Discussion
Assertion (A): Rice husk is burnt under controlled conditions when the ash is wanted for cement.
Reason (R): Silica crystallises above about 700 degrees and loses its pozzolanic activity.
Select the correct answer from the options below.
Answer: B
Correct answer: Both A and R are true, and R is the correct explanation of A
Both statements are true and the reason is precisely why the practice exists. Rice husk ash is pozzolanic only while its silica is amorphous, because amorphous silica reacts with the calcium hydroxide released as cement hydrates.
Above about 700 degrees the silica crystallises to cristobalite and tridymite, which are unreactive, and the ash becomes an inert filler. Controlled combustion - limiting both temperature and residence time - is therefore what separates a saleable pozzolan from a waste to be disposed of.
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