Heat of Hydration in a cement at early age is maximum for 

Concept:

The composition of portland cement is rather complicated but basically, it consists of the following four major compounds:

1.  Tricalcium silicate(C₃S): 3CaO.SiO₂ (alite)
2. Dicalcium silicate(C₂S): 2CaO.SiO₂ (belite)
3. Tricalcium aluminate(C₃A): 3CaO.Al₂O₃ (aluminate)
4. Tetracalcium alumino ferrite(C₄AF): 4CaO.Al₂O₃.FeO₃ (ferrite)

The brown crystals are alite (C3S), the blue crystals are belite (C2S), and the bright interstitial material is mainly ferrite (C4AF), with small dark inclusions of aluminate (C3A).

• The two silicates, namely C3S and C₂S, which together constitute about 70 to 80 percent of the cement, control most of the strength-giving properties.
• Tricalcium silicate (C3S) having a faster rate of reaction accompanied by greater heat evolution develops early strength.
• Dicalcium silicate (C₂S) hydrates and hardens slowly and provides much of the ultimate strength.
• The compound Tricalcium aluminate(C3A) is characteristically fast-reacting with water and may lead to an immediate stiffing of paste, and this process is termed a flash set.
• Tricalcium aluminate (C3A) is mostly related to the heat of hydration. It releases a large amount of heat during the first few days of hardening which generally results in early-age cracking.
• Cement with high content of C3A is not suitable for mass concreting and is better to be mixed with fly ash.​​