Pozzolan is a siliceous or siliceous and aluminous material that, when mixed with calcium hydroxide, exhibits cementitious properties. These materials are naturally occurring or derived as byproducts from industrial processes—examples include volcanic ash, fly ash, and silica fume. When added to concrete, pozzolans react with calcium hydroxide produced during the hydration of Portland cement to form additional calcium silicate hydrate (CSH), the binding phase that contributes significantly to the strength and durability of concrete. This pozzolanic reaction is essential, especially under severe environmental conditions, as it enhances the material’s properties by refining the microstructure and reducing the permeability, which in turn increases resistance to aggressive agents.