Production of high strength concrete incorporating an agricultural waste- Rice husk ash

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1 From the SelectedWorks of Hilmi Mahmud 2010 Production of high strength concrete incorporating an agricultural waste- Rice husk ash Hilmi Mahmud, University of Malaya Available at:

2 Production of high strength concrete incorporating an agricultural waste - rice husk ash Hilmi Bin Mahmud, Norjidah Anjang Abdul Hamid and Koay Yew Chin Department of Civil Engineering, Faculty of Engineering University of Malaya Kuala Lumpur, Malaysia hilmi@um.edu.my Abstract-Rice husk which is an agricultural waste, constitutes about one-fifth of the 500 million tonnes of rice produced annually worldwide. Normally, the residue is disposed off by burning at the mill sites and the resultant rice husk ash (RHA) is dumped on a waste land. This generates environmental, pollution and land dereliction problems. Under controlled burning and if sufficiently ground, the highly reactive ash that is produced can be used as a supplementary cementing material or in the production of high strength concrete (HSC). This paper shows that it is relatively easy to produce high strength Grade 80 concrete incorporating RHA. Data on the strength property and durability showed that RHA concrete is just as good as that of silica fume. The potential of RHA as an alternative material to produce cheaper HSC is highlighted in this paper and should be exploited by the rice and construction industries. Keywords - high strength concrete; rice husk ash; workability; compressive strength; durability INTRODUCTION Currently the trend in concrete construction has shifted towards high strength concrete. In Malaysia, the use of 80 MPa concrete in the construction of the Petronas Twin Towers, is a typical example. To develop high strength concrete (HSC), two ingredients namely superplasticizer and silica fume (SF) are essential. However, SF is expensive and the need to find an alternative indigenous material to this material exists. In Malaysia, where rice is the staple food, the use of rice husk ash (RHA), a product of combustion of rice husk, is such an example. It is known that rice husk ash has many properties similar to that of silica fume [1,2]. Researches have shown that incorporation of RHA can produce high strength and durable concrete [3,4,5]. Based on an earlier work [4], four optimum mixtures, i.e. a control OPC, RHA loa and CSF loa (concrete containing 10% addition of RHA or SF respectively) and RHA 5R (concrete containing 5% replacement of cement with RHA) were selected for further studies on strength and durability (initial water absorption (lsa) and water absorption) of HSC. The beneficial effects of incorporating RHA in HSC mixes are highlighted in the paper. A. Materials I. EXPERIMENTAL PROGRAMME Ordinary Portland cement conforming to BS 12, densified silica fume and RHA were used. RHA was obtained from laboratory incineration of the husk in a furnace [3]. After incineration, the ash particles were ground until they met the fly ash fineness requirement [6]. The specific surface area of silica fume, cement and RHA using the nitro en absorption method were 22.5 m 2 /g, 3.2 m 2 /g and m /g respectively. The specific gravity and average particle size of RHA were 2.58 and 13.2 Ilm respectively. Chemical analysis reveals that the material was primarily composed of silica, as presented in Table 1. The fine aggregate was 5mm maximum size mining sand, having specific gravity (S.G) of 2.65, 1.2% water absorption and fineness modulus of 2.5. Single-sized 20 mm crushed granite with S.G of 2.67 and water absorption of 0.57% was used as coarse aggregate. A sulphonated naphthalene formaldehyde based superplasticizer was used as chemical admixture. Tap water was used for mixing. TABLE I. CHEMICAL COMPOSITION OF OPC, RHA AND SF Chemical OPC RHA SF Compound (%) (%) (%) Si Ah Fe CaO MgO Na K LOI B. Mixture Proportioning The mixture proportioning was based on an earlier publication [4]. They were designed to achieve target mean strength of 80 N/mm 2 at 28 days with water to binder (w/b) ratio of All mixes (including control OPC) contained total cementitious material content of 518 kg/m3 and coarse to fine aggregate content ratio of 67:33. RHAIOA, CSFIOA and RHA5R mixes contained RHA or SF at 5% or 10%, either as addition (A) or replacement (R) of cement. Varying amounts of superplasticizer were added to all the mixes to maintain workability in the region of mm slump /10/ $ IEEE 106

