Which constituent in cement is to be kept minimum to avoid a sulphate attack?
🔬 Understanding Sulphate Attack
Sulphate attack is a chemical process that can cause serious damage to concrete. It occurs when sulphate ions from external sources (like soil, groundwater, or seawater) penetrate the concrete and react with compounds in the hydrated cement paste. This reaction forms new, expansive products, primarily ettringite. The growth of these ettringite crystals exerts immense internal pressure, leading to expansion, cracking, loss of strength, and eventually, the disintegration of the concrete. Therefore, for structures in sulphate-rich environments, it's crucial to use cement that can resist this attack.
📝 Detailed Analysis of the Bogue's Compounds
(c) C₃A (Tricalcium Aluminate)
This is the correct answer. Tricalcium Aluminate (C₃A) is the most vulnerable compound to sulphate attack. It reacts directly with sulphate ions and calcium hydroxide (a byproduct of cement hydration) to form calcium sulphoaluminate (ettringite), which has a volume about 227% greater than the original C₃A. This massive increase in volume is the primary cause of the destructive expansion. To make cement resistant to sulphates, the most effective strategy is to limit the amount of C₃A. Sulphate Resisting Cement (SRC) is manufactured by keeping the C₃A content below 5%.
(a) C₂S (Dicalcium Silicate)
This is incorrect. C₂S is the least reactive compound and has a very low susceptibility to sulphate attack. In fact, its proportion is often increased in SRC to ensure good long-term strength.
(b) C₃S (Tricalcium Silicate)
This is incorrect. While the hydration of C₃S produces calcium hydroxide, which is a necessary ingredient for the destructive reaction, C₃S itself is not the primary target for reduction. Controlling the C₃A is far more effective.
(d) C₄AF (Tetracalcium Aluminoferrite)
This is incorrect. C₄AF is less reactive than C₃A and contributes much less to sulphate attack. While it does contain alumina, its reaction is slower and less expansive.
📊 Summary: Role of Bogue's Compounds in Sulphate Attack
| Compound | Susceptibility to Sulphate Attack | Content in Sulphate Resisting Cement (SRC) |
|---|---|---|
| C₃A (Tricalcium Aluminate) | Highest (Primary Culprit) | Minimized (< 5%) |
| C₄AF (Tetracalcium Aluminoferrite) | Moderate | Relatively unchanged |
| C₃S (Tricalcium Silicate) | Low (Indirect role) | Slightly reduced |
| C₂S (Dicalcium Silicate) | Very Low | Increased |
💡 Study Tips
- A is for Attack: Remember that C₃A (Aluminate) is the primary target for sulphate Attack.
- The SRC Recipe: To make Sulphate Resisting Cement, the recipe is simple: keep C₃A as low as possible.
- Think Beyond Cement Type: As the notes mention, other factors also help resist sulphate attack: making a dense, impermeable concrete (low water-cement ratio, good compaction) and using air-entraining agents can also significantly improve durability.