Ancient Pompeii construction site reveals the process for creating Roman concrete

By Taylor Nicioli, CNN

(CNN) — Along with its many other innovations, the Roman Empire revolutionized architecture with never-before-seen features, such as large-scale arches and dome roofs. And many of these structures still stand today despite being more than 2,000 years old.

None of it would have been possible without the Romans’ infallible building material: self-healing concrete. Now, an ancient construction site has revealed the recipe for creating this sturdy foundation.

At the time Mount Vesuvius erupted in AD 79, covering Pompeii in as much as 6 meters (19.7 feet) of volcanic ash, construction workers were in the process of repairing and renovating a house. International researchers excavated the site in 2023, revealing some completed walls and others that were half-built, as well as raw materials and tools.

“When I entered this archaeological site in Pompeii, everything was so vivid and also kind of perfectly preserved, to be able to just reconstruct clearly what was going on there,” said Admir Masic, an associate professor of civil and environmental engineering at the Massachusetts Institute of Technology and lead author of a new study documenting the discovery. “They’re frozen in time. It’s literally a time capsule.”

The findings, which were published December 9 in the journal Nature Communications, are the clearest evidence of mixing processes that the ancient Romans used to create concrete, according to a release from MIT — and they allow researchers to “make conclusions that we were not able to make, or at least not with this certainty about the Roman technology,” Masic told CNN.

Uncovering an active construction site

About one-third of Pompeii remains to be excavated, enabling scientists to continue making new discoveries about the ancient Roman way of life. The active construction site described in the new study was first investigated in the late 1880s, but excavations were halted and did not begin again until 2023. It was then that Masic’s team realized the magnitude of its discovery.

“This is typical for Pompeii. Archaeologists are just slowly but surely, you know, uncovering parts,” Masic said. “I think there is this kind of standard, very cautious way of excavating, because once excavated, you actually break that time capsule and things start to degrade. … You basically remove that protection that ensures that everything is perfectly preserved.”

After excavations, the study authors performed analysis on evidence found at the site, including piles of mixed dry materials that builders had been using to create the concrete, a wall that was in the process of being built and other structural walls that were already completed.

But this discovery was not the first that Masic made on the recipe for Roman concrete. A paper he authored in 2023 had analyzed samples from a 2,000-year-old city wall in the archaeological site of Privernum in central Italy. In that article, he identified lime clasts in the wall — small, white mineral chunks that give the concrete a self-healing ability. When cracks formed, water or rainfall could be added, which would dissolve the lime, allowing the mineral to fill and seal the fractures as it dried and recrystallized.

Differing recipes

Masic and his team determined that these minerals were added through a process known as “hot-mixing” in which the lime fragments were combined with dry ingredients such as volcanic ash. Water was then added, creating a chemical reaction that produced heat and trapped the lime clasts in the concrete.

However, Masic’s team was initially unsure if the city wall was representative of all Roman architecture, as the concrete recipe differed from one described in the first century manuscript “De architectura” by the famed ancient Roman architect Vitruvius.

Vitruvius described water being added to the lime before any other materials instead of the hot-mixing method. The newly excavated construction site, however, shows that the materials were mixed when they were dry, which confirmed that Romans had used the hot-mixing process instead of Vitruvius’ method, according to the study.

“It’s really difficult to think that Vitruvius was wrong. And I respect Vitruvius, and he inspired literally all my work,” Masic said. He added that it is possible that Vitruvius’ method was used elsewhere throughout the Roman Empire, or that scholars have misinterpreted his writings or not fully examined them.

John Senseney, an associate professor of ancient history at the University of Arizona in Tucson, said he does not find it surprising that Vitruvius’ methods are not representative of the process used in the construction site.

“Expecting scientific discoveries to conform to what Vitruvius writes would be misguided. Vitruvius’s corpus was indeed authoritative for humanist architects during the Renaissance over a thousand years later, but you’d be hard pressed to find much in Roman imperial era buildings that followed that reflect his prescriptions. If Roman builders had any detailed awareness of what he wrote, they invariably discarded it,” Senseney said in an email. He was not involved with the new study.

“Discoveries like this throw light on the incredible contributions of common workers and even enslaved persons in ancient history, which is very difficult to appreciate directly in the written works of elite authors,” Senseney added. He pointed to ancient buildings such as the Pantheon and Colosseum that reflect the “expertise and innovation” of these everyday people who were masters at their craft.

“Studies like this allow us to see them and the wonders they gave to their world, and to us in return. When we recognize that, we’re better able to appreciate the stunning achievements that everyday people make possible in our own world,” Senseney said.

Masic said he hopes the discovery will inspire other scholars to look further into Vitruvius’ work in relation to the Roman architecture that stands today. He also wants to examine how the ancient processes could translate into and possibly improve modern practices.

“I will never forget being able to just open a time capsule and travel in time and feel like I am in 79 AD looking at people making their concrete,” he said.

“That’s what really fascinates me in dissecting these, particularly when it comes to ancient Roman concrete and infrastructure Romans built that is still standing here after 2,000 years — I’m not sure how much of our things will be there in 2,000 years from now.”

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