The ancient Romans used a concrete called opus caementicium that contained seawater and volcanic ash (pozzolana), and marine structures built with it have been found to be stronger after 2,000 years of seawater exposure than when they were first poured, because seawater triggers the continued growth of a rare mineral called tobermorite within the concrete.
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What we found
Ancient Roman engineers accidentally created a concrete that grows stronger the longer seawater touches it — a feat modern materials science only explained in 2017. Seawater reacts with volcanic ash inside the concrete to grow microscopic crystals that seal cracks from within, while our modern Portland cement erodes and cracks within decades. Follow for a new fact every day.
Transcript
What if the concrete you poured got stronger the longer you left it underwater? Two thousand years ago, the Romans figured this out by accident. They mixed seawater with volcanic ash called pozzolana and lime to build harbour walls and piers across the Mediterranean. Modern engineers assumed seawater would destroy it — but when researchers drilled cores from submerged Roman harbour structures in 2017, they found something impossible by today's standards. The seawater was slowly reacting with the volcanic ash inside, growing a rare mineral called tobermorite — microscopic interlocking crystals that seal cracks from within. The result: the Roman concrete was measurably denser and stronger than on the day it was poured. Today's Portland cement, by contrast, erodes and cracks within decades in seawater. We spent two thousand years building with an inferior material. Follow for a fact every day.
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