Current rice production in the world is estimated to be 700 million tons. Rice husk ash constitutes about 20% of the weight of rice Bulk density of rice husk is low and lies in the range 90–150 kg/m3. It is an organic source of silica which has very high insulation properties. This rice husk is mostly used as a fuel in the boilers in different industries. The ash becomes 25% by weight of rice husk when burnt in boilers. RHA is the product of incineration of rice husk. Most of the evaporable components of rice husk are slowly lost during burning where the primary residues are the silicates. The characteristics of the ash are dependent generally on the composition of the rice husks its burning temperature and the time of burning. Every 100 kg of husks burnt in a boiler 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 used in as a local fuel. The partial burning contributes to air pollution. The chemical composition of RHA is significantly dependent on combustion conditions, and steps to be taken to control the burning temperature to keep silica in an amorphous state. The ash obtained from uncontrolled combustion i.e. in open-field burning or in industrial furnaces where the temperatures can be greater than 700°C–800°C. In this case there will be availability of significant amounts of cristobalite and tridymite which are nonreactive silica minerals. In order to get pozzolanic activity, such ashes will be required to be converted to a very fine particle size which is likely to make their use financially not feasible . Under controlled combustion i.e. burning temperatures in the range of 500°C–700°C for a period of about 1 hour. We get amorphous silica which 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. It is used as a pozzolan in the construction industry. In the construction industry, RHA can be used as a partial replacement for cement. Rice husk is mostly used as a fuel in the boilers in different industries.
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