Discussion
Arrange the steps of recovering pozzolanic silica from rice husk in order:
1. Burn under controlled conditions below about 700 degrees
2. Collect and clean the husk at the mill
3. Grind the ash to the required fineness
4. Recover the amorphous ash from the furnace
Select the correct sequence.
Answer: D
Correct answer: 2, 1, 4, 3
The sequence begins at the mill, where the husk is collected and cleaned (2) - soil and grit would contaminate the ash and dilute the silica.
It is then burnt under controlled conditions below about 700 degrees (1), which is the critical step, since a higher temperature crystallises the silica and destroys the pozzolanic activity. The amorphous ash is recovered from the furnace (4) and finally ground to the required fineness (3), because reactivity depends on surface area as well as on being amorphous.
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