Evidence of Roman engineering ingenuity is not in short supply. From Rome’s Pantheon to the Pont du Gard aqueduct in southern France to the Alcántara Bridge on the Iberian Peninsula, large-scale infrastructure projects were the backbone of the Roman Empire. The development of a durable form of cement was the key. By 150 B.C.E., such cement was in widespread use from Carthage to Corinth, but the precise manner in which Romans made it has long been unknown archaeologists.

Then, in 2023, a team from Massachusetts Institute of Technology had a breakthrough. After analyzing samples taken from a city wall in Priverno, southwest Italy, Admir Masic and his fellow researchers discovered that one secret behind the strength and longevity of Roman cement was a process called “hot-mixing.”

This involved Roman builders pre-mixing dry quicklime and dry volcanic ash before adding water. The addition of water sets off a heat-producing chemical reaction, but as the concrete sets, the process traps and preserves small clumps of lime, known as lime clasts, which remain highly reactive. When, in time, the concrete cracks and water enters in, these clumps dissolve and crystallize to fill the gaps, a process researchers describe as “self-healing.”

The newly excavated Regio IX site in Pompeii showing materials prepared for the ongoing reconstruction works. Photo: Archaeological Park of Pompeii.

Eye-opening as the discovery was, it directly contradicted the preeminent source on Roman architecture: De Architectura, a 1st-century B.C.E. treatise by the architect and military engineer Marcus Vitruvius Pollio. To make cement, Vitruvius wrote, Romans created a paste-like substance by adding water to lime before mixing in other materials. The kicker? The writings of Vitruvius had catalyzed Masic’s interest in Roman architecture in the first place. “It was difficult to suggest his description may be wrong,” Masic said in a statement. “I have a lot of respect for Vitruvius.”

No matter. Masic has now found further evidence of Romans using “hot mixing” at the construction site of a house that was under renovation in Pompeii when Mount Vesuvius erupted in 79 C.E. It was, Masic noted, a “time capsule” and besides demonstrating that the builders at hand were organized and eager to recycle materials, the site offered a unique opportunity to analyze clearly delineated piles of building materials, the results of which were published in Nature on December 9.

“Microstructural and chemical analysis of materials collected from previously constructed walls, walls under construction, and adjacent dry, raw material piles show unequivocally how quicklime was pre-mixed with dry pozzolan [volcanic ash] before adding water in the creation of Roman concrete,” the authors wrote in the paper.

Step-by-step guide to Roman cement making. Photo: Admir Masic et. al.

One key to the discovery was the team’s demonstrations that lime clasts found in both newly constructed walls and mortar repairs had the distinctive physical and chemical characteristics that could only result from hot mixing. Earlier researchers had thought these lime clasts were impurities from incomplete mixing.

The findings may have implications for modern-day builders. Cement today is made by heating limestone and clay in large kilns to create clinker, a material which is ground into powder and mixed with water at building sites to make concrete. Though strong, it is not long-lasting and the 2,000-year-old technique may yet prove informative. At the very least, researchers noted, the newly learned recipe could be used for restoration at Roman sites.

“The analysis of raw materials and Roman concrete in the context of Pompeii also has the potential to significantly influence the development of their modern material analogs,” the researchers wrote. “Contributing to the development of restoration and repair mortars that are compatible with ancient materials and, at the same time, more durable and sustainable from an environmental perspective.”