pozzolaan

Pozzolaan, in English often referred to as pozzolanic material, denotes a siliceous or aluminous material that, in the presence of water and calcium hydroxide, reacts to form cementitious compounds. It does not harden on its own but contributes to strength and durability when blended with cement, by consuming calcium hydroxide and producing additional calcium silicate hydrates and related phases. The term originates from Pozzuoli, Italy, where ancient volcanic ashes were used in cement.

Pozzolaan materials are broadly categorized into natural and artificial types. Natural pozzolans include volcanic ash, pumice, tume, and diatomaceous earth, which contain reactive silica or alumina. Artificial pozzolans include fly ash from coal-fired power plants, silica fume, metakaolin (calcined clay), and calcined natural clays. Some materials, such as granulated blast-furnace slag, can exhibit pozzolanic behavior under certain conditions and are commonly used as supplementary cementitious materials (SCMs).

In concrete technology, pozzolaan are added to Portland cement to form blended cements. They reduce the amount of Portland cement required, decrease heat of hydration, lower permeability, and improve sulfate resistance and long-term strength. They can also enhance resistance to alkali-silica reaction and reduce the environmental footprint by utilizing industrial by-products or natural minerals. However, early-age strength development can be slower, and the performance depends on material variability and proper proportioning.

Testing and standards exist to assess pozzolanic activity and compatibility with cement, including activity index tests, the Frattini and Chapelle tests, and standards such as ASTM C618 for fly ash and related European and international specifications that define acceptable pozzolanic materials and usage in cement blends.