Scientists excavating the ancient Roman city of Pompeii have unearthed a construction site preserved exactly as it was when Mount Vesuvius erupted in 79 AD, offering unprecedented insight into the durable, self-healing concrete that revolutionised Roman architecture.

The remarkable discovery reveals a building project abruptly halted by the volcanic catastrophe.

Researchers found rooms with unfinished walls, alongside piles of premixed dry materials and tools for weighing and measuring, all laid out for concrete preparation.

"Studying it truly felt as if I had traveled back in time and was standing beside the workers as they mixed and placed their concrete," remarked Admir Masic, a Massachusetts Institute of Technology professor of civil and environmental engineering and lead author of the study published in Nature Communications.

Roman concrete was an indispensable material, enabling the construction of monumental structures such as the Colosseum, the Pantheon, public baths, aqueducts, and bridges – feats of engineering unparalleled at the time.

Its unique ability to harden underwater also made it crucial for building harbours and breakwaters.

The precise methods employed by the Romans have long been a subject of academic debate, with recent archaeological findings often challenging accounts from the 1st-century BC treatise by architect Vitruvius.

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The Pompeii discovery showed the Romans used a technique called "hot mixing" in which a material called quicklime - dry limestone that was previously heated - is combined directly with water and a blend of volcanic rock and ash, producing a chemical reaction that naturally heats the mixture.

That differs from the method described by Vitruvius, who wrote about a century earlier.

"Pompeii preserves buildings, materials and even work in progress in the precise state they were in when the eruption occurred. Unlike finished structures that have undergone centuries of repair or weathering, this site captures construction processes as they happened," Masic said.

"For studying ancient technologies, there is simply no parallel," Masic said. "Its exceptional preservation offers a true ‘snapshot’ of Roman building practice in action."

The building under construction combined domestic rooms with a working bakery with ovens, grain-washing basins and storage. The evidence there indicated that the technique outlined by Vitruvius, known as slaked lime, was not used for building walls.

That method may have been outdated by the time of the project in Pompeii.

"Imagine what 100 years of difference could mean for the building technology. A good analogy could be the early telephones. In the 1920s-30s: rotary dialing, long-distance copper lines. In the 2020s: smartphones using packet-switched digital signals and wireless networks," Masic said.

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The hot-mixing technique contributed to the self-healing properties of the concrete, chemically repairing cracks. The concrete contains white remnants of the lime used to make it, called "lime clasts," which can dissolve and recrystallise, healing cracks that may form with the infiltration of water.

The Romans industrialised concrete, beginning in the 1st centuries BC and AD.

"This allowed builders to construct massive monolithic structures, complex vaults and domes, and harbors with concrete that cured underwater. Concrete fundamentally expanded what could be built and how cities and infrastructures were conceived," Masic said.

The new understanding of Roman concrete may have relevance for modern architects.

"Modern concretes generally lack intrinsic self-healing capability, which is increasingly important as we seek longer-lasting, lower-maintenance infrastructure," Masic said.

"So while the ancient process itself is not a direct replacement for modern standards, the principles revealed can inform the design of next-generation durable, low-carbon concretes."