Roman builders combined the semicircular arch with opus caementicium, a concrete of lime, volcanic ash (pozzolana) and aggregate that set hard and could be cast into mass form. This allowed barrel vaults, groin vaults and the unreinforced dome, culminating in the 43-metre coffered span of the Pantheon completed around 125 CE under Hadrian. The same technology produced basilicas, baths, amphitheatres such as the Colosseum, and the substructures of harbours and roads. Vitruvius described the materials and methods in his treatise around 25 BCE.
Concrete and the arch let Rome enclose large interior spaces and build durable infrastructure at imperial scale, setting the structural vocabulary of Western monumental architecture for two millennia.