Which of the following cement is supposed to be the best cementing material and is well burnt cement?
Correct Answer: A. Tricalcium silicate
📚 Detailed Explanation: Cement Compounds — Why C3S is the Best
Portland cement clinker contains four main compounds formed during the high-temperature burning process (clinkering at 1400–1500°C). Each compound contributes differently to strength development, heat of hydration, and durability. A well-burnt clinker maximises the formation of Tricalcium Silicate (C3S), which is why it is prized in good-quality cement.
Why A (Tricalcium silicate, C3S) is correct: C3S hydrates rapidly and contributes the most to both early and long-term strength through the formation of calcium silicate hydrate (C-S-H) gel. It is produced in maximum quantities when burning is complete and the kiln temperature is adequate. Under-burning leaves unreacted lime (CaO) and produces more C2S (dicalcium silicate), which hydrates slowly and gives lower early strength.
The Four Main Clinker Compounds
| Compound | Notation | Rate of Hydration | Contribution |
|---|---|---|---|
| Tricalcium silicate | C3S | Fast | High early & long-term strength |
| Dicalcium silicate | C2S | Slow | Long-term strength, low heat |
| Tricalcium aluminate | C3A | Very fast | Early stiffening; susceptible to sulphate attack |
| Tetracalcium alumino ferrite | C4AF | Moderate | Minor strength; gives grey colour |
Key Concepts for Students
- C3S is the best cementing compound: high content (55–65%) in OPC indicates well-burnt, good-quality cement.
- C2S (the slow one) dominates in low-heat cement and is responsible for the long-term strength of concrete dams.
- C3A must be limited in sulphate-resistant cement (≤5%) to prevent ettringite formation and expansion.