Roman concrete could heal its own cracks, and some has lasted 2,000 years in the sea.
Modern concrete exposed to seawater can deteriorate within decades. Roman harbour concrete has survived two millennia. Research suggests seawater reacting with volcanic ash grew reinforcing crystals inside it, and in 2023 scientists found lumps of lime, once dismissed as sloppy mixing, that reseal cracks when water seeps in.
Roman concrete, opus caementicium, was in widespread use from about 150 BC. Like modern concrete it combined a hydraulic mortar with aggregate, but the aggregate included large chunks of rock, tile and even rubble from demolished buildings, so it was laid in place rather than poured. Where possible, builders used pozzolana, a volcanic ash from around Pozzuoli near Naples. The architect Vitruvius recommended one part lime to three of pozzolana for buildings, and one to two for underwater work.
That underwater ability was transformative. Hydraulic concrete sets even when submerged, and by the late 2nd century BC the Romans were building harbours with it. At Caesarea, between 22 and 15 BC, enormous quantities of pozzolana were shipped from Italy to build a harbour. Studies suggest that seawater seeping into tiny cracks reacted with minerals in the volcanic rock to grow a rare crystal, aluminous tobermorite, which may help resist fracture; some call it a candidate for the most durable building material in human history.
A 2023 study added a twist. The small lumps of lime scattered through Roman concrete had been read as a sign of poor mixing. Instead, they appear to come from 'hot mixing' with quicklime, and when water enters a crack, they supply calcium that recrystallises as calcium carbonate and seals it. Research suggests this self-healing may matter more than the ash alone.
Romans also engineered with it. The Pantheon's dome, the world's largest unreinforced concrete dome, uses heavy travertine in the foundations and light pumice and tuff near the top. Today researchers and some builders are experimenting with Roman-style mixes, using coal fly ash instead of volcanic ash, because they need less cement and could last far longer.
Source: Wikipedia — Roman concrete · Text summarised from Wikipedia (CC BY-SA 4.0)
