How did ancient Greek and Roman builders make hydraulic cement?

Ancient Greek and Roman builders developed hydraulic cement by combining lime with specific volcanic materials, a process that allowed the resulting mortar and concrete to harden even under water.123

The key ingredient was pozzolana (or pulvis puteoli), a fine volcanic earth first identified near Puteoli (modern Pozzuoli), Italy. Roman engineers traditionally mixed two parts of this volcanic ash with one part of hydrated lime.42

Recent research has highlighted a technique known as "hot mixing," where quicklime was added directly to volcanic ash, water, sand, and aggregate. This process left behind tiny chunks of limestone called "lime clasts". These clasts acted as calcium reservoirs, providing the material with a unique, self-healing property: when cracks formed, water would dissolve the calcium from the clasts, which then recrystallized to seal the damage.5

By mixing this pozzolanic cement with aggregates such as broken tuff, travertine, brick, or marble, the Romans created opus caementicium, a durable concrete that enabled the construction of monumental structures like the Pantheon and the Baths of Caracalla.423