caementicium

Caementicium is the term used for the lime-based hydraulic concrete developed by ancient Rome. It consisted of a binder of lime putty mixed with pozzolanic ash and an aggregate such as rubble, brick, or small stones. The addition of volcanic ash from areas around Pozzuoli and other Italian sources gave the mixture hydraulic set, allowing it to harden in water and over time without complete drying.

In construction, caementicium was typically used as the core or infill of walls and structures (opus caementicium), which were then faced with dressed stone or brick. This technique enabled the creation of large spans, thick vaults, and durable foundations, contributing to the longevity of many Roman buildings, aqueducts, harbors, and monuments such as the Pantheon and the Colosseum.

Historical context and significance: caementicium formed a cornerstone of Roman engineering from roughly the 3rd century BCE through the imperial period, facilitating complex architecture and rapid urban development. Although knowledge of the precise Roman recipes declined after the Western Roman Empire, the concept of hydraulic concrete influenced later engineering. In modern times, Roman caementicium is studied as an early example of hydraulic cement and is often contrasted with later Portland cement-based concretes.