What is the effect on the strength of concrete on addition of pozzolanas, keeping other factors same?
Correct Answer: C. Early strength decreases but ultimate strength remains almost same
📚 Detailed Explanation: Pozzolanas and Concrete Strength
Why C is correct — early strength ↓, ultimate strength same: Pozzolanas (Class F fly ash, GGBS, silica fume, volcanic ash) are not cementitious by themselves. They react with calcium hydroxide [Ca(OH)₂] produced during cement hydration to form additional calcium silicate hydrate (C-S-H) in a secondary pozzolanic reaction:
Ca(OH)₂ + SiO₂ (pozzolana) → C-S-H gel
Ca(OH)₂ + SiO₂ (pozzolana) → C-S-H gel
This reaction is slower than OPC hydration, so early-age strength is reduced. However, over time (28–90 days and beyond), the accumulated secondary C-S-H fills pores and the ultimate strength approaches or equals that of plain OPC concrete.
Pozzolana Effect on Strength Over Time
| Age | Pozzolanic Concrete Strength vs. Plain OPC |
|---|---|
| 1–7 days (early) | Lower — pozzolanic reaction barely started |
| 28 days | Slightly lower |
| 90+ days | Approximately same or slightly higher |
- Pozzolanas: early strength decreases; ultimate (long-term) strength remains approximately same.
- Key benefit of pozzolanas: reduced heat of hydration, improved durability, lower permeability.