How it began
Roman concrete, or opus caementicium, was developed by the ancient Romans on the territory of present-day Italy. Britannica records that during the 3rd century BCE the Romans used pozzolana in place of sand in concrete and mortared rubblework. The material was in widespread use from about 150 BC. Pozzolana is a volcanic earth named for the beds at Pozzuoli on the Bay of Naples. Hydraulic concrete was first used in coastal underwater structures, probably in the harbors around Baiae before the end of the 2nd century BC.
Before it existed
The most common cements of the pre-Roman world were slaked lime mortars, which set differently from hydraulic cements. Roman builders replaced the sand in such mortars with volcanic material. Writing around 25 BC, Vitruvius distinguished the materials suited to lime mortars and recommended pozzolana from Pozzuoli for structural work.
How it works
The concrete consisted of an aggregate bound by a hydraulic mortar of lime, volcanic ash and water that hardened over time. Aggregate included pieces of rock, ceramic tile and brick rubble, and in Rome often tuff. Because the pieces were large, the material was laid rather than poured. Vitruvius specified one part lime to three parts pozzolana for buildings and one to two for underwater work. Research published in 2023 found that lumps of lime in the mix, called lime clasts, allow cracks to reseal.
How it spread
The technique traveled with Roman building across the empire. The harbor of Caesarea, built from 22 to 15 BC, used underwater concrete on a large scale, with large quantities of pozzolana imported from Puteoli. After the fire of 64 AD, rebuilding in Rome relied largely on brick-faced concrete. The dome of the Pantheon in Rome, graded with lighter aggregate toward the top, is the largest unreinforced concrete dome.
Where it stands today
Many Roman bridges, reservoirs, aqueducts and buildings made with the material still stand. For many years researchers assumed that volcanic ash from the Pozzuoli area was the key to this durability. A 2023 study reported by MIT pointed instead to the lime clasts and their self-healing effect. Pozzolana cement is still used in some countries, and Roman pozzolanic cement is chemically close to modern cements with added slag or fly ash.
