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Here’s what you’ll learn when you read this story:
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A recently discovered site in Pompeii demonstrates a theory of why Roman concrete has stood the test of time.
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The hot mixing process of concrete creation found in the ancient city was the final evidence needed to fully understand Roman concrete processes.
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The researchers were also able to identify Roman volcanic ash mixed with lime.
Roman concrete has always been remarkably durable. Researchers believe a newly discovered ancient site in Pompeii offers an answer to why.
There is no dispute about the stability of Roman-made concrete, from the Pantheon—Rome’s unreinforced concrete dome dedicated in AD 128 that still stands today—to countless ancient aqueducts that still carry water. What is being discussed, however, is how the Romans did it.
For decades, researchers credited a special Italian volcanic ash from Naples for the unique mixture, since the Romans were the first to introduce ash into concrete. Others consider lime as a key component. In 2023, a team of international researchers led by MIT’s Admir Masic, a professor of civil and environmental engineering, published a study claiming that it was actually the combination of fly ash and lime in a “hot mix” process that produced a self-healing concrete.
Now they have proof, thanks to the discovery of a construction site in Pompeii that was frozen in time by the infamous eruption of Mount Vesuvius in AD 79.
“There is the historical importance of this material and then there is the scientific and technological importance of understanding it,” Masic said in a statement. “This material can heal itself over thousands of years, it’s reactive, and it’s extremely dynamic. It’s survived earthquakes and volcanoes. It’s survived under the sea and it’s 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 building practices.”
The strength comes from its self-healing properties, which help fill cracks as they form. Masic clarified this aspect in 2023 when he narrated Popular Mechanics that using lime and fly ash helps, but hot-mixing, a concrete-making process that forms reactive calcium, is essential.
The process involved shards of lime mixed with volcanic ash and other dry ingredients before water was added. Once water is added to the dry mix, it heats up, and as the concrete hardens, the hot mix captures and retains the highly reactive lime as small, white, gravel-like features. These lime clasts can redissolve and fill cracks. Creates spontaneous self-healing of cracks before they spread.
Masic and his colleagues were certain of this in 2023, but had no historical evidence to support it. The famous ancient Roman architect Vitruvius wrote the world’s first architectural theory book, About architecture in the first century BC and explained that the Romans added water to lime to create a paste-like material before mixing it with other ingredients.
“With great respect for Vitruvius, it was difficult to suggest that his description could be inaccurate,” Masic said.
But Pompeii shows a different process. The team studied constructed walls alongside unfinished walls that had been under construction, all adjacent to piles of dry raw materials. In a December 2025 study published in Communication of nature Of the discovery, the team wrote that the site can “show unequivocally how premixed the smooth lime with dry pozzolana was before adding water in the creation of Roman concrete.”
That’s the hot mix.
“We were blessed to be able to open this time capsule of a construction site and find piles of materials ready to be used for the wall,” Masic said. The concrete at the site contained lime clasts that Masic had found in previous research, and the intact fragments of smooth lime were premixed with other ingredients, a critical first step in making hot-mixed concrete.
The team created tools for differentiating the materials at the site and then studied them to differentiate between hot-mixed lime and slaked lime originally described by Vitruvius. Masic noted that Vitruvius also mentioned latent heat during the mixing process, suggesting that hot mixing was at play.
“These results showed 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 adding water afterwards to create a cementing matrix,” Masic said.
Further investigation of the volcanic ash confirmed that it contained pumice, key particles that reacted chemically with the surrounding mixtures to form new mineral deposits for self-healing properties.
“This is relevant because Roman cement is durable, self-healing and a dynamic system,” Masic said. “The way these pores in volcanic ingredients can be filled by recrystallization is a dream process that we want to translate into our modern materials. We want materials that regenerate themselves.”
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