The compound of Portland cement which reacts immediately with water and also sets first is ......
🔥 The Race of Hydration
When water is added to cement, a "race" begins between the four main Bogue's compounds to react. The compound that reacts the fastest is responsible for the initial stiffening, or "setting," of the cement paste. The slower-reacting compounds contribute to the long-term hardening and strength gain.
📝 Detailed Analysis of the Options
(a) Tri-calcium silicate (C₃S)
This compound is the primary contributor to early strength (hardening), but it reacts much more slowly than the aluminates. Its main hydration activity picks up after the initial set has occurred.
(b) Tri-calcium aluminate (C₃A)
This is the correct answer. C₃A is the most reactive of the four Bogue's compounds. It reacts almost instantaneously with water, releasing a large amount of heat. This rapid reaction is what causes the initial set of the cement. Its reaction is so fast that gypsum must be added to control it and prevent a "flash set."
(c) Di-calcium silicate (C₂S)
This is the slowest reacting compound. Its hydration is responsible for the long-term strength gain of concrete, continuing for months or even years.
(d) Free lime (CaO)
Free lime is an uncombined residue from manufacturing, not one of the primary reactive compounds. Its slow hydration can cause unsoundness (unwanted expansion) and it does not contribute to the initial set.
📊 Rate of Hydration and Contribution
| Compound | Rate of Hydration | Primary Contribution |
|---|---|---|
| Tri-calcium Aluminate (C₃A) | Fastest | Initial Set, High Heat |
| Tetracalcium Aluminoferrite (C₄AF) | Fast | Initial Set, Color |
| Tri-calcium Silicate (C₃S) | Moderate | Early Strength (Hardening) |
| Di-calcium Silicate (C₂S) | Slowest | Ultimate Strength (Hardening) |
💡 Study Tips
- "A" for Aluminate, "A" for Action: Remember that the Aluminate (C₃A) is the first to take action, causing the initial set.
- "S" for Silicate, "S" for Strength: Remember that the Silicates (C₃S and C₂S) are responsible for the strength that develops later.
- Setting vs. Hardening: This question highlights the difference. Setting is the initial stiffening (C₃A's job). Hardening is the gain of strength (the silicates' job).