The chemical reaction between cement and water is known as Hydration of Cement.
Heat of hydration of Ordinary Portland Cement during 7 days is about 89-90cal/gm and during 28 days is about 90-100 cal/gm
Water-About 23% of by weight of cement is required to carry out the complete hydration.
About 15% of water is used up in filling voids of the cement particles, hence effectively 38% of water (by weight of cement) is required to carry out complete hydration.