Roman concrete can heal its own cracks, which is why the Pantheon is still standing after 1,900 years.
Researchers found that lumps of lime in Roman concrete dissolve when water enters a crack and then recrystallise, sealing the gap. The Pantheon's dome remains the largest unreinforced concrete dome in the world.
Why is this true?
The Romans mixed volcanic ash with quicklime, and the hot mixing process left reactive lime fragments behind. Modern concrete is stronger when new but usually needs steel reinforcement and starts to decay within decades.
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