Pompeii building site reveals how the Romans made concrete

A newly excavated ancient building site in Pompeii, frozen in time after the eruption of Vesuvius, has allowed archaeologists to finally identify the methods used to make Roman concrete.

Pompeii, near the modern city of Naples, Italy, was buried and preserved under volcanic ash in 79 AD. Excavations at the construction site first began in the 1880s and then stopped until early 2023, when a new major excavation was undertaken.

After new excavations, archaeologists discovered an almost perfectly preserved concrete workshop with marks on the walls detailing the work schedule and quantities of materials. Also found were concreting tools and piles of quicklime, as well as recycled roof tiles.

I admire Masic MIT says the team was stunned by how “exceptionally well preserved” the site was and that it offered insight into Roman concreting techniques in a way that “no laboratory reconstruction could have replicated.”

“The materials were the same as they were at the time of the eruption, which froze the city in time,” says Masik. “Studying it, I truly felt as if I had traveled back in time to 79 AD and stood next to the workers as they mixed and laid the concrete.”

The study's findings suggest that long-held ideas about how the Romans made concrete now need to be reconsidered.

Historical texts