Sources of rice husk ash (RHA) will be in the rice growing regions of the world, as for example China, India, and the far-East countries. RHA is the product of incineration of rice husk. Most of the evaporable components of rice husk are slowly lost during burning and the primary residues are the silicates. The characteristics of the ash are dependent on composition of the rice husks, burning temperature, and burning time. Every 100 kg of husks burnt in a boiler for example will yield about 25 kg of RHA. In certain areas, rice husk is used as a fuel for parboiling paddy in rice mills, whereas in some places it is field-burnt as a local fuel. However, the combustion of rice husks in such cases is far from complete and the partial burning also contributes to air pollution. The calorific value of rice husks is about 50% of that of coal, and assuming that husks have about 8%–10% of moisture content and zero bran, the calorific value is estimated to be 15 MJ/kg. Under controlled burning conditions, the volatile organic matter in the rice husk consisting of cellulose and lignin are removed and the residual ash is predominantly amorphous silica with a (microporous) cellular structure (Fig. 13.1). Due to its highly microporous structure, specific surface area of RHA as determined by the Brunauer–Emmett–Teller (BET) nitrogen adsorption method can range from 20 to as high as 270 m2/g, while that of silica fume, for example is in the range of 18–23 m2/g.

The chemical composition of RHA is significantly dependent on combustion conditions, and the burning temperature must be controlled to keep silica in an amorphous state. The ash obtained from uncontrolled combustion (as in open-field burning or in industrial furnaces at temperatures greater than 700°C–800°C) will contain significant amounts of cristobalite and tridymite which are nonreactive silica minerals. In order to develop pozzolanic activity, such ashes will be required to be ground to a very fine particle size which is likely to make their use financially unviable. Under controlled combustion (burning temperatures in the range of 500°C–700°C for a period of about 1 hour), amorphous silica is the major constituent of ash whose reactivity is attributed to the presence of this form of silica and to its very large surface area resulting from the microporous structure of ash particles. Although reactivity of a pozzolanic material improves upon increasing its fineness, Mehta and Monteiro (1997) reckon that grinding RHA to a high degree of fineness is not advisable since this material derives its pozzolanic activity from the internal surface area of its microporous particles which is already very high. When obtained from controlled combustion, the specific surface (as measured using nitrogen adsorption) of RHA can be as high as 50,000 m2/kg even though the particle size may be in the range of 10–75 μm, which is large when compared to that of silica fume for example. The average composition of well-burnt RHA is 90% amorphous silica, 5% carbon, and 2% K2O.