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Roman concrete
Roman concrete, also called opus caementicium, was a material used in construction during the late Roman Republic through the whole history of the Roman Empire. Roman concrete was based on a hydraulic-setting cement. Recently, it has been found that it materially differs in several ways to modern Portland cement. It is widely acknowledged that roman concrete is the most durable type of cement of its kind due to its use of volcanic ash. By the middle of the 1st century, the material was used frequently as brick-faced concrete, although variations in aggregate allowed different arrangements of materials. Further innovative developments in the material, called the Concrete Revolution, contributed to structurally complicated forms, such as the Pantheon dome, the world's largest unreinforced concrete dome. Roman concrete was also used to make roads.
==Historic references==
Vitruvius, writing around 25 BC in his ''Ten Books on Architecture'', distinguished types of aggregate appropriate for the preparation of lime mortars. For structural mortars, he recommended ''pozzolana'', which are volcanic sands from the sandlike beds of Pozzuoli brownish-yellow-gray in color near Naples and reddish-brown at Rome. Vitruvius specifies a ratio of 1 part lime to 3 parts pozzolana for cements used in buildings and a 1:2 ratio of lime to pulvis Puteolanus for underwater work, essentially the same ratio mixed today for concrete used at sea.〔Heather Lechtman and Linn Hobbs "Roman Concrete and the Roman Architectural Revolution", ''Ceramics and Civilization Volume 3: High Technology Ceramics: Past, Present, Future'', edited by W.D. Kingery and published by the American Ceramics Society, 1986; and Vitruvius, Book II:v,1; Book V:xii2〕
By the middle of the 1st century, the principles of underwater construction in concrete were well known to Roman builders. The City of Caesarea was the earliest known example to have made use of underwater Roman concrete technology on such a large scale.〔Lechtman and Hobbs "Roman Concrete and the Roman Architectural Revolution"〕
Rebuilding Rome after the fire in 64 AD, which destroyed large portions of the city, the new building code by Nero consisted of largely brick-faced concrete. This appears to have encouraged the development of the brick and concrete industries.〔
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