Which of the following compounds of Portland cement reacts immediately with water and sets early?
🔬 Understanding the Speed of Hydration
When water is mixed with cement, the different Bogue's compounds start to react (hydrate) at very different speeds. The compound that reacts the fastest is responsible for the initial stiffening of the concrete paste, a process known as the initial set. This immediate reaction is crucial as it marks the beginning of the transition from a plastic, workable mix to a solid material. The question asks to identify this most reactive compound.
📝 Detailed Analysis of Reaction Speeds
(c) Tricalcium aluminate (C3A)
This is the correct answer. Tricalcium aluminate (C3A) is by far the most reactive of the four main Bogue's compounds. It reacts almost instantaneously with water, releasing a large amount of heat in a short period. This rapid reaction is the primary cause of the initial set of the cement. In fact, its reaction is so fast that gypsum has to be added to the cement to slow it down to a manageable rate (the 30-minute initial set time).
(b) Tetracalcium aluminoferrite (C4AF)
This is incorrect. While C4AF also reacts relatively early in the hydration process, its reaction is significantly slower than that of C3A. It contributes to the early stiffening but is not the compound that reacts "immediately."
(d) Tricalcium silicate (C3S)
This is incorrect. C3S is responsible for early strength gain, but its reaction begins after the initial set caused by C3A. It hydrates steadily over the first few weeks, not immediately upon contact with water.
(a) Dicalcium silicate (C2S)
This is incorrect. C2S is the slowest reacting compound. It hydrates very slowly over months and years, contributing to long-term strength, and plays no role in the immediate setting of cement.
📊 Order of Reaction Speed
| Compound | Abbreviation | Speed of Reaction | Primary Role |
|---|---|---|---|
| Tricalcium Aluminate | C3A | Immediate / Very Fast | Causes Initial Set |
| Tetracalcium Aluminoferrite | C4AF | Fast | Early stiffening |
| Tricalcium Silicate | C3S | Medium | Early Strength (1-28 days) |
| Dicalcium Silicate | C2S | Slow | Ultimate Strength |
💡 Study Tips
- Immediate = C3A: Always associate Tricalcium Aluminate (C3A) with the most immediate, fastest reaction and the initial set.
- Think "A for Action": The 'A' in C3A can stand for "Action" because it's the first compound to take action when water is added.
- Setting vs. Strength: Remember that the fastest setting compound (C3A) is not the one that gives the most strength. The silicates (C3S and C2S) are the primary strength-givers.