Mineral hydration

Mineral hydration is an inorganic chemical reaction where water is added to the crystal structure of a mineral, usually creating a new mineral, usually called a hydrate.

There are two main ways in which minerals hydrate. One is conversion of an oxide to a double hydroxide, as in the hydration of calcium oxide - CaO - to calcium hydroxide - Ca(OH)₂, the other is incorporation of water molecules directly into the crystalline structure of the mineral, as in the hydration of feldspars to clay minerals.

Some mineral structures, for example, montmorillonite, are capable of including a variable amount of water without significant change to the mineral structure.

Hydration is the mechanism by which Portland cement develops strength.