http://www.sciencedaily.com/releases/2015/07/150709144850.htmnew discovery about Roman Concrete
Lake Avernus, the Phlegraean Fields, are something you might remember from history books—or Roman studies: the lake no bird could fly over, the Fields of Fire that figure in Virgil’s Aeneid and the contact with the Cumaean Sibyl, or prophet…
Well, the Campi Flegrei are still active, and it is a European supervolcano, a little removed from Vesuvius, itself a very dangerous volcano.
“This is the same chemical reaction that the ancient Romans unwittingly exploited to create their famous concrete, but in Campi Flegrei it happens naturally,” Vanorio said.
In fact, Vanorio suspects that the inspiration for Roman concrete came from observing interactions between the volcanic ash at Pozzuoli and seawater in the region. The Roman philosopher Seneca, for example, noted that the “dust at Puteoli becomes stone if it touches water.”
I would not characterize that as “unwittingly”. As I recall, the presence of volcanic ash in Roman concrete was discovered in 2001, arguably ending the dark ages. Fly ash–coal burning waste–also works. Even earlier uses of concrete have survived.