Roman concrete composition and earliest surviving structures

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Key facts

• Roman concrete was composed of water, lime, pozzolana (a volcanic sand), and aggregate (gravel, stone fragments, or rubble).
• Pozzolana deposits were concentrated in the volcanic landscape of Italy, especially around the Bay of Naples region.
• The architect Vitruvius documented pozzolana's properties, noting that the substance hardened underwater when mixed with lime and rubble.
• Vitruvius's writings survived into the medieval period through monks copying them by hand.
• Volcanic sands other than Italian pozzolana were also used, including those around the island of Santorini.
• Romans developed concrete around 200–250 BC, and by about 150 BC they were using it in construction.
• The Mediterranean town of Baiae features the oldest surviving concrete dome, built approximately 200 years before the Pantheon.
• The harbor complex known as the Portus was constructed first by Claudius and later expanded by Trajan.
• The Pantheon dome was constructed with lighter weight by embedding empty vessels into the concrete as coffering to reduce density.
• Roman concrete was placed by hand and tamped down rather than poured or pumped.

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