The chemical reaction that takes place between cement when mixed with water is exothermic. The intensity of this reaction is measured in this test. The value of heat of hydration can be important where the cement maybe incorporated into concrete which will be poured in large volumes. In such cases there can be a considerable build up of heat as the reaction takes place, which, if excessive, could caused cracking in the structure. Heat of hydration of cement is expressed in calories per gram.