In the process of hydration of OPC, to complete all chemical reaction, the water requirement (expressed as the percentage of cement) is ……..
Correct Answer: D. 35 to 45%
📚 Detailed Explanation: Water Requirement for Complete OPC Hydration
This question expresses the same concept as Q8 but in percentage terms rather than w/c ratio. Both are different ways of stating the same physical requirement.
Why D (35 to 45%) is correct: The total water needed for complete hydration of OPC = 35% to 45% of cement mass = w/c ratio of 0.35 to 0.45. Breakdown:
• Non-evaporable (chemically combined) water: ~23% of cement mass (w/c = 0.23)
• Gel water (adsorbed in C-S-H): ~12–22% of cement mass
• Total: ~35–45%
Options A (5–8%) and B (8–16%) are only sufficient for early reactions, not complete hydration. Option C (20–25%) covers chemical water but not gel water.
• Non-evaporable (chemically combined) water: ~23% of cement mass (w/c = 0.23)
• Gel water (adsorbed in C-S-H): ~12–22% of cement mass
• Total: ~35–45%
Options A (5–8%) and B (8–16%) are only sufficient for early reactions, not complete hydration. Option C (20–25%) covers chemical water but not gel water.
Water Requirements for OPC Hydration
| Water Type | % of Cement Mass | W/C Equivalent |
|---|---|---|
| Non-evaporable (chemical) | ~23% | ~0.23 |
| Gel water | ~12–22% | ~0.12–0.22 |
| Total (complete hydration) | 35–45% | 0.35–0.45 |
Key Concepts for Students
- 35–45% of cement mass = w/c 0.35–0.45 — required for complete hydration.
- Practical w/c is typically 0.40–0.55 to balance complete hydration + workability + strength.