The Pantheon in Rome has a huge concrete dome. It was finished about 1,900 years ago and is still standing. Roman concrete has small white lumps of lime in it. People once thought these lumps were mistakes.

But a 2023 study showed that when water gets into a crack, the lumps can dissolve a little and fill the crack. The concrete can fix itself!

Some ancient Roman buildings, such as the Pantheon, have lasted for nearly 2,000 years, even though their concrete contains no steel bars. Roman concrete was made from lime, volcanic ash, and pieces of rock or brick. It often contains small white lumps of lime, which experts long believed were signs of careless mixing.

In 2023, however, a team at MIT suggested that the lumps came from "hot mixing" with quicklime, which releases heat when water is added. In tests, when cracks formed and water flowed through, the lime dissolved and formed new crystals that filled the cracks within about two weeks. Concrete without the lumps did not heal.

The Pantheon's 43-meter dome, completed about 1,900 years ago, is still the largest unreinforced concrete dome in the world, while many modern structures need major repairs within decades. Roman concrete combined lime, volcanic ash, and rock fragments. In seawater structures, reactions with the ash slowly grew platy minerals that made the material stronger. A 2023 study led by MIT researchers looked at millimeter-scale white lime lumps, traditionally dismissed as a sign of poor mixing.

After analyzing samples from an ancient city wall, the team concluded that the lumps reflect hot mixing, in which quicklime reacts with water and releases heat. When cracks later let water in, the lumps can dissolve and recrystallize as calcium carbonate, sealing the gaps. In replica samples, cracked concrete with lumps stopped leaking within about two weeks.

Samples without lumps kept leaking. This healing has been shown only in laboratory tests. Nobody can directly measure how much it helped over two thousand years, and the Roman buildings standing today are only the ones that survived. Modern concrete is also often stronger in compression. A 2025 analysis of an unfinished construction site at Pompeii supported one part of the story, that Romans premixed quicklime and ash dry.