Cement is primarily used in construction for the production of concrete and mortar, and is characterized as either hydraulic or non-hydraulic. Hydraulic cements (e.g. Portland cement) harden when mixed with water. The chemical reaction that results when the anhydrous cement powder is mixed with water produces hydrates that are not water-soluble. Non-hydraulic cements (e.g. lime and gypsum plaster) must be kept dry in order to retain their strength.