pozzolana

Pozzolana is a natural or artificial siliceous or aluminous material that, in the presence of calcium hydroxide and water, reacts to form cementitious compounds. The name derives from Pozzuoli, Italy, where volcanic ash from the Phlegraean Fields was historically quarried and used in mortars and concretes.

In lime-based systems, pozzolanic materials react with calcium hydroxide released during lime hydration to produce calcium silicate hydrates and related cementitious compounds, contributing to strength and durability. Common natural pozzolans include volcanic ash, pumice, and tuff; artificial pozzolanic materials include fly ash, metakaolin, and silica fume. The pozzolanic reaction progresses over time, improving long-term strength, reducing permeability, and enhancing resistance to sulfate attack and freeze–thaw cycles when properly proportioned.

Historical use dates to classical Rome, where volcanic ash-enabled concretes endured long after exposures that would damage ordinary lime mortars. In modern construction, pozzolanic materials are used with Portland cement to reduce clinker content, improve workability, and increase durability. They are especially valued for lowering heat of hydration and for enabling sustainable concrete by reducing cement production impacts.

Standards and classification vary by region. In many countries, natural pozzolanic materials are defined and validated for use in cementitious mixes under specific standards (for example, ASTM C618 in the United States). Proper particle size, chemistry, and reactivity are essential for effective performance, and the benefits depend on dosage and mixing practice.