answers: about 150 BC
context: pozzolana for cement used in buildings and a 1:2 ratio of lime to pozzolana for underwater work, essentially the same ratio mixed today for concrete used in marine locations. 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. 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 </s> and other waterside construction. 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. 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 ("pulvis puteolanus" in Latin), the volcanic sand from the beds of Pozzuoli, which are brownish-yellow-gray in color in that area around Naples, and reddish-brown near Rome. Vitruvius specifies a ratio of 1 part lime to 3 parts </s> of these structures survive to this day. In the Ancient Egyptian and later Roman eras, builders re-discovered that adding volcanic ash to the mix allowed it to set underwater. German archaeologist Heinrich Schliemann found concrete floors, which were made of lime and pebbles, in the royal palace of Tiryns, Greece, which dates roughly to 1400–1200 BC. Lime mortars were used in Greece, Crete, and Cyprus in 800 BC. The Assyrian Jerwan Aqueduct (688 BC) made use of waterproof concrete. Concrete was used for construction in many ancient structures. The Romans used concrete extensively from 300 BC to 476 AD, a
question: when was the earliest use we know about for concrete