Concrete was the foundation of the ancient Roman empire. It enabled Rome’s storied architectural revolution as well as the construction of buildings, bridges, and aqueducts, many of which are still used some 2,000 years after their creation.
In 2023, MIT Associate Professor Admir Masic and his collaborators published a paper describing the manufacturing process that gave Roman concrete its longevity: Lime fragments were mixed with volcanic ash and other dry ingredients before the addition of water. Once water is added to this dry mix, heat is produced. As the concrete sets, this “hot-mixing” process traps and preserves the highly reactive lime as small, white, gravel-like features. When cracks form in the concrete, the lime clasts redissolve and fill the cracks, giving the concrete self-healing properties.
There was only one problem: The process Masic’s team described was different from the one described by the famed ancient Roman architect Vitruvius. Vitruvius literally wrote the book on ancient architecture. His highly influential work, “De architectura,” written in the 1st century B.C.E., is the first known book on architectural theory. In it, Vitruvius says that Romans added water to lime to create a paste-like material before mixing it with other ingredients.
“Having a lot of respect for Vitruvius, it was difficult to suggest that his description may be inaccurate,” Masic says. “The writings of Vitruvius played a critical role in stimulating my interest in ancient Roman architecture, and the results from my research contradicted these important historical texts.”
Now, Masic and his collaborators have confirmed that hot-mixing was indeed used by the Romans, a conclusion he reached by studying a newly discovered ancient construction site in Pompeii that was exquisitely preserved by the volcanic eruption of Mount Vesuvius in the year 79 C.E. They also characterized the volcanic ash material the Romans mixed with the lime, finding a surprisingly diverse array of reactive minerals that further added to the concrete’s ability to repair itself many years after these monumental structures were built.
“There is the historic importance of this material, and then there is the scientific and technological importance of understanding it,” Masic explains. “This material can heal itself over thousands of years, it is reactive, and it is highly dynamic. It has survived earthquakes and volcanoes. It has endured under the sea and survived degradation from the elements. We don’t want to completely copy Roman concrete today. We just want to translate a few sentences from this book of knowledge into our modern construction practices.”
The findings are described in a forthcoming paper in Nature Communications. Joining Masic on the paper are first authors Ellie Vaserman ’25 and Principal Research Scientist James Weaver, along with Associate Professor Kristin Bergmann, PhD candidate Claire Hayhow, and six other Italian collaborators.
Uncovering ancient secrets
Masic has spent close to a decade studying the chemical composition of the concrete that allowed Rome’s famous structures to endure for so much longer than their modern counterparts. His 2023 paper analyzed the material’s chemical composition to deduce how it was made.
That paper used samples from a city wall in Priverno in southwest Italy, which was conquered by the Romans in the 4th century B.C.E. But there was a question as to whether this wall was representative of other concrete structures built throughout the Roman empire.
The recent discovery by archaeologists of an active ancient construction site in Pompeii (complete with raw material piles and tools) therefore offered an unprecedented opportunity.
For the study, the researchers analyzed samples from these pre-mixed dry material piles, a wall that was in the process of being built, completed buttress and structural walls, and mortar repairs in an existing wall.
“We were blessed to be able to open this time capsule of a construction site and find piles of material ready to be used for the wall,” Masic says. “With this paper, we wanted to clearly define a technology and associate it with the Roman period in the year 79 C.E.”
The site offered the clearest evidence yet that the Romans used hot-mixing in concrete production. Not only did the concrete samples contain the lime clasts described in Masic’s previous paper, but the team also discovered intact quicklime fragments pre-mixed with other ingredients in a dry raw material pile, a critical first step in the preparation of hot-mixed concrete.
Bergman, an associate professor of earth and planetary sciences, helped develop tools for differentiating the materials at the site.
“Through these stable isotope studies, we could follow these critical carbonation reactions over time, allowing us to distinguish hot-mixed lime from the slaked lime originally described by Vitruvius,” Masic says. “These results revealed that the Romans prepared their binding material by taking calcined limestone (quicklime), grinding them to a certain size, mixing it dry with volcanic ash, and then eventually adding water to create a cementing matrix.”
The researchers also analyzed the volcanic ingredients in the cement, including a type of volcanic ash called pumice. They found that the pumice particles chemically reacted with the surrounding pore solution over time, creating new mineral deposits that further strengthened the concrete.
Rewriting history
Masic says the archaeologists listed as co-authors on the paper were indispensable to the study. When Masic first entered the Pompeii site, as he inspected the perfectly preserved work area, tears came to his eyes.
“I expected to see Roman workers walking between the piles with their tools,” Masic says. “It was so vivid, you felt like you were transported in time. So yes, I got emotional looking at a pile of dirt. The archaeologists made some jokes.”
Masic notes that calcium is a key component in both ancient and modern concretes, so understanding how it reacts over time holds lessons for understanding dynamic processes in modern cement as well. Towards these efforts, Masic has also started a company, DMAT, that uses lessons from ancient Roman concrete to create long-lasting modern concretes.
“This is relevant because Roman cement is durable, it heals itself, and it’s a dynamic system,” Masic says. “The way these pores in volcanic ingredients can be filled through recrystallization is a dream process we want to translate into our modern materials. We want materials that regenerate themselves.”
As for Vitruvius, Masic guesses that he may have been misinterpreted. He points out that Vitruvius also mentions latent heat during the cement mixing process, which could suggest hot-mixing after all.
The work was supported, in part, by the MIT Research Support Committee (RSC) and the MIT Concrete Sustainability Hub.
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Written by Zach Winn, MIT News
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