Which of the following compound affects the ultimate strength of cement?
🔬 Understanding Early vs. Ultimate Strength
The strength of cement does not develop all at once. It's a gradual process. We typically distinguish between early strength (the strength gained in the first 28 days) and ultimate strength (the long-term strength that continues to develop over months or even years). Different chemical compounds within the cement are responsible for these different phases of strength gain. The question is specifically about the compound responsible for the long-term, or ultimate, strength.
📝 Detailed Analysis of Compounds
(a) Dicalcium silicate (C2S)
This is the correct answer. Dicalcium silicate (C2S) hydrates and hardens very slowly. In the first 28 days, its contribution to strength is minimal. However, after this period, it continues to hydrate and contribute significantly to strength gain. This slow, progressive hardening is what gives cement its ultimate strength. The presence of C2S ensures that the concrete continues to get stronger over a very long period.
(d) Tricalcium silicate (C3S)
This is incorrect for ultimate strength. Tricalcium silicate (C3S) is the primary contributor to the early strength of cement. It hydrates rapidly and is responsible for most of the strength development within the first 7 to 28 days. While it provides the bulk of the standard 28-day strength, its contribution levels off, and C2S takes over for long-term strength gain.
(c) Tricalcium aluminate (C3A)
This is incorrect. C3A reacts almost instantly with water and is mainly responsible for the initial set. It contributes very little to strength at any stage, either early or ultimate.
(b) Tetracalcium alumino-ferrite (C4AF)
This is incorrect. C4AF is often considered a "filler" compound. It has a slow rate of hydration and contributes very little to the overall strength of the cement.
📊 Summary: Strength Contribution of Bogue's Compounds
| Compound | Abbreviation | Rate of Hydration | Strength Contribution |
|---|---|---|---|
| Dicalcium Silicate | C2S | Slow | Ultimate / Long-term Strength |
| Tricalcium Silicate | C3S | Fast | Early Strength (1-28 days) |
| Tricalcium Aluminate | C3A | Very Fast | Negligible (causes initial set) |
| Tetracalcium Aluminoferrite | C4AF | Medium | Very little |
💡 Study Tips
- Think "S" for Strength: Both strength-giving compounds are Silicates (C2S and C3S).
- Link the Number to the Timeline: C3S acts faster (within the first ~3 weeks). C2S acts slower (contributes to strength after ~2 weeks and beyond). Think of it as "3 comes before 2" in the timeline of strength gain.
- Ultimate = Long-Term: "Ultimate strength" means the final, long-term strength. This requires a compound that works slowly and progressively, which is the role of Dicalcium Silicate (C2S).