In the broad sense, a material which can bind other materials together into a hardened, cohesive mass. Cements in general may be organic or inorganic, including various plasters and glues, but the most important classes of cements used worldwide are those which are hydraulic; that is, harden through addition of water to form a water-insoluble final product. The dominant hydraulic cement used worldwide is Portland cement (Hewlett 1998), which consists primarily of hydraulic calcium silicates in addition to calcium sulfate, aluminate, and aluminoferrite phases (ASTM International 2016). Alternatives to Portland cement in some applications include gypsum or lime (particularly as plasters), geopolymers, calcium aluminate or sulfoaluminate cements, and magnesia-based cements. However, considering the current domination of cement usage by Portland cement, this is the material described in detail here.