Good cement contains
Correct Answer: A. high percentage of C3S
📚 Detailed Explanation: Role of C3S in Cement Quality
Portland cement is made up of four main clinker compounds: C3S (alite), C2S (belite), C3A (aluminate), and C4AF (ferrite). Each compound hydrates at a different rate and contributes to concrete strength, heat evolution, and durability differently.
Why A (high C3S) defines a good cement: C3S is the single most important compound in Portland cement. It reacts rapidly with water to produce C-S-H gel (the main strength-giving phase) and calcium hydroxide. C3S governs both the rate and magnitude of early strength gain (first 28 days). Ordinary Portland Cement (OPC) typically contains 45–65% C3S. High C3S variants (like rapid-hardening cement) exceed 65%. Options B, C, D: C2S is slow-reacting; C3A must be limited (it causes sulphate attack susceptibility and rapid early reaction); C4AF contributes little to strength.
Bogue Compounds in Portland Cement
| Compound | Formula | % in OPC | Role |
|---|---|---|---|
| C3S (Alite) | Ca3SiO5 | 45–65% | Early + final strength (key compound) |
| C2S (Belite) | Ca2SiO4 | 15–30% | Late strength (slow hydration) |
| C3A (Aluminate) | Ca3Al2O6 | 5–12% | Very early flash set; heat; sulphate vulnerability |
| C4AF (Ferrite) | Ca4Al2Fe2O10 | 8–12% | Minor strength; grey colour in cement |
Key Concepts for Students
- C3S = strength: this is the most tested fact about cement compounds in civil engineering exams.
- Rapid-hardening cement (RHC / IS 8041) has higher C3S and finer grinding to accelerate early strength.
- C3A must be limited to ≤5% in sulphate-resistant cement to prevent expansive ettringite formation in sulphate environments.