As a precaution in cold weather concreting, cement containing ________ should be selected.
Correct Answer: B. Higher C₃S and lower C₂S
📚 Detailed Explanation: Cold Weather Requires Cement with High C₃S and Low C₂S
Why B (Higher C3S and lower C2S) is correct: In cold weather, the rate of cement hydration slows dramatically, leaving fresh concrete vulnerable to freezing before it gains adequate strength. To combat this, a cement with a higher proportion of Tricalcium Silicate (C3S) is selected because it hydrates rapidly, generates significant heat of hydration, and produces high early compressive strength. Dicalcium Silicate (C2S) is a slow hydration compound — more C2S means less early strength, which is precisely what you do NOT want in cold weather.
Cement Compound Hydration Properties
| Compound | Symbol | Rate of Hydration | Heat Generated | Early Strength | Long-term Strength |
|---|---|---|---|---|---|
| Tricalcium Silicate | C3S (Alite) | Rapid | High (500 J/g) | High | Good |
| Dicalcium Silicate | C2S (Belite) | Very slow | Low (260 J/g) | Very low | High |
| Tricalcium Aluminate | C3A | Very rapid | Very high (865 J/g) | Very early (flash set if uncontrolled) | Susceptible to sulphate attack |
| Tetracalcium Aluminoferrite | C4AF | Moderate | Moderate (420 J/g) | Moderate | Moderate |
Why High C₃S is Critical for Cold Weather Concreting
| Benefit of High C₃S | How It Helps in Cold Weather |
|---|---|
| Rapid hydration | Generates significant heat of hydration quickly; raises internal concrete temperature; counters ambient cold |
| High early strength | Concrete reaches structural strength before temperatures can drop to freezing; reduces the vulnerable plastic stage duration |
| More heat per unit mass | Self-heating effect reduces dependence on external heating measures |
Cements Suitable for Cold Weather Concreting
| Cement Type | C₃S Content | Suitability for Cold Weather |
|---|---|---|
| Rapid Hardening Portland Cement (RHPC) | Very high (>60%) | Excellent — designed for high early strength |
| Ordinary Portland Cement (OPC 53 grade) | High (≈55–65%) | Good — standard choice with adequate early strength |
| OPC 33 grade | Moderate | Acceptable with precautions |
| Sulphate-Resisting Cement (SRPC) | Low C3A; moderate C3S | Avoid in very cold — lower heat generation |
| Low Heat Cement | Low C3S; high C2S | ✗ NOT suitable — slow heat generation, slow early strength |
- Cold weather concreting requires cement with higher C3S and lower C2S.
- High C3S = rapid hydration + high heat of hydration + high early strength — all critical before potential freezing.
- Low Heat Cement (high C2S) should be avoided in cold weather; it is ideal for mass concrete where heat dissipation is needed.