The setting of cement and hardening of concrete
Cement hardens when it comes into contact with water.
This hardening is a process of crystallization. Crystals form (after a certain length of time which is known as the initial set time) and interlock with each other. Concrete is completely fluid before the cement sets, then progressively hardens. The cement and water mixture that has crystallized in this way encloses the aggregate particles and produces a dense material. The concrete continues to harden over several months.
Hardening is not a drying process and can very well take place in water.
Heat speeds up the setting and hardening of cement, and cold slows it down and can even completely stop the processes.
In order to crystallize or hydrate) cement requires a quantity of water equal to 25% of its weight. But in order for it to be laid and remain sufficiently workable, twice this amount is usually required. However, too much water can reduce the strength and durability of the concrete. The paste (i.e. the mixture of cement and water) acts both as a lubricant and an adhesive.
Hardening does not start as soon as cement and water are mixed, but after a certain length of time known as the "initial set time" during which the concrete can be mixed, transported, laid and vibrated.