Reactivity of Cement Compounds with Water

In ordinary Portland cement, various compounds are responsible for its reactivity when mixed with water. The most significant compound that reacts with water in cement is C3A (tricalcium aluminate).

1. Tricalcium Aluminate (C3A)
- Tricalcium aluminate is one of the main components of Portland cement, typically comprising around 5-10% of the cement composition.
- When water is added to cement, tricalcium aluminate rapidly reacts to form calcium aluminate hydrate and a large amount of heat is released. This exothermic reaction is known as the heat of hydration.
- The reaction of C3A with water is highly exothermic, and it contributes to the quick setting of cement and the development of early strength in concrete.
- However, if the amount of C3A is too high in cement, it can lead to the formation of excessive amounts of ettringite, a secondary crystalline phase. This can cause expansion and cracking in concrete, known as "delayed ettringite formation" or "DEF."

Other Cement Compounds:
- C3S (tricalcium silicate): Although tricalcium silicate is the main compound in Portland cement (typically comprising around 40-60% of the cement composition), it is not the first one to react with water.
- C2S (dicalcium silicate): Dicalcium silicate is another major compound in cement, but it reacts more slowly with water compared to C3A.
- C4AF (tetracalcium aluminoferrite): Tetracalcium aluminoferrite is also present in cement, but it has a minimal contribution to the early reactivity with water.

Conclusion:
In ordinary Portland cement, the first compound to react with water is C3A. Its reaction is highly exothermic and leads to the quick setting and early strength development of concrete. However, it is important to control the amount of C3A to prevent issues such as delayed ettringite formation.