Cement hardens through a process called hydration. This process begins when water is mixed in with cement and an aggregate such as gravel or crushed limestone is added to form concrete.
Cement is made of five main compounds commonly referred to as clinker. When water is added to clinker, these compounds reacts to create calcium and hydroxide ions as well as heat. As this process progresses the calcium hydroxide crystallizes and causes the mixture to harden. Heat will speed up the hardening process and cold will slow it down, and potentially stop the reaction. Too much water in the mixture ultimately leads to weaker cement, but cement hardens even if all of the water is not consumed by the reaction or evaporated by the heat.