The cement compound that reacts and sets immediately after addition of water is:
Try it first: pick an option, then read the explanation.
Understanding Hydration Reactivity
The process of cement hardening, known as hydration, begins the moment water is added. The four main compounds in cement (Bogue's compounds) participate in this reaction, but at vastly different speeds. The compound that reacts the fastest is responsible for the initial stiffening of the concrete, known as the initial set. Identifying this "first responder" is key to understanding the early behavior of cement.
Detailed Analysis of Compounds
(b) C3A (Tricalcium Aluminate)
This is the correct answer. Tricalcium Aluminate is the most reactive compound in Portland cement. It reacts almost instantaneously and violently with water, releasing a large amount of heat. This rapid reaction is what causes the initial setting of the cement. Its reaction is so fast that it would cause a "flash set" (instant hardening) if not controlled by adding gypsum during the grinding process.
(a) C2S (Dicalcium Silicate)
This is incorrect. C2S is the slowest reacting compound. It hydrates and hardens over a long period (months to years) and is responsible for the ultimate, long-term strength of the concrete.
(c) C3S (Tricalcium Silicate)
This is incorrect. While C3S is responsible for early strength, its reaction is much slower than C3A. It begins to hydrate significantly after the initial set has occurred and continues for the first few weeks.
(d) C4AF (Tetracalcium Aluminoferrite)
This is incorrect. C4AF also reacts relatively early, but its reaction is less vigorous and contributes less to the overall properties compared to C3A. C3A is definitively the first and fastest to react.
Order of Hydration Speed
| Compound | Full Name | Speed of Reaction | Primary Role |
|---|---|---|---|
| C3A | Tricalcium Aluminate | Immediate / Very Fast | Initial Setting / Flash Set |
| C4AF | Tetracalcium Aluminoferrite | Fast | Early Stiffening |
| C3S | Tricalcium Silicate | Medium | Early Strength |
| C2S | Dicalcium Silicate | Slow | Ultimate Strength |
Study Tips
- Immediate = C3A: This is a fundamental concept. The fastest, most immediate reaction comes from C3A.
- Think "A for Action": The 'A' in C3A can help you remember it's the first to spring into "Action" when water is added.
- Setting vs. Strength: Reiterate the difference. The fastest reaction (C3A) causes setting (stiffening), while the slower reactions of the silicates (C3S and C2S) provide the actual strength.