Concrete was the foundation of the Roman Empire. For centuries, researchers have tried to uncover the secret behind the self-healing material that allowed Roman structures to defy time. As it turns out, the answer was waiting in a half-renovated living room in Pompeii.
By analyzing raw materials frozen in place by the eruption of Vesuvius, MIT researchers have finally confirmed that ancient builders harnessed a volatile, high-heat mixing technique. This process grants concrete the ability to repair its own cracks — a breakthrough that could help our own modern infrastructure withstand the elements.
A Time Capsule
When Mount Vesuvius erupted in 79 AD, it didn’t just destroy the nearby cities of Pompeii and Herculaneum. By burying them under a thick layer of ash and pumice, it preserved them. The eruption stopped life in its tracks, cementing bread in ovens and residents in their final moments.
It also froze construction projects.
In one Pompeii home, archaeologists discovered a renovation interrupted by the apocalypse. They found piles of limestone, heaps of sand, and tools scattered exactly where workers dropped them. This offered a forensic level of detail that completed structures simply can’t provide. It’s one thing to analyze a finished wall; it’s another to find the raw ingredients lying on the floor.
The scene was so striking that it brought tears to the eyes of Admir Masic, an MIT Associate Professor who has spent nearly a decade hunting the secrets of Roman engineering.
“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 remembers.
Hot Mixing
In 2023, Masic and colleagues published a paper describing robust chemical analyses on Roman concrete. They proposed a manufacturing process that explained why the material was so durable: “hot mixing.”
The theory was that builders mixed lime fragments with volcanic ash and other dry ingredients before adding water. When they eventually added the water, the chemical reaction generated immense heat. This preserved the lime as small, white, gravel-like chunks. When cracks inevitably formed in the concrete later on, water would seep in, hit those lime chunks, and dissolve them, essentially recrystallizing to fill the crack.
It was a stunningly beautiful mechanism. But there was one problem: it contradicted the history books.
For centuries, historians relied on the writings of Vitruvius, a famed Roman architect who wrote the definitive guide on building in the 1st century B.C.E. Vitruvius explicitly wrote that builders should create a wet paste with lime and water before mixing it with sand — a process known as slaking.
“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.”
This is why the Pompeii find is the smoking gun.
The team found intact lime fragments premixed with other ingredients in a dry raw material pile. It clearly shows that the Romans on-site were preparing to “hot mix” cement, defying Vitruvius’s instructions. Masic’s initial hypothesis was confirmed.
“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.”
Perhaps Vitruvius was simply describing a different method, or perhaps “hot mixing” was a trade secret kept off the books. Either way, we now have physical proof it existed.
This Could Matter Today
Roman concrete was so innovative that it could even be of use to us today.
Modern Portland cement is a major polluter, responsible for about 8% of global carbon dioxide emissions. And despite that high cost, our concrete rots. It cracks, steel reinforcement rusts, and buildings fail.
“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.”
The Roman method offers a path toward resilience. “We don’t want to completely copy Roman concrete today,” Masic explains. We just want to translate some parts of it into our modern construction practices.
The builders in Pompeii knew what they were doing. They built a legacy that would outlast their empire and their language, eventually inspiring technology two millennia later. It kind of makes you wonder: what other secrets does Pompeii still hold?