Steam curing is NOT used with which type of cement?
Correct Answer: C. High alumina cement
📚 Detailed Explanation: Steam Curing Incompatible with High Alumina Cement
Why C (High Alumina Cement) is correct: High Alumina Cement (HAC) hydrates very rapidly and generates a large amount of heat. Its initial hydration products (CAH10 and C2AH8) are metastable. When exposed to elevated temperatures (as in steam curing), these metastable phases undergo conversion to stable but much weaker phases (C3AH6), causing drastic strength reduction of 50% or more.
The HAC Conversion Reaction
| Initial Hydration Products (HAC) | After Conversion at High Temp. | Effect on Strength |
|---|---|---|
| CAH10 (strong, metastable) | C3AH6 (weak, stable) + AH3 + H2O | -50% or more strength loss |
| C2AH8 (strong, metastable) | C3AH6 + AH3 + H2O | Increased porosity; more susceptible to chemical attack |
Cement Compatibility with Steam Curing
| Cement Type | Steam Curing | Reason |
|---|---|---|
| OPC (33, 43, 53 grade) | Suitable | Standard hydration products stable under steam |
| Rapid Hardening Cement (RHC) | Suitable | Faster initial hydration but products stable under heat |
| PPC (Portland Pozzolana) | Suitable | Pozzolanic reaction enhanced by heat; stable products |
| PSC (Portland Slag) | Suitable | Slag activation improved by steam |
| High Alumina Cement (HAC) | Not suitable | Conversion reaction causes severe, irreversible strength loss |
- Steam curing is incompatible with High Alumina Cement due to the conversion reaction.
- HAC achieves very high early strength (80% of 24-hour strength) naturally — steam curing is unnecessary and harmful.
- The conversion effect was famously implicated in the collapse of HAC roof beams in the UK in the 1970s.