Pozzolane

Pozzolane, or pozzolana, refers to siliceous or siliceous and aluminous materials that, in the presence of moisture, react with calcium hydroxide released during cement hydration to form cementitious compounds. The term originates from Pozzuoli, Italy, where ancient Romans used volcanic ash in concrete. Natural pozzolane include volcanic ash and pumice, diatomaceous earth, and certain clays; artificial pozzolane include fly ash from coal combustion, silica fume, metakaolin, and calcined clays. Pozzolane are not cementitious on their own but gain binding properties when combined with lime or Portland cement.

In modern concrete, pozzolane are typically used as supplementary cementitious materials (SCMs) to partially replace Portland cement. Benefits include enhanced long-term strength, reduced heat of hydration, lower permeability, and improved resistance to chemical attack and sulfate exposure. They can also improve workability and reduce risks such as alkali-silica reaction, depending on material and mix design. The rate of strength development varies; some pozzolanic reactions contribute mainly to late-age strength.

Standards classify pozzolanic materials for cementitious use and set limits for silica, alumina, and other properties. Natural pozzolane are defined separately from artificial pozzolane; Portland-pozzolana cements blend clinker with pozzolanic materials to meet specified performance. Pozzolane are widely used to promote sustainable concrete, enabling lower cement content while maintaining or improving durability, especially in aggressive environments.