Ancient Roman concrete marine structures built thousands of years ago are stronger now than when they were first built.
So how has Roman concrete outlasted the empire, while modern concrete mixtures erode within decades of being exposed to seawater?
Scientists have uncovered the chemistry behind how Roman sea walls and harbour piers resisted the elements, and what modern engineers could learn from it.
Romans built their sea walls from a mixture of lime (calcium oxide), volcanic rocks and volcanic ash, a study, published in the journal American Mineralogist, found.
Elements within the volcanic material reacted with sea water to strengthen the concrete structure and prevent cracks from growing over time.
"It's the most durable building material in human history, and I say that as an engineer not prone to hyperbole," Roman monument expert Phillip Brune told the Washington Post.
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