Pozzolana are rich in:
Correct Answer: C. Silica, alumina and alkali
📚 Detailed Explanation: Chemical Composition of Pozzolanas
Why C (Silica, alumina and alkali) is correct: ASTM C618 and IS 1344 define pozzolanas as materials containing silicon dioxide (SiO₂) + aluminium oxide (Al₂O₃) + iron oxide (Fe₂O₃) in reactive (amorphous) form, totalling at least 70% for Class F, 50% for Class C. However, the classical textbook phrasing for Indian exams specifies “silica, alumina, and alkali” as the key constituents.
Option D includes “iron” — while iron oxide (Fe₂O₃) is present and counted in the ASTM total, the standard Indian textbook answer for “pozzolanas are rich in” is: silica, alumina, and alkali. Option D overstates and is not the standard answer.
Pozzolana Composition
| Constituent | Oxide | Role |
|---|---|---|
| Silica | SiO₂ | Primary reactive constituent; reacts with Ca(OH)₂ |
| Alumina | Al₂O₃ | Secondary reactive; forms calcium aluminate hydrates |
| Alkali | Na₂O, K₂O | Activates pozzolanic reaction |
Common Pozzolanas and Sources
| Pozzolana | Source |
|---|---|
| Fly ash (Class F/C) | Coal thermal power plants |
| GGBS | Blast furnace slag |
| Silica fume | Silicon metal / ferrosilicon production |
| Rice husk ash | Burnt agricultural waste |
| Volcanic ash / Trass | Natural volcanic deposits |
- Pozzolanas = rich in silica, alumina, and alkali.
- No independent cementitious value; must react with calcium hydroxide (liberated by OPC hydration) + water.