Discussion
Three of the following raise the value of rice husk ash. Which destroys it?
Burning at 900 degrees or above
Controlled combustion below about 700 degrees
Grinding to a high fineness
Keeping the silica amorphous
Answer: A
Correct answer: Burning at 900 degrees or above
Burning at 900 degrees or above crystallises the silica into cristobalite and tridymite, which are chemically unreactive. The ash then has no pozzolanic activity and is worth little more than an inert filler.
The other three all preserve or add value: controlled combustion below about 700 degrees keeps the silica amorphous, grinding to a high fineness increases the surface area available to react, and it is the amorphous state itself on which the whole value depends.
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