Pozzolans are siliceous materials that can be used as an inexpensive substitute for cement in mortar mixtures. Some forms of it occur naturally and others are manmade. Pozzolans commonly used in modern are silicates or aluminosilicates that have been converted to amorphous or glass phases.
Naturally occurring pozzolan deposits are mainly of volcanic origin. They are tuffs, volcanic ash and pumice. Due to frequent volcanic eruption, volcanic debris is found abundantly in all around the world. The meaningful use of such volcanic debris can transform them into natural resources, and can not only provide lower cost cement and concrete but can also help to decrease environmental hazard.
In general, a good pozzolan has low quantities of clay minerals, low quantities of alkali feldspar, high quantities of zeolite minerals, and volcanic glass. The amorphous or glassy form allows the silicates to react readily as the concrete cures. For use in modern cement and concrete applications, pozzolans must be low in alkalis (Na2O and K2O), which cause long-term durability problems in concrete by expansion due to the alkali-silica reaction (ASR). Chemically, pozzolans are comprised principally of oxides of silicon, aluminum and calcium. In addition, it should exhibit high porosity and specific surface area.