Rice Husk Ash
Rice husk is an agricultural residue obtained from the outer covering of rice grains during milling process. Current rice production in the world is more than 700 million tons. Rice husk constitutes about 20% of the weight of rice. It contains about 50% cellulose, 25-30% lignin, and 15-20% of silica.
Rice husk ash (RHA) is generated by burning rice husk. On burning, cellulose and lignin are removed leaving behind silica ash. The controlled temperature and environment of burning yields better quality of rice-husk ash as its particle size and specific surface area are dependent on burning condition. The ash produced by controlled burning of the rice husk between 550°C and 700°C incinerating temperature for 1 h transforms the silica content of the ash into amorphous phase. The reactivity of amorphous silica is directly proportional to the specific surface area of ash. The ash so produced is pulverized or ground to required fineness and mixed with cement to produce blended cement.
The production of rice husk ash is primarily in areas where rice crops are abundant. Fully burned rice husk ash could be gray, purple or white, depending on the impurities present and the burning conditions. In open field burning or in uncontrolled combustion environments, the ash will remain mostly un-reactive because of the unfavorable mineralogical composition. Partially burned rice husk ash contains carbon, and is therefore black in colour. The silica in rice husk ash can is amorphous or crystalline, depending on the manner in which it is burned and cooled. If the ash is formed in open field burning or in uncontrolled combustion environments, it will retain a large proportion of non-reactive silica in the form of cristobalite and tridymite, and would require grinding to develop pozzolanic activity.
Rice husk ash reactivity is attributed to its high content of amorphous silica, and to its very large surface area, governed by the porous structure of the particles [19, 36]. Generally, reactivity is favored also by increasing the fineness of the
R. Siddique and M. Iqbal Khan, Supplementary Cementing Materials, Engineering Materials, DOI: 10.1007/978-3-642-17866-5_5,
© Springer-Verlag Berlin Heidelberg 2011 pozzolanic material. However, Mehta  reported that grinding of RHA to a high degree of fineness should be avoided, since it derives its pozzolanic activity mainly from the internal surface area of the particles.
The form of silica obtained after combustion of rice husk depends on the temperature and duration of combustion of rice husk. Mehta  reported that amorphous silica can be produced by maintaining the combustion temperature below 500°C under oxidizing conditions for prolonged periods or up to 680°C with a hold time less than 1 min. Yeoh et al.  reported that RHA can remain in the amorphous form at combustion temperatures of up to 900°C if the combustion time is less than 1 h, while crystalline silica is produced at 1,000°C with combustion time greater than 5 min. Chopra et al.  observed that at burning temperatures up to 700°C, the silica is in amorphous form. Hwang and Wu  studied the effect of burning temperatures and the chemical composition of rice husk ash. They observed that at 400°C, polysaccharides begin to depolymerize. Above 400°C, dehydration of sugar units occurs. At 700°C, the sugar units decompose. At temperatures above 700°C, unsaturated products react together and form a highly reactive carbonic residue. The X-ray data and chemical analyses of RHA produced under different burning conditions given by Hwang and Wu  showed that the higher the burning temperature, the greater the percentage of silica in the ash. K, S, Ca, Mg as well as several other components were found to be volatile.
Della et al.  presented the processing and characterization of high specific surface area silica from RHA. They reported that a 95% silica powder could be produced after heat-treatment at 700°C for 6 h. And specific surface area of particles was increased after wet milling from 54 to 81 m2/g.