Roman concrete and Alcatraz

Washington Post:
Ancient Romans made the world’s ‘most durable’ concrete. We might use it to stop rising seas.

The mixture of volcanic ash and quicklime reacts with seawater to create a rare crystal called tobermorite, which may resist fracturing. One engineer called it "the most durable building material in human history."
What makes the Roman concrete so remarkable is not only its strength but also that it was mixed with sea water.  Alcatraz stands as a monument to the failure of concrete in the US when mixed with sea water.  That made it vulnerable to attempted escapes by prisoners who dug away at the concrete with items as mundane as a spoon.

The challenge for duplicating their concrete will be in finding enough volcanic ash to build the structures envisioned.  There should be little doubt about the Roman method.  Many of the structures built with it are still standing.

The BBC has more on the Roman concrete.

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