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Department of Geological Sciences, University College London, Gower Street, London WC1E 6BT, UK
The realization that the addition of volcaniclastic material to hydrated lime cements produced a concrete that was not only waterproof but would also set under water revolutionized the building programmes of the Roman Empire. The material became known as pulvis Puteolis (dust of Puteoli) from the Latin name for modern Pozzuoli on the Bay of Naples. Pozzuoli itself is the root of ‘pozzolana’, meaning any material (in most natural cases, volcanic in origin) that is capable of producing a hydraulic (waterproof) set when combined with lime cement. A reaction between quicklime (CaO) and volcanic ash promotes the growth of phases that, rather than reverting to calcium carbonate on curing, as in non-hydraulic cements, produce a hard, watertight material. Although volcanic materials were the aggregates of choice in the building projects of Imperial Rome, being locally derived, abundant and relatively easily quarried, their hydraulic properties were soon fully realized, perhaps as a result of observations of naturally calcite-cemented scoria and ash flows in sea water or from experimental building. Away from the Imperial centre, terrestrial architecture involving waterproofing (baths and cisterns) effectively used waste ceramic sherds as a substitute for volcanic ash. However, for marine and riparian architecture, volcanically derived pozzolanic cements were the preferred material of use.
