<P> Roman concrete, also called opus caementicium, was a material used in construction during the late Roman Republic until the fading 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 from modern concrete which is based on Portland cement . Roman concrete is durable due to its incorporation of volcanic ash, which prevents cracks from spreading . By the middle of the 1st century, the material was used frequently, often brick - faced, 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 and oldest unreinforced concrete dome . </P> <P> Roman concrete was normally faced with stone or brick, and interiors might be further decorated by stucco, fresco paintings, or thin slabs of fancy colored marbles . Made up of aggregate and cement, like modern concrete, it differed in that the aggregate pieces were typically far larger than in modern concrete, often amounting to rubble, and as a result it was laid rather than poured . Some Roman concretes were able to be set underwater, which was useful for bridges and other waterside construction . </P> <P> It is uncertain when Roman concrete was developed, but it was clearly in widespread and customary use from about 150 BC; some scholars believe it was developed a century before that . </P> <P> 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 . </P>

When was the earliest use we know about for concrete