Pozzolans are materials which, while not inherently cementitious, contain reactive constituents that combine with lime at ordinary temperatures in the presence of water to form stable, insoluble hydraulic compounds. Pozzolans have been subjected to heat, either naturally, as with volcanic earths, or artificially through the firing of clays such as brick and tile. Heating renders certain constituents such as silicates and aluminates reactive towards alkalis.

When mixed with lime, hydraulic compounds are produced creating a hydraulic set, enhancing durability and performance.