Along with many other innovations, the Roman Empire revolutionized architecture with never-before-seen features such as large-scale arches and domed roofs. And many of these structures are still standing today, despite being over 2,000 years old.
None of this would have been possible without the infallible building blocks of the Romans: self-healing concrete. Now, an ancient construction site has revealed the recipe for creating this strong foundation.
During the eruption of Vesuvius in 79 AD. covering Pompeii In as much as 6 meters (19.7 ft) of volcanic ash, builders were repairing and updating the house. International researchers excavated the site in 2023, discovering some completed walls and others half-built, as well as raw materials and tools.
“When I walked into this archaeological site in Pompeii, everything was so vibrant and beautifully preserved that I could just clearly reconstruct what was going on there,” said Admir Masic, assistant professor of civil and environmental engineering at MIT and lead author of the new study documenting the discovery. “They're frozen in time. It's literally a time capsule.”
The results, published December 9 in the journal Nature Communications magazineare clear evidence of the mixing processes that the ancient Romans used to create concrete, according to release from MIT — and they allow researchers to “draw conclusions that we weren't able to draw, or at least not with as much confidence, about Roman technology,” Masik told CNN.
Active Construction Site Detection
About a third of Pompeii remains to be excavated, allowing scientists to continue to make new discoveries about the ancient Roman way of life. The active construction site described in the new study was first explored in the late 1880s, but excavations were stopped and did not resume until 2023. It was then that Masik's team realized the scale of their discovery.
“This is typical of Pompeii. Archaeologists are slowly but surely, you know, uncovering some parts,” Masic said. “I think there is this standard, very careful way of excavating because once you excavate, you basically destroy the time capsule and everything starts to fall apart… Essentially, you remove those protections that ensure everything is perfectly preserved.”
After the excavation, the study authors analyzed evidence found at the site, including piles of mixed dry materials that builders used to create concrete, a wall that was in the process of being built, and other structural walls that had already been completed.
But this discovery was not the first that Masik made using a Roman concrete recipe. A paper he wrote in 2023 analyzed samples of a 2,000-year-old city wall in archaeological site Privernum in central Italy. In this article, he discovered limestone chunks in the wall—small white mineral lumps that give concrete its self-healing ability. When cracks formed, water or rain could be added, which would dissolve the lime, allowing the mineral to fill and seal the cracks as it dried and recrystallized.
Various recipes
Masik and his team determined that these minerals were added through a process known as “hot mixing,” in which lime fragments were mixed with dry ingredients such as volcanic ash. Water was then added, causing a chemical reaction that produced heat and trapped the lime chunks in the concrete.
However, Masic's team was initially unsure whether the city wall was representative of all Roman architecture, since the recipe for concrete was different from that described in a first-century manuscript.”About architecture» by the famous ancient Roman architect Vitruvius.
Vitruvius described how water was added to lime before any other materials instead of the hot mixing method. However, a recently excavated construction site shows that the materials were mixed when they were dry, confirming that the Romans used a hot mixing process instead of Vitruvius' method, according to the study.
“It's very hard to think that Vitruvius was wrong. And I respect Vitruvius, and he inspired literally all my work,” Masic said. He added that it was possible that Vitruvius' method was used somewhere throughout the Roman Empire, or that scholars had misinterpreted his writings or had not fully studied them.
John Senseney, an assistant professor of ancient history at the University of Arizona in Tucson, said he doesn't think it's surprising that Vitruvius' methods don't reflect the process used on the construction site.
“To expect scientific discoveries to match what Vitruvius writes would be a mistake. Vitruvius' corpus was indeed authoritative for Renaissance humanist architects more than a thousand years later, but you would be hard-pressed to find much in subsequent buildings of the Roman imperial era that reflected his recipes. If Roman builders had any detailed knowledge of what he wrote, they would invariably discard it,” Senseney said in an email. He was not involved in the new study.
“Discoveries like these shed light on the incredible contributions of ordinary workers and even enslaved people to ancient history, which are very difficult to appreciate directly in the written works of elite authors,” Senseney added. He pointed to ancient buildings such as the Pantheon and the Colosseum that reflect the “expertise and innovation” of these ordinary people who were masters of their craft.
“Research like this allows us to see them and the wonders they gave to their world and to us in return. When we realize this, we can better appreciate the amazing achievements that ordinary people make possible in our own world,” Senseney said.
Masic said he hopes the discovery will inspire other scholars to further study Vitruvius's work in relation to modern Roman architecture. He also wants to explore how ancient processes can be transformed into modern methods and possibly improved.
“I will never forget being able to just open a time capsule and travel back in time and feel like I was in 79 A.D. looking at people making concrete,” he said.
“That's what really fascinates me about their analysis, especially when it comes to ancient Roman concrete and infrastructure that the Romans built that is still there 2,000 years later—I'm not sure how much of our stuff will still be there in 2,000 years.”
Taylor Nicoli is a freelance journalist based in New York.
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