Pozzolans

Pozzolans are siliceous or aluminous materials that, in finely divided form and in the presence of water, react with calcium hydroxide released during cement hydration to form additional cementitious compounds. They do not harden on their own but contribute to strength and durability when used with lime or cementitious binders, particularly Portland cement.

Chemically, pozzolanic materials react with calcium hydroxide to produce calcium silicate hydrates and calcium aluminate hydrates, refining the cement paste microstructure and reducing porosity. This reaction enhances long-term strength and improves resistance to chemical attack and permeability.

Pozzolans are typically categorized as natural or artificial. Natural pozzolans include volcanic ash (pozzolana), pumice, diatomaceous earth, and calcined clays. Artificial or industrial pozzolans include fly ash from coal combustion, metakaolin, silica fume (micro silica), and other reactive siliceous or aluminous materials. The classification reflects both composition and origin rather than a single chemical formula.

Historically, the term derives from Pozzuoli, near Naples, where ancient Roman concrete used volcanic ash to create hydraulic cement. Modern use combines pozzolanic materials with Portland cement to form supplementary cementitious materials (SCMs) that enhance performance.

In construction, pozzolans are used to replace part of the Portland cement in concrete, mortars, and grouts. Benefits include improved long-term strength, reduced heat of hydration, lower permeability, better sulfate and alkali-silica resistance, and often improved workability. Proportions and reactivity influence early strength development and durability, so mix design and curing conditions are important.

Standards and specifications exist internationally, such as ASTM C618 in the United States for natural pozzolanic materials, with separate standards for silica fume and related products, and European norms that classify supplementary cementitious materials.