6 Utilization of Unprocessed Rice Husk Ash as a Cementitious Material in Concrete (A Comparison with Silica Fume) Mohammad Qamruddin, Master of Civil Structures, Civil Engineering, Faculty of Engineering, Jawaharlal Nehru Engineering College, Dr. Babasaheb Ambedkar Marathwada University, Aurangabad, Maharashtra, India. Prof. L. G. Kalurkar, Master of Civil Structures, Civil Engineering, Faculty of Engineering, Jawaharlal Nehru Engineering College, Dr. Babasaheb Ambedkar Marathwada University, Aurangabad, Maharashtra, India. ABSTRACT Fast depleting natural resources, huge consumption of energy, and environmental hazards involved in the production of cement has inspired for searching the substitution by other material with similar material, especially in developing countries. Rice husk ash (RHA), an agricultural waste, is classified as a highly active pozzolan because it contains a very high amount of amorphous silica and a large surface area. The objective of the study is to investigate the mechanical properties of concrete with different replacement levels of ordinary Portland cement by rice husk ash and silica fume individually and in combination. The standard cubes (150mmx150mmx150mm) cylinders (150mmdia x 300mmheight) were cast. The compressive strength at 7days and 28 days have been obtained with normal curing regime. For RHA a maximum increase in compressive strength was 16% whereas for silica fume it was approximately 20% compared with nominal mix when used individually. Combination of RHA and silica fume did not show any exciting results. Keywords Rice husk ash, Silica fume, cementitious material, concrete. 1. INTRODUCTION With growing environmental consciousness at all levels of society, the pollution and health hazards especially associated with the concrete and cements industries, is coming under intense scrutiny from environmentalists and the governments. The developed countries are farther ahead in tackling the problem by using industrial and agricultural wastes in their industries. These industrial and agricultural wastes are mostly the byproducts of oil and coal burning by-products, slag, rice husk ash, bagasse, fly ash, cement dust, stone crusher dust, marble dust, brick dust, sewer sludge, glass, tires, etc. Million tons of these waste materials are abundantly available and discarded every year in the world. They pose environmental problems like air pollution and leaching of hazardous and toxic chemicals (arsenic, beryllium, boron, cadmium, chromium, chromium(vi), cobalt, lead, manganese, mercury, molybdenum, selenium, strontium, thallium, and vanadium, along with dioxins and polycyclic aromatic hydrocarbon compounds, etc.) when dumped in landfills, quarries, rivers and oceans [1,2]. Consequently air and water pollution have been inextricably linked to environmental problems and climate change. The production of cement (key binding component of concrete) is costly, consumes high energy, depletes natural resources and emits huge amounts 0of greenhouse gases (1 ton of cement production emits approximately 1 ton of CO 2 ). Consequently, environmental degradation, serious pollution and health hazards associated with cement and concrete industries. Rice Husk is one of the waste materials in the rice growing regions. This not only makes the purposeful utilization of agricultural waste but it will also reduce the consumption of energy used in the production of cement. Therefore Rice Husk is an agro based product which can be used as a substitute of cement without sacrificing the strength and durability. Generally the Rice Husk Ash is used while burning the raw clay bricks in the Brick Kilns. Till recently it is also used in Hotels for cooking but now it is replaced by LPG Gas. Since Rice Husk has negligible protein content, it is not useful for animal feeding. Rice Husk Ash is obtained from burning of Rice Husk, which is the by-product of rice milling. It is estimated that 1,000 kg of rice grain produce 200 kg of Rice Husk; after Rice Husk is burnt, about 20 percent of the Rice Husk or 40 kg would become RHA. Rice Husk Ash contains as much as 80-85% silica which is highly reactive, depending upon the temperature of incineration. Due to relative high water demand, the lime Rice Husk Ash cement developed lower compressive strengths. However, the strength characteristics are considered adequate for general masonry work. The water demand for normal consistency tends to increase with increasing Ash content of the blended cements. However, this can be corrected by application of certain water reducing admixtures. The investigations as outlined above point
