The setting and hardening of cement paste is mainly due to the hydration and hydrolysis of:
🔥 Understanding Bogue's Compounds
The properties of cement are governed by four main chemical compounds, known as Bogue's compounds. Their reaction with water dictates the cement's behavior.
- Tricalcium Silicate (C₃S): Responsible for early strength (first 7 days).
- Dicalcium Silicate (C₂S): Responsible for long-term, ultimate strength.
- Tricalcium Aluminate (C₃A): Reacts fastest, responsible for initial set and high heat of hydration.
- Tetracalcium Aluminoferrite (C₄AF): Reacts fast, but contributes little to strength. Acts as a flux in manufacturing.
📝 Detailed Analysis of the Options
This is a nuanced question. "Setting" refers to the initial stiffening, while "hardening" refers to strength gain. The question asks about the combined process, but the initial phase is dominated by one compound.
(a) Tri calcium silicate (C₃S) & (c) Di calcium silicate (C₂S)
These compounds are the primary contributors to strength (a key part of "hardening"), but they react slower than the aluminates. C₃S provides early strength (up to 28 days), and C₂S provides long-term strength. They are not the primary cause of the initial "set".
(b) Tetra calcium alumina ferrite (C₄AF)
This compound also reacts very quickly upon adding water, but its contribution to both setting and strength is minimal compared to C₃A and the silicates.
(d) Tri calcium aluminates (C₃A)
This is the correct answer. C₃A is the most reactive compound. It hydrates almost instantly when water is added, releasing a large amount of heat. This rapid reaction is primarily responsible for the initial setting of the cement paste. While the ultimate hardening and strength come from the silicates, the initial setting and hardening process is initiated and dominated by the hydration of C₃A.
📊 Bogue's Compounds: Reaction Speed and Role
| Compound | Abbreviation | Rate of Hydration | Primary Role |
|---|---|---|---|
| Tricalcium Aluminate | C₃A | Fastest | Initial Setting, High Heat Release |
| Tetracalcium Aluminoferrite | C₄AF | Fast | Fluxing agent, minor setting role |
| Tricalcium Silicate | C₃S | Moderate | Early Strength (Hardening) |
| Dicalcium Silicate | C₂S | Slowest | Ultimate Strength (Long-term Hardening) |
💡 Study Tips
- "A" for Aluminate, "A" for Action: Remember that the Aluminates (C₃A and C₄AF) are the first to react and cause the initial action (setting).
- "S" for Silicate, "S" for Strength: Remember that the Silicates (C₃S and C₂S) are responsible for the long-term strength development.
- Setting vs. Hardening: Clearly distinguish that setting is the initial stiffening (caused by C₃A), while hardening is the gain of strength over time (caused by C₃S and C₂S).