The term ‘heat of hydration’ is applied to the heat generated by the chemical reactions, which occur in setting concrete between the water and cement. The heat causes the concrete first to expand and then to shrink as it cools. If there is a temperature gradient across the concrete or the concrete is otherwise restrained, cracking may occur. The two factors, which are most likely to cause excessive generation of heat, are very large pours and high cement contents. Some specifications set a maximum cement content in order to limit the generation of heat.