A pozzolan is a material which, when combined with calcium hydroxide, exhibits cementitious properties. Pozzolans are commonly used as an addition to Portland cement to increase the long-term strength and other material properties. Pozzolans are primarily vitreous siliceous materials which react slowly with calcium hydroxide to form calcium silicates; other cementitious materials may be formed depending on the constituents of the pozzolan. The pozzolanic reaction is slower than the rest of the reactions which occur during cement hydration, and thus the short-term strength of concrete made with pozzolans may not be as high as concrete made with purely cementitious materials.
The first known pozzolan was pozzolana, a volcanic ash, for which the category of materials was named. The most commonly-used pozzolan today is fly ash, though silica fume, ground granulated blast furnace slag and other materials are also used as pozzolans.