A construction site nearly 2,000 years before Pompeii’s fall in AD 79 has revealed new evidence of the secrets behind ancient Rome’s ultra-durable concrete.
Last year, archaeologists discovered an intact construction site beneath the volcanic ash that buried Pompeii – a rare snapshot of Roman architecture frozen in time.
The site includes neatly arranged piles of materials, including the ingredients used to mix the famously durable concrete behind monuments like the Pantheon, whose massive unreinforced dome has stood for thousands of years.
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A new analysis reveals that the secret is a technique that MIT materials scientist Admir Masic calls “thermal mixing.”
It involves directly mixing the ingredients of concrete: a mixture of volcanic ash called pozzolana with quicklime, which reacts with water to create intense heat inside the mixture.
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“The benefits of thermal mixing are twofold,” said Massik, who first discovered the technology experimentally in 2023.
“First, when the entire concrete is heated to high temperatures, it creates chemical reactions that would not be possible when using only hydrated lime, producing high-temperature-related compounds that would not otherwise form. Second, because all reactions are accelerated, this elevated temperature significantly reduces curing and setting times, allowing for faster construction.”
A third and crucial benefit is that the surviving lime blocks or gravel give the concrete its remarkable self-healing capabilities. This is probably the main reason why ancient Roman monuments remained standing even as other civilizations collapsed.
When cracks form in concrete, they propagate preferentially toward lime debris, which has a larger surface area than other matrix particles. When water enters a crack, it reacts with the lime to form a calcium-rich solution that dries and hardens into calcium carbonate, cementing the crack back together and preventing it from spreading further.
Some neatly arranged building materials found at the site. (Pompei Archaeological Park)
“This material is of historical importance, and understanding it is also of scientific and technological importance,” Massik said. “This material can repair itself over thousands of years, it is reactive, and it is highly dynamic. It has survived earthquakes and volcanoes. It has withstood the test on the seafloor and survived degradation from natural conditions.”
While thermal mixing technology provided a solution to the difficulties posed by Roman concrete, it presented a new one: the recipe did not match the description of how the building material was made in a treatise from 1 B.C. architecture Designed by architect Vitruvius.
The Vitruvian method first mixes lime with water in a process called slaking, and then mixes slaked lime with volcanic ash. However, this process does not produce the lime debris observed in authentic Roman concrete samples.
This mismatch has long puzzled scientists. Vitruvius’s writings represent the most complete surviving documentation of Roman architecture and construction. He describes a method called cement works Walls were built, but physical samples of ancient buildings contradicted his instructions.
Materials from Pompeii shed light on the mystery. Masic and his team conducted isotope analysis of five piles of dried material, identifying volcanic ash made from pumice and lime, quicklime and even lime debris.
Most obviously, these dry ingredients were pre-mixed—conclusive archaeological evidence.
Under a microscope, mortar samples from the walls revealed telltale signs of thermal mixing: broken limestone fragments, calcium-rich reaction edges that grew into volcanic ash particles, and tiny crystals of calcite and aragonite that formed within pumice vesicles.
Raman spectroscopy confirmed the mineral transformation, while isotope analysis revealed the chemical pathway of carbonization over time.
“Through these stable isotope studies, we can track these key carbonation reactions over time, allowing us to differentiate between hot-mixed lime and the hydrated lime originally described by Vitruvius,” Massik said.
“These results indicate that the Romans prepared binding materials by calcining limestone (quicklime), grinding [it] Achieved to a certain size, it is dry mixed with volcanic ash and then finally water is added to form a cementitious matrix. “
This doesn’t necessarily mean that Vitruvius was wrong – he may have described an alternative method of making concrete, or his work may have been misunderstood – but it does suggest that the most durable forms of materials must come from thermal mixing techniques.
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Researchers believe this information could be incorporated into the way we build concrete, leaving monuments many centuries after the fall of the Roman Empire as reminders not only of its grandeur but also of the ingenuity of its people.
Modern concrete is one of the most widely used building materials in the world. It is also very lacking in durability, tending to collapse within decades under environmental stress. Producing it is also bad for the environment, requiring huge resource costs and leading to greenhouse gas emissions.
Simply improving the durability of concrete has the potential to make it more sustainable.
“We don’t want to completely replicate Roman concrete today. We just want to translate a few sentences from this book of knowledge into our modern building practices,” said Masik, who has started a company called DMAT to do just that.
“Filling these pores in volcanic components through recrystallization is a dream process that we want to translate into modern materials. We want materials that can regenerate themselves.”
The study was published in nature communications.
