SummaryRice husk ash is an attractive pozzolan. Due to its low cost and high activity it has a promising perspective in sustainable construction. In combination with lime, its effect in soil improvement can be equal to cement treatment but its production process consumes much less energy. The main component of the rice husk ash is silica, which is the element that governs the reactivity of the ash. A delicate burning process is required to eliminate the organic components in the rice husk but keep the silica to be amorphous so that a highly reactive rice husk ash can be obtained. A too high temperature would transform amorphous silica to crystalline silica, which would reduce the reactivity. The suggested burning process in literature is 2 hours at 500oC. However, due to the exothermic property of the burning rice husk it is difficult to control the exact burning temperature, hence there is still a possibility that the carbon and the crystallized silica are present and hinder the activity of the rice husk ash. Based on the silica state and the carbon content, the rice husk ash is classified in three types: C-RHA which is collected from a quick and open-air burning and contains a large amount of carbon; Cr-RHA which is collected form slow burning at above 600oC and contains a large amount of crystallized silica; and A-RHA which is collected from the suggested burning process which is 500oC in 2 hours and is considered to be the most active.
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