7 towards encouraging trend. Normally fly Ash may be used for partially replacing cement to the extent of about 25% of cement. Reactions that take place in the preparation of Rice Husk Ash concrete are; Silicon burnt in the presence of Oxygen gives Silica. Si + O2 SiO2 C3S (Cement) + H2O CSH + Ca (OH) 2 The highly reactive silica reacts with Calcium hydroxide released during the hydration of cement, resulting in the formation of Calcium Silicates responsible for strength. SiO2 + Ca (OH)2 CSH + SiO2 Mehta, P.K., (1) has conducted investigations on Portland Rice Husk Ash cements up to 50% of Ash showed higher compressive strength than the control Portland cement even at as early as 3 days. Mehta, and Pirtz (2) in a concrete mixture, when 30% Rice Husk Ash by weight of the total cementing material was present, the 7 days and the 28 days compressive strengths were higher. Subba Rao.et.al (3) studied the reaction product of lime and silicate from Rice Husk Ash and showed that it is Calcium Silicate Hydrate (C- S-H) which accounts for the strength of lime Rice Husk Ash cements. 2. MATERIAL PROPERTIES INVESTIGATED FOR THIS RESEARCH: Table 1:- Material Properties:- 1 Nominal Max.Size of coarse aggregate 20 mm 2 Slump range (Medium) 50-75 mm IS 10262-2009 (Pg.02) 3 Finness Modulus Of Fine Aggregate 2.88 Confirming to zone II (IS 383-1970) 4 Finness Modulus Of Steel Slag aggregate 2.86 Confirming to zone II (IS 383-1970) 5 Finness Modulus Of Coarse Aggregate 5.12 Confirming to zone II (IS 383-1970) 6 Specific Gravity Of Fine Aggregate 2.65 7 Specific Gravity Of Steel Slag aggregate 2.67 8 Specific Gravity Of Coarse Aggregate 2.75 9 Specific Gravity Of Cement 3.15 10 Water Absorption of fine aggregates 4.7 % 11 Water Absorption of coarse aggregates 1.65 % 3 CHEMICAL COMPOSITION AND PHYSICAL PROPERTIES OF RICE HUSK ASH USED IN THE STUDY: Table 2:- Chemical Composition of rice husk ash:- Constituent SiO 2 Al 2 O 3 Fe 2 O 3 CaO MgO SO 3 Na 2 O K 2 O Percentage ( % ) 94.84 0.39 0.54 1.32 0.40 0.01 0.11 1.45 Table 3:- Physical Properties of rice husk ash Physical Properties Steel Slag Colour Grey Appearance Amorphous specific gravity 2.17 4 RHA CHARACTERISTICS A residual RHA obtained from open filed burning from local resource was used. The material was carefully homogenised packed to enhance the transport to the laboratory. Grinded RHA (GRHA): after drying and homogenization process the RHA was ground in a laboratory ball mill by one hour for optimization. The size and shape of the natural rice husk ash particles make difficult the development of pozzolanic reactions and the water demand strongly increases. The GRHA was passed through IS 90 micron sieve and this was used for the research. Adopting an adequate sequence for concrete mixing process, where the RHA and the coarse aggregates are mixed during a certain time and after that the rest of component materials are incorporated, the RHA characteristics can be improved. Table 2 and 3 presents some chemical composition of the ash and the physical characteristics. 5. EXPERIMENTAL PROGRAM Materials used in this study were OPC 53 grade cement confirming to IS 8112 and fine aggregate and coarse
