Which of the following compounds of cement is responsible for early strength ?
🔬 Understanding Bogue's Compounds and Strength Development
The strength of cement develops over time through the hydration of its four main Bogue's compounds. The overall strength development is a tale of two stages: early strength, which is critical for construction timelines (like removing formwork), and later or progressive strength, which contributes to the long-term durability of the concrete. Each Bogue's compound has a specific role in this process.
📝 Detailed Analysis of the Compounds
(b) C₃S (Tricalcium Silicate)
This is the correct answer. Tricalcium Silicate, also known as Alite, is the most important compound for strength development, particularly in the early stages. It constitutes about 40-50% of OPC. C₃S hydrates relatively quickly and is primarily responsible for the strength gained within the first 28 days. Cements with a higher percentage of C₃S, like Rapid Hardening Cement, gain strength much faster.
(d) C₂S (Dicalcium Silicate)
This is incorrect. Dicalcium Silicate (Belite) hydrates very slowly. Its role is to provide progressive or long-term strength (strength gained after 28 days and up to a year). It has very little contribution to the early strength of cement.
(c) C₃A (Tricalcium Aluminate)
This is incorrect. Tricalcium Aluminate (Celite) is the fastest to react, but it contributes very little to strength, especially compared to the silicates. Its main role is to govern the initial set or flash set of the cement. It also generates a large amount of heat.
(a) C₄AF (Tetracalcium Aluminoferrite)
This is incorrect. Tetracalcium Aluminoferrite (Felite) has the poorest cementing properties of the four compounds. It hydrates at a moderate rate but its contribution to overall strength is minimal. Its primary role is to act as a flux in the kiln and give cement its characteristic greyish color.
📊 Summary: Function of Bogue's Compounds
| Compound (Common Name) | Approx. Quantity | Primary Function |
|---|---|---|
| Tricalcium Silicate (Alite) | ~40% | Early Strength (up to 28 days) |
| Dicalcium Silicate (Belite) | ~32% | Progressive / Later Strength |
| Tricalcium Aluminate (Celite) | ~10% | Initial Setting / Flash Set |
| Tetracalcium Aluminoferrite (Felite) | ~8% | Poorest cementing value, gives color |
💡 Study Tips
- Silicates for Strength: Remember that the two silicate compounds (C₃S and C₂S) are the primary strength-givers.
- The "3" is Fast, the "2" is Slow: A simple mnemonic: C₃S gives strength fast (early). C₂S gives strength slow (later).
- A is for Action (Initial): C₃A (Aluminate) is all about the initial action - the first reaction with water that causes setting. It's fast but doesn't provide much lasting strength.