pozzolan

Pozzolan is a siliceous or aluminous material that, when finely divided and in the presence of moisture, chemically reacts with calcium hydroxide released during cement hydration to form cementitious compounds. In this way, pozzolanic materials contribute to the strength and durability of cementitious systems, even though they are not Portland cement themselves.

The term has historical roots in the volcanic ash used by Romans to produce hydraulic lime cements at Pozzuoli, Italy. Natural pozzolans include volcanic ash, pumice, tuff, and diatomaceous earth. Artificial pozzolans comprise fly ash from coal-fired power plants, metakaolin and calcined clays, silica fume, and other reactive silica/alumina-rich materials. Pozzolanic materials are commonly blended with Portland cement or used in lime-based mortars to enhance performance.

Uses and benefits include improved workability, reduced heat of hydration, increased long-term strength, lower permeability, and enhanced resistance to sulfate attack. They also reduce the amount of Portland cement needed, contributing to reduced embodied energy and lower carbon dioxide emissions. Standards such as ASTM C618 define and classify pozzolanic materials for use in concrete, and European and international standards (for example EN 197-1) provide guidance on acceptable pozzolanic materials and their compositions.

Limitations can include slower early strength gain and sensitivity to curing conditions. The effectiveness of a pozzolan depends on its chemical composition, fineness, and interaction with specific cements and admixtures. Proper selection and proportioning are essential to maximize benefits in a given mix.