Nobody knows 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 start to harden as soon as it was mixed with water.
They made extensive use of the still-wet slurry as mortar for their brick- and stoneworks. But they had also learned the value of stirring in pumice, pebbles or pot shards along with the water: Get the proportions right, and the cement would eventually bind it all into a strong, durable, rock-like conglomerate called opus caementicium or — in a later term derived from a Latin verb meaning “to bring together” — concretum.
The Romans used this marvelous stuff throughout their empire — in viaducts, breakwaters, coliseums and even temples like the Pantheon, which still stands in central Rome and still boasts the largest unreinforced concrete ...
