Opus caementicium

Opus caementicium was the core of every Roman wall after the 2nd century BC. Many walls made in opus caementicium were covered with other materials to make a more robust and workable surface.

It was a construction technique using an aggregate, water and a binding agent. The aggregate functioned as a filler, such as gravel, chunks of bricks or stones and rubble. The binding agent is usually called mortar such as lime, gypsum, clay (pozzolana). Nowadays cement is used.