The volcanic ash used by Romans in concrete to help it set underwater was pozzolana.
Pozzolana is a volcanic ash or fine volcanic material rich in reactive silica and alumina. When mixed with lime and water, it undergoes a pozzolanic reaction, forming cementitious compounds that harden hydraulically. Unlike ordinary lime mortar, this mixture can set in damp conditions and beneath water.
The name comes from Pozzuoli, ancient Puteoli, near Naples, where important deposits were available. Roman builders used different colored varieties, including black, white, gray, and red ash. The material became especially valuable in harbor walls, piers, and other marine structures, where ordinary mortar would remain vulnerable to water.
Pozzolana is often confused with pumice, tufa, or travertine. Those are volcanic or limestone building materials, but they do not name the specific reactive ash used as the binder ingredient. Roman concrete also contained aggregates such as stone, brick fragments, or tufa; pozzolana’s special role was in producing the hydraulic mortar. At Cosa and Caesarea, Roman marine concrete demonstrates how effectively this technology worked.