Opus caementitium
Innovation started in the 3rd or 2nd century BC with the development of Roman concrete as a readily available adjunct to, or substitute for, stone and brick. More daring buildings soon followed, with great pillars supporting broad arches and domes. The freedom of concrete also inspired the colonnade screen, a row of purely decorative columns in front of a load-bearing wall. In smaller-scale architecture, concrete's strength freed the floor plan from rectangular cells to a more free-flowing environment.