When Mount Vesuvius erupted in AD 79, a building crew in Pompeii left a wall half-finished, with piles of raw materials still on site. Studying that site, an MIT-led team found quicklime that had been mixed dry with volcanic ash before water was added, a method called hot mixing.

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In 79 AD, Mount Vesuvius erupted and stopped these Roman builders mid-job. The half-finished job they left solved a 2,000-year-old mystery. Rome's own building manual said to soak the lime in water first. But the piles at Pompeii show something else. They mixed raw quicklime dry with volcanic ash, then added water. The mix got scorching hot, and it left little white lumps of lime in the concrete. For years, those lumps looked like sloppy mixing. They're a built-in repair kit. When a crack lets water in, the lime dissolves and seals it shut. Concrete that heals itself, and we're only now copying it.