Discussion
Statement I: Rice husk ash is valuable in cement only if the husk is burnt under controlled conditions.
Statement II: Burning above about 700 degrees crystallises the silica and destroys its pozzolanic activity.
In the light of the above statements, choose the correct answer.
Answer: C
Correct answer: Both Statement I and Statement II are correct
Both statements are correct and the second is the reason for the first. Rice husk ash is pozzolanic only while its silica is amorphous, since amorphous silica reacts with the calcium hydroxide released during cement hydration.
Above about 700 degrees the silica crystallises into cristobalite and tridymite, which are unreactive. The ash then becomes an inert filler worth very little. Controlled combustion - holding the temperature down and the residence time short - is therefore what separates a valuable product from a waste.
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