Vitruvius specifies a ratio of 1 part lime to 3 parts pozzolana for mortar used in buildings and a 1:2 ratio for underwater work. For structural mortars, he recommended pozzolana ( pulvis puteolanus in Latin), the volcanic sand from the beds of Pozzuoli, which are brownish-yellow-gray in colour in that area around Naples, and reddish-brown near Rome. Vitruvius, writing around 25 BC in his Ten Books on Architecture, distinguished types of materials appropriate for the preparation of lime mortars. The so-called "Temple of Mercury" in Baiae, a Roman frigidarium pool of a bathhouse built in the 1st century BC containing the oldest surviving concrete dome, and largest one before the Pantheon. Roman concretes, like any hydraulic concrete, were usually able to set underwater, which was useful for bridges and other waterside construction. Roman concrete differs from modern concrete in that the aggregates often included larger components hence, it was laid rather than poured. The most prominent example of these is the Pantheon dome, the world's largest and oldest unreinforced concrete dome. Further innovative developments in the material, part of the so-called concrete revolution, contributed to structurally complicated forms. It was often used in combination with facings and other supports, and interiors were further decorated by stucco, fresco paintings, or coloured marble. Roman concrete was in widespread use from about 150 BC some scholars believe it was developed a century before that. Recent research has shown that the incorporation of lime clasts allowed the concrete to self-repair cracks. The addition of ash prevented cracks from spreading. Its strength was sometimes enhanced by the incorporation of pozzolanic ash where available (particularly in the Bay of Naples). Many buildings and structures still standing today, such as bridges, reservoirs and aqueducts, were built with this material, which attests to both its versatility and its durability. Like its modern equivalent, Roman concrete was based on a hydraulic-setting cement added to an aggregate. Roman concrete, also called opus caementicium, was used in construction in ancient Rome. Caesarea harbour: an example of underwater Roman concrete technology on a large scale
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