Stone Was Made, Not Only Carved: The Rumour of Roman Concrete and Egyptian Pyramids

In 2023, researchers from MIT and collaborating laboratories analyzed a roughly 2-centimeter fragment of Roman concrete collected at Privernum, Italy. The fragment contained calcium-rich lime clasts that had often been treated as signs of poor mixing. The team argued that Roman builders may have mixed quicklime directly with volcanic ash and stone fragments. When water enters a crack, a lime clast can dissolve and form calcium carbonate again, filling the gap. In tests with replica material, cracks stopped passing water after two weeks.

That concrete was not an abstract idea. Roman builders used lime and volcanic ash binders with water and aggregate in domes, ports, sewers and walls. Some structures exposed to seawater have survived for about two thousand years. Roman concrete therefore shows that ancient builders could control chemical reactions to make a durable building material instead of only stacking stones cut in advance.