8 aggregate confirming to IS 383-1970.The cement and aggregate were tested to fulfill the IS requirements. Rice husk ash was replaced with cement as 10%, 15%, 20% and 25%, similarly silica fume was also replaced with cement in different batches with percentage replacements as10%, 15%, 20% and 25%. In the third step rice husk ash and silica fume were replaced in combination as Rice husk ash and silica fume were replaced according to following table Table 4:- Batch Cement(%) RHA(%) Silica fume(%) 1 100 0 0 2 90 10 0 3 85 15 0 4 80 20 0 5 75 25 0 6 90 0 10 7 85 0 15 8 80 0 20 9 75 0 25 10 90 5 5 11 85 7.5 7.5 12 80 10 10 13 75 12.5 12.5 14 75 15 10 15 70 20 10 16 65 25 10 Designed concrete mix of M-20 grade having mix proportion 1:1.90:2.96 with w/c ratio 0.5 same for different percentages of rice husk ash and silica fume were used. The concrete ingredients namely, cement and coarse aggregates and rice husk ash and silica fume according to batches were first mixed in the dry state and water was added last. Cylinders of size 150mm diameter x 300mm length for split tensile strength, Cubes of size 150x150x150 mm for compressive strength were cast replacing rice husk ash and silica fume by weight of cement. All the samples were watered cured for 7 days and 28 days. For each batch of RHA and silica fume percentage replacement 6 specimens were cast. Details of the experimental investigation of effect of different percentages replacement of fine aggregate by steel slag are given elsewhere. 5.1 Testing Programme 5.1.1 Compressive Strength The cube specimen was placed in the machine, of 1000kN capacity. The load was applied at a rate of approximately 140 kg/sq.cm/min until the resistance of the specimen to the increasing load can be sustained. Results are presented in Tables 5 and 6. 5.1.2 Splitting Tensile Strength: The cylinder specimen was placed horizontally in the centering with packing skip or loading pieces carefully positioned along the top and bottom of the plane of loading of the specimen. The load was applied without shock and increased continuously at a nominal rate within the range 1.2 N/mm2/min to 2.4 N/mm2/min until failure the specimen. The maximum load applied was recorded at failure. TEST RESULTS FOR COMPRESSIVE STRENGTH:
9 TEST RESULTS FOR SPLITTING TENSILE STRENGTH:
10 6. DISCUSSIONS ON TEST RESULTS 6.1 Compressive Strength of Concrete For the given water cement ratio optimum replacement for RHA was found to be 15% whereas that for the silica fume was found to be at 20%. An increase of 16% in the compressive strength was observed with RHA and that with silica fume was approximately 20%. The results of combination of RHA and silica fume were found to be decreasing. Neither of the combination used gave higher compressive strength at 28 days. Reason for the same could not be understood. 6.2 Splitting tensile Strength of Concrete As the replacement level increases there is increase in splitting tensile strength at 28 days age of strength. The maximum splitting tensile strength for the replacement with RHA and silica fume individually was 4.54 and 4.78 respectively. An increase of 14% and 20% was observed for RHA and silica fume individually. RHA and silica fume in combination showed a decreasing trend of strength. 7. CONCLUSIONS 1. Replacement of cement with Rice Husk Ash leads to increase in the compressive strength at 14% at 15% replacement. 2. Optimum replacement level for silica fume was found to be 20% where 17% increase in splitting tensile strength is observed. 3. Same trend as seen in compressive strength was observed in case of splitting tensile strength at 28 days of age. An increase of 14% and 20% was observed at the replacement level of 15% and 20% for RHA and silica fume respectively. 4. The strength achieved for the replacement of combination of RHA and SF was even less than the nominal mix but cannot be authenticated due to less number of tests. REFERENCES [1] P.K.Mehta, Properties of Blended Cements Made from Rice Husk Ash, ACI Journal/September 1977 pp. 440-442. [2] P.K.Mehta and D.Pritz, Use of Rice Husk Ash to Reduce Temp in HSC, Journal/February 1978 pp. 60-63. [3] James, J., Subba rao.m., Reactive of Rice Husk Ash, Cement and Concrete Research, Vol.16, 1986. [4] Rahman M.N., Curing of RHA Mix sand concrete blocks, International Journal of Structures, Vol. 8, No.1, 1988. [5] Seshagiri Rao M.V., Chankravarthi R.K., Narasimha Murthy, Rice Husk Ash Blend Cement National Seminar on Recent Advances in Civil Engg. With Special. Reference to Building Industry, JNTU College of Engg. & Tech., Hyderabad1992.