The hydration, time-averaged and dynamic structures of adsorbed water on clay minerals have been reviewed to clarify the nature of clay hydration and the difference in structure between adsorbed and free water. Hydration of the clay minerals involves three kinds of water adsorption on the surface in interlamellar, intraaggregate and interaggregate pore spaces. The hydration mechanism and factors influencing hydration are quite different in these hydration modes. Water molecules in the clay interlayers have statistically and dynamically a more rigid structure than that in bulk liquid water. Especially, the thermal motion of water bound to divalent cations is restricted and similar to that of normal ice (Ih).
