What is "pozzolan"

n. Any material which, when combined with calcium hydroxide, exhibits cementitious properties; often used as an extender with Portland cements.

Pozzolans are a broad class of siliceous or siliceous and aluminous materials which, in themselves, possess little or no cementitious value but which will, in finely divided form and in the presence of water, react chemically with calcium hydroxide at ordinary temperature to form compounds possessing cementitious properties. The quantification of the capacity of a pozzolan to react with calcium hydroxide and water is given by measuring its pozzolanic activity. Pozzolana are naturally-occurring pozzolans of volcanic origin.