Ancient Roman architecture
Although concrete had been used on a minor scale in Mesopotamia, Roman architects perfected Roman concrete and used it in buildings where it could stand on its own and support a great deal of weight. The first use of concrete by the Romans was in the town of Cosa sometime after 273 BC. Ancient Roman concrete was a mixture of lime mortar, aggregate, pozzolana, water, and stones, and was stronger than previously-used concretes. The ancient builders placed these ingredients in wooden frames where they hardened and bonded to a facing of stones or (more frequently) bricks. The aggregates used were often much larger than in modern concrete, amounting to rubble.
Pozzolana
The pozzolanic reaction is the chemical reaction that occurs in portland cement containing pozzolans. It is the main reaction involved in the Roman concrete invented in Ancient Rome. At the basis of the pozzolanic reaction stands a simple acid-base reaction between calcium hydroxide (as Portlandite) and silicic acid.