3 The mixture proportion of the four selected mixes (per cubic meter of concrete) is shown in Table 2. TABLE II. MIXTURE PROPORTION FOR HSC Mix wlb Cement RHA Water Coarse Fine No ratio (kg) or SF (kg) (kg) OPC RHAIOA SFIOA RHA5R C. Tests Workability of the mixes was measured by the slump test. Compressive strength and water absorption tests were performed on 100mm cubes. Initial surface absorption test (ISAT) was conducted on 150 mm cubes. Except for compressive strength specimens where they were also subjected to three days water curing followed by air drying (partial water curing), all other specimens were subjected to continuous water curing until day of testing. For the durability tests, prior to testing, all specimens were dried in a laboratory oven at ±l 05 C until a constant mass was achieved. ISAT was performed by measuring the absorption of water from a pressure head of 200 mm into the concrete from the top surface. The flow, in ml/m2/s was calculated at intervals of 10, 30, 60 and 120 minutes. Water absorption test was conducted by completely immersing dried cube specimens in water at room temperature for 48 hours. II. RESULTS AND DISCUSSIONS A. Workability and Compressive Strength Workability of all the selected concrete mixtures are shown in Table 3. Although the w/b ratio of all the concrete mixtures were low at 0.31, workability in the range of mm slump can be achieved with the aid of a superplasticizer (Sp). For similar workability, the amount of Sp needed for RHA or SF mixtures was similar. Compared to the silica fume mix, at the same binder content, incorporation of RHA will not require additional amount of Sp for constant workability. Table 3 shows that at 28 days, all the concrete specimens achieved the target strength of 80 MPa. Although specimens subjected to partial water curing showed marginally lower strengths than those continuously cured in water, the difference is small, i.e between 1-5% of each other. This result shows that for a country having high humidity such as that in Malaysia (R.H 90%), curing of specimens for just three days may not be detrimental to the strength development of HSC. There is enough moisture from the atmosphere to provide hydration to normal or pozzolanic concrete alike. Irrespective of the type of curing, compared to the OPC mixture, concrete containing RHA (RHA1OA and RHA5R) showed higher strengths at all ages. The beneficial pozzolanic effect of incorporating RHA to the short and long term strengths of concrete is highlighted by this result. This finding is similar to that found in SF concrete. For similar pozzolanic material content, strengths of RHA and SF concrete are similar. Although the strength of the former is slightly lower, the difference is insignificantly small (between 1-5%), irrespective of age and type of curing. This result shows that RHA is the cheaper alternative material to SF in producing HSC of this magnitude. Compared to the replacement mix (RHA 5R), the addition mix (RHA loa) concrete exhibited marginally higher strengths. This is expected as the hydration reaction of the replacement mix is slower due to the reduced cement content (part of cement was replaced by RHA). However when compared to the control concrete, at 180 days, both RHA concrete under water curing were able to achieve strength of 90 MPa or better, which was not achievable in the control OPC concrete. Similar to that found in SF concrete, the incorporation of a pozzolanic material is beneficial to the long term strength development of HSC. B. Initial Surface Absorption (ISA) Test The results of the initial surface absorption test for selected HSC mixtures are presented in Table 4. Data show that at all ages, ISA values decreased with increasing age. This is because the outer zone of the concrete surface became saturated as the capillaries were filled with water, making it more difficult for water to be absorbed by the inner pores [7]. Irrespective of the age of the specimen, compared to the control OPC concrete, there is a marked reduction in the ISA values for RHA and SF concrete, indicating the importance of incorporating pozzolanic materials to reduce the permeability of concrete. For the RHA mixtures, marginally lower ISA values were exhibited by the RHA 'replacement' mixture. However at 28 and 90 days, similar ISA values of 0.09 and 0.06 mllm 2 /s respectively were recorded for both the specimens, 2 hours from the start of the test. This result compares well with that reported for HSC containing 5% and 10% RHA as 'replacement' of cement [8]. Under the same curing condition and at the stated ages mentioned earlier, ISA values for SF loa samples were marginally lower at 0.08 mllm% and 0.04 mllm% respectively. Presence of RHA or SF acts as pozzolanic micro filler to reduce the size of capillary pores of the total cementitious paste, causing the pore system to become segmented through partial blocking. In this case, the capillary pores are only interconnected by gel pores, which are much smaller and almost impermeable. This modification of microstructure by RHA in the interfacial zone of aggregate and cement paste produced a more dense concrete with less pores [9]. TABLE III. STRENGTH OF HSC MIXES 107

4 2010 2nd International Conforence on Chemical, Biological and Environmental Engineering (ICBEE 2010) Mix No w/b Sp Slump Compressive StrenJ4h (MPa) Type of Curing ratio (%) (mm) 7 days 28 days 90 days 180 days SpOPC Water Partial Water RHA loa Water Partial Water SF loa Water Partial Water RHA5R Water Partial Water TABLE IV. ISAT RESULTS FOR HSe Flow (ml/m 2 /s) Mix Age Time mins) No (Days) ope RHAIOA SF loa marked decrease in volume of pore space within the concrete. RHA loa achieved the lowest absorption in comparison with the other mixes. SF loa and RHA 5R mixes showed similar absorption values but when compared to the control concrete, significantly lower values were observed. The pozzolanic reaction by the two materials reduced the ease of water that can be absorbed by the concrete because of the formation of the dense product of reaction, thereby decreasing the pore volume [8]. This result reinforced the fact that inclusion of RHA or SF is beneficial in improving the durability of HSC. 9 8 r o Age (days) - cope RHA10A C SF10A crha5 R I I RHA5R Figure 1. Water absorption of HSe C. Water Absorption The volume of pore space in concrete, as distinct from the ease with which fluid can penetrate it, can be measured by its absorption. The percentage of water absorbed by all the HSC mixtures is shown in Fig. 1. It is noticeable that all the specimens, with the exception of OPC concrete at 7 days, have low absorption characteristic of about 8 % or less. At 28 days and beyond, concrete containing RHA or SF have water absorption of less than 7%. It can be seen from Fig. 1 that the results of the water absorption is similar to that of the initial surface absorption, whereby mixes containing RHA and SF exhibited much lower values than that of the control OPC concrete. For all the specimens, the decrease in absorption values becomes insignificant from 28 days onwards implying that there is no III. CONCLUSIONS From the results obtained from the various tests on selected HSC mixtures, the following conclusions can be drawn: 1. To achieve a required workability, the use of RHA will not require any much more superplasticizing admixture than that of silica fume. 2. Concrete containing RHA, either as an additional admixture or as partial replacement of cement can routinely produce strength of 80 N/mm 2 at 28 days. 3. Irrespective of age and type of curing, for similar pozzolanic content, strength of RHA concrete is marginally lower than that of SF concrete. However, the difference in strength between the two is insignificantly small (between 1-5%). 4. Both RHA and SF mixtures enhanced the durability of concrete by reducing its water absorption characteristics. 108

5 Inclusion of these pozzolanic materials significantly decreases the initial surface absorption and water absorption of concrete, when compared to the control ope concrete. 5. RHA is just as good as SF in producing durable high strength concrete of Grade 80. Since RHA is an agricultural waste and can be produced at a much lower cost than SF, it is the attractive alternative indigenous material in the production of HSC. This aspect should be exploited fully by the rice and construction industries. ACKNOWLEDGMENT The work described above is supported by an R&D grant "Development of high performance and durable rice husk ash concrete". The authors would like to thank the Ministry of Science, Environment and Innovation of Malaysia for awarding the grant. The help of BASF (M) Sdn. Bhd. in supplying the superplasticizer and silica fume, as well as the National Rice Board of Malaysia for supplying the raw rice husk are highly appreciated. [I] REFERENCES DJ. Cook., "Rice husk ash", in Cement Replacement Materials, Concrete Technology and Design, Vol. 3, Ed: R.N.Swamy, Surrey University Press, UK, 1986 [2] P.K.Mehta., "Rice husk ash - a unique supplementary cementing material", Proc. Int. Symp. on Advances in Concrete Technology, Greece, 1992, pp [3] RB.Mahmud., B.S.Chia and N.B.A.A.Hamid., "Rice husk ash - an alternative material in producing high strength concrete", Proc. Int. Conf. on Eng. Materials, Ottawa, Canada, Vol. 2,1997, pp [4] H.B.Mahmud., YC.Koay, N.B.A.A.Hamid and M.F.M.zain.., "Use of rice husk ash to produce high strength high performance G80 concrete", Proc. 6th Int. Symp. on Utilization of High Strength / High Performance Concrete, Leipzig, Germany, Vol. 2, 2002, pp [5] M.S. Ismail and A.M. Waliuddin, "Effect of rice husk ash on high strength concrete" Construction and Building Materials, Vol. 93, 1996, pp [6] BS 3892: Part I, "Pulverised fuel ash for use as a cementitious component in structural concrete," British Standards Institution, London., 1982 [7] A.E. Long., G.D Henderson. and F.R. Montgomery, "Why assess the properties of near surface concrete?", Construction and Building Materials, Vol. 15,2001, pp [8] P.R.S. Speare., K.Eleftheriou and S. Siludom., "Durability of concrete containing rice husk ash as an additive", Proc. Int. Seminar on Exploiting Wastes in Concrete, Dundee, UK, Eds. R. K. Dhir and T. G. Jappy, Thomas Telford, UK, 1999, pp [9] R.N.Swamy, "Durable concrete structure - The challenge to design and construction", Proc., Fifth Int. Conf. on Concrete Eng. and Tech., Kuala Lumpur, Malaysia, 1997, pp