be independent of the incineration temperature in the range of 650–800◦C.

4.5. Splitting tensile strengthThe results of STS tests carried out on soil-RHAr-lime specimens for 14, 28, and 56 days aresummarised in Table 5. The trend of the STS with the RHAr and lime content and curing time(Figure 12) verified the occurrence of alkaline reactions between the amorphous silica of the RHArand the calcium ions of the lime that generate cementitious products. The STS values increase

Table 5. STS values of soil-RHAr-lime mixtures.

15RHAr-5L 20RHAr-5L 20RHAr-10LMaterialTime STS STSAvge SD STSAvge STS STSAvge(days) (kPa) (kPa) SD (kPa) (kPa) SD (kPa) (kPa) SD

32.3 36.5 48.714 30.4 32.0 1.5 27.9 30.4 5.3 51.2 46.2 6.7

33.3 26.9 38.552.3 30.6 65.7

28 60.1 56.1 3.9 46.4 46.1 15.4 74.5 69.2 4.755.9 61.5 67.479.0 52.0 76.9

56 87.2 82.4 4.3 64.3 62.6 2.3 95.5 95.0 17.881.1 61.0 112.5

Figure 12. STS of soil-RHAr-lime mixtures as a function of RHAr and lime content and curing time.

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Figure 13. Relationship between UCS and STS of soil-RHAr-lime mixtures.

with an increase in the RHAr and lime content and with the curing time. The STS gain is fast upto 14 days for both mixtures with 5% of lime and up to 28 days for the remaining mixture. Fromthis curing time, the strength gain ratio is lower and it tends to be constant. The curing time wouldbe extremely important to the gain of STS.

Thompson (1965) found a linear relationship between the STS and UCS in soils stabilised withlime with a typical ratio of 0.13 despite the lime content and curing time. Other researches hadfound similar relations for cases of soil stabilisation with fly ash and lime (Ceratti, 1979; Kolias,Kasselouri-Rigopoulo, & Karahalios, 2005; Thome, 1994) with a ratio between 0.10 and 0.15. Asimilar linear relationship between the UCS and STS was defined for the case of soil stabilisingwith RHA and lime as is presented in Figure 13. However, it must be noted that the resultant ratio(0.27) is quite greater than those ratios are reported by other authors. More research should bedone to adjust the relationship between UCS and STS.

5. ConclusionsThe following conclusions can be drawn on the basis of the results obtained from the tests doneon mixtures of soil with RHAr, RHACT, and lime.

The organic content of the RHA was allowed for incineration controlled temperatures above650◦C which indicates a low limit of temperature necessary to produce a highly reactive RHA.Amorphous silica was found in the RHA for incineration controlled temperatures up to 800◦C,meanwhile for 900◦C, the silica is rather crystalline. An upper limit temperature to reach a highlyreactive RHA would be of 800◦C. For the purpose of stabilisation of sandy soils, the optimumincineration temperature range of RHA is of 650–800◦C. The RHA would be of highest pozzolanicreactivity within this temperature range.

The MDD of mixtures of soil with RHA and lime is lesser than that for control soil and theOMC being greater. The specific gravity of RHA and lime are lower than the soil specific gravityof soil so when they are added to the soil the specific gravity of the mixture is reduced. Both theash and lime are water consuming, being a part of this water needed for alkaline reactions. Fromthis, we see that a greater water content to compact the soil-RHA-lime mixtures is required.

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The mineralogy of mixtures of soil with RHACT and lime after 28 days were quite similar. Thepozzolanic reactivity of the RHACT is independent of controlled incineration temperature in therange of 650–800◦C.

The UCS of mixtures of sandy soil with RHACT and lime was greater than that of mixturesof sandy soil with RHAr and lime, including those tested after 56 days. The highest UCS valuesobtained for mixtures with RHACT in the temperature range of 650–800◦C demonstrates the higherpozzolanic reactivity of the RHACT with respect to the RHAr. Higher strengths were reached withless RHACT and lime content and therefore the RHA burned at controlled temperature into therange of 650◦C and 800◦C is a more economic alternative. The similar unconfined compressivestrength values of mixtures with RHACT confirm the independence of the pozzolanic reactivitywith respect to the burning temperature in the range of 650–800◦C.

STS was developed in mixtures of soil with RHA and lime which indicates the occurrence ofalkaline reactions between the amorphous silica of the RHAr and the calcium free ions of the limethat generate cementitious products. The tensile strength gain ratio is significant at early ages andtends to be constant for middle age. The curing time is extremely important for the STS gain.

The control of the rice husk incineration allows the production of more reactive ashes thanthose residuals done in an uncontrolled process which provides remarkable strength increases.The optimum range of incineration temperature is of 650–800◦C. As in this temperature range,the produced ashes presents similar pozzolanic reactivity, rice husk incineration at temperaturesslightly higher than 650◦C may be most suitable due to economical and environmental advantages,deriving of the less energetic consume.

The obtained results indicate that the stabilisation of sandy soils with RHA and lime wouldprovide an alternative material for sub-base and base layers of low-volume traffic pavements inregions where good performance materials are not available, such as rice production regions. Theuse of such material would provide significant improvement of the road networks with socio-economic consequences. Also this would contribute to the preservation of the environment byemploying of a residue and reducing the exploitation of deposits of non-renewable resources suchas soils and rocks.

AcknowledgementsThe authors are grateful to the CAPES of the Brazilian Ministry of Education, to the Postgraduation Programin Civil Engineering of the Federal University of Rio Grande do Sul (Brazil) and the Geotechnical Departmentof University of the Republic of Uruguay for having funded and supported the research. Also, we thankArrozur S.A. of Uruguay for providing the ash.

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