Roman concrete (opus caementicium)

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A structural material made by binding rubble aggregate (caementa) in a mortar of lime and volcanic ash. The key ingredient is pozzolana - the Romans used pulvis puteolanus from the Bay of Naples - which reacts with lime to form a hydraulic binder that sets in the absence of air and in water. It is not modern concrete: there is no reinforcement, it is placed in layers into formwork rather than poured as a fluid, and its tensile strength is negligible, so a Roman concrete structure must be shaped so that only compression occurs. Roman builders exploited what the material does allow, which is variable density: the Pantheon dome uses heavy travertine and tuff aggregate low down and light pumice and scoria near the crown, so the shell gets lighter exactly where lightness matters most. A 2023 study in Science Advances by Seymour, Masic and colleagues argued that the millimetre-scale lime clasts long dismissed as poor mixing are the product of hot mixing with quicklime and give the material a self-healing capacity, cracks recrystallising as calcium carbonate when water passes.