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.
Self-healing material
The ancient Romans used a form of lime mortar that has been found to have self-healing properties. By 2014, geologist Marie Jackson and her colleagues had recreated the type of mortar used in Trajan's Market and other Roman structures such as the Pantheon and the Colosseum and studied its response to cracking. The Romans mixed a particular type of volcanic ash called Pozzolane Rosse, from the Alban Hills volcano, with quicklime and water. They used it to bind together decimeter-sized chunks of tuff, an aggregate of volcanic rock.
As a result of Pozzolanic activity as the material cured, the lime interacted with other chemicals in the mix and was replaced by crystals of a calcium aluminosilicate mineral called Strätlingite. Crystals of platey strätlingite grow in the cementitious matrix of the material including the interfacial zones where cracks would tend to develop. This ongoing crystal formation holds together the mortar and the coarse aggregate, countering crack formation and resulting in a material that has lasted for 1,900 years.