Latest results from a recently discovered ancient Roman construction site confirm earlier findings.

Back in 2023, we reported on MIT scientists’ conclusion that the ancient Romans employed “hot mixing” with quicklime, among other strategies, to make their famous concrete, giving the material self-healing functionality. The only snag was that this didn’t match the recipe as described in historical texts. Now the same team is back with a fresh analysis of samples collected from a recently discovered site that confirms the Romans did indeed use hot mixing, according to a new paper published in the journal Nature Communications. //

Then archaeologists discovered the remains of what was once an active construction site in Pompeii, with tools and piles of raw materials scattered about, a half-built wall, completed buttresses, and even mortar repairs to an existing wall. Masic described it as a veritable “time capsule” holding even more secrets about how the Romans made their concrete.

Masic et al.’s latest isotopic analysis of samples taken from the site confirms that the concrete had the same lime clasts as those used to build Privernum. Intact quicklime fragments showed they had been premixed with other dry raw materials—a crucial early step in a hot mixing process. Furthermore, the volcanic ash used in the cement contained pumice, and those pumice particles would chemically react with the surrounding solution over time to create new, strengthening mineral deposits. As for Vitruvius, Masic suggests that the historian may have been misinterpreted, pointing to a passing mention of latent heat during the cement mixing process that might indicate hot mixing.