A crumbly white speck inside ancient concrete may be part of why some Roman structures endured for centuries.
Roman builders mixed lime and volcanic ash into concrete used for roads, buildings, and harbors. Researchers had long noticed small white fragments, called lime clasts, but often treated them as signs of poor mixing. A 2023 study led by researchers at MIT proposed a different explanation after analyzing samples from an archaeological site in Privernum, Italy.
The team argued that Romans used a hot-mixing process involving quicklime. In laboratory tests, concrete made with the proposed approach healed deliberately created cracks within two weeks, while a comparison sample did not. That is evidence for a possible self-healing mechanism, not proof that every Roman structure or recipe worked the same way. Researchers continue to investigate why particular ancient constructions lasted so long.
Sometimes a feature that looks like a flaw turns out to be a clue to how a material was made.