Pozzolana is a fine sandy volcanic ash, originally discovered and dug at Pozzuoli in the region around Vesuvius, but later at a number of other sites. Vitruvius speaks of four types of pozzolana. It is found in all the volcanic areas of Italy in various colours: black, white, grey and red.

Mixed with lime it acts like Portland cement and makes a strong mortar that will also set under water. It transformed the possibilities for making concrete structures, although it took the Romans some time to discover its full potential. Typically it was mixed two-to-one with lime just prior to mixing with water. The Roman port at Cosa was built of Pozzolana that was poured underwater, apparently using a long tube to carefully lay it up without allowing sea water to mix with it. The three piers are still visible today, with the underwater portions in generally excellent condition after 2100 years.

Pozzolana is still produced today, although the volcanic ash has been replaced in most cases with coal ash from thermal generating stations (known as fly ash or fly-ash), ground slag from various mining processes, or silica fume, ash produced when heating quartz. Modern Pozzolana's are actually a mix of Pozzolana and Portland, more correctly known as Portland-Pozzolana. In addition to underwater use, pozzolana's high acidity makes it especially resistant to common forms of corrosion from sulphates. Once fully set, Portland-Pozzolana is stronger than Portland alone due to it being less pourous, which also makes it more resistant to water leakage and spalling.