Some Roman harbor structures, built from concrete more than two thousand years ago, still stand in seawater that crumbles modern concrete within decades. The contrast has drawn engineers back to the Roman recipe, which mixed volcanic ash with lime and seawater rather than the sand and fresh water used today. Why the ancient material has endured is a matter of active debate.
One account points to a chemical reaction with the sea itself. As seawater seeps through the concrete, these researchers find, it dissolves components of the volcanic ash and prompts new minerals to grow inside the cracks. Far from weakening the structure, the intruding water knits it back together, so that the material actually grows stronger over the centuries it spends submerged. On this view the sea is a partner in the concrete's durability.
A second account is wary of resting too much on that reaction. The surviving harbors, these researchers observe, are the ones that happened to last; countless lesser Roman structures have long since vanished, leaving a misleadingly sturdy sample. Much of the strength, they add, may owe simply to the massive thickness of the walls and to careful siting in calm water. The crack-sealing chemistry is real, they grant, but it need not be the main reason these particular ruins survive.
According to the passage, the first account holds that seawater seeping into the concrete
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