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Pozzolans are siliceous and aluminous materials blended with Portland cement to improve concrete performance. They occur naturally, such as volcanic ash and pumicite, or are synthesized from industrial by-products like fly ash collected from coal-fired power plant flue gases. Pozzolans react with calcium hydroxide during cement hydration to form cementitious compounds that enhance strength and durability.

Pozzolanic hydration is slower than Portland cement hydration, resulting in longer setting times and extended curing periods. However, Portland-pozzolan cements achieve high long-term strength, depending on the pozzolan proportion used. The strength activity index, defined in ASTM C 618-93, measures a pozzolan's effectiveness by comparing compressive strength of mixtures with and without pozzolan replacement.

Pozzolans have lower specific gravity compared to Portland cement. When cement is replaced on a weight basis, the volume of pozzolan needed to match that weight is higher. This volumetric difference is important for concrete mix design calculations and material proportioning in construction applications.

Pozzolans are added to cement used in roller-compacted concrete, mass concreting, and chemically resistant concrete. Their slower hydration produces low heat development, making them ideal for large-volume pours. Fly ash and other pozzolans also improve sulfate resistance and long-term durability while reducing strength and heat of hydration compared to ordinary Portland cement.

Common pozzolans include fly ash from coal combustion, rice husks burned between 500 and 700°C to yield amorphous material, metakaolin from calcined kaolinitic clay, and silica fume from silicon and ferrosilicon production. Silica fume consists of highly reactive glassy particles that fill voids in concrete, significantly enhancing its strength and durability.

Pozzolanic activity is assessed using the strength activity index, which calculates the ratio of compressive strength in cement mixtures partially replaced with pozzolans to that without any replacement. This metric, standardized in ASTM C 618-93, quantifies a pozzolan's effectiveness in contributing to concrete strength development and is essential for quality control in cement blending.

In the presence of moisture, finely divided pozzolans react with calcium hydroxide released during hydration of cement to produce additional cementitious compounds. This pozzolanic reaction improves concrete's mechanical properties and durability. The reaction proceeds more slowly than primary cement hydration, contributing to extended curing times but ultimately higher long-term strength.