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Nobody is aware who did it first, or when. But by the 2nd or 3rd century BCE, Roman engineers were routinely grinding up burnt limestone and volcanic ash to make caementum: a powder that would begin to harden as soon as it was mixed with water
They. made wide use of the still-wet slurry as mortar for their brick- and stoneworks. But they had additionally discovered the value of stirring in pumice, pebbles, or pot shards along with the water: Get the proportions right, and the cement would ultimately bind it all into a strong, durable, rock-like conglomerate known as opus caementicium or—in a later term derived from a Latin verb meaning “to convey together”—concretum
The. Romans used this marvelous stuff all through their empire—in viaducts, breakwaters, coliseums, and even temples like the Pantheon, which nonetheless stands in central Rome and nonetheless boasts the largest unreinforced concrete dome in the world
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