<P> By the middle of the first 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 . </P> <P> For rebuilding Rome after the fire in 64 AD, which destroyed large portions of the city, Nero's new building code largely called for brick - faced concrete . This appears to have encouraged the development of the brick and concrete industries . </P> <P> Roman concrete, like any concrete, consists of an aggregate and hydraulic mortar--a binder mixed with water that hardens over time . The aggregate varied, and included pieces of rock, ceramic tile, and brick rubble from the remains of previously demolished buildings . </P> <P> Gypsum and quicklime were used as binders . Volcanic dusts, called pozzolana or "pit sand", were favored where they could be obtained . Pozzolana makes the concrete more resistant to salt water than modern - day concrete . The pozzolanic mortar used had a high content of alumina and silica . Tuff was often used as an aggregate . </P>

What was concrete made out of in ancient rome
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