Discussion
Effect of combustion temperature on rice husk ash.
| Temperature (C) | Silica form | Pozzolanic activity |
|---|---|---|
| 500 | Amorphous | High |
| 700 | Mostly amorphous | Good |
| 900 | Partly crystalline | Poor |
| 1100 | Crystalline | Negligible |
What combustion practice does the table recommend?
Answer: D
Correct answer: Burn below about 700 degrees
Reading down the table, pozzolanic activity falls from high at 500 degrees to negligible at 1100, exactly as the silica passes from amorphous to crystalline. The recommendation follows directly: keep the combustion below about 700 degrees.
Crystalline silica cannot react with the calcium hydroxide released during cement hydration, so the ash becomes an inert filler. Burning hotter does not purify the silica or increase its yield; it destroys the one property that gives the ash its value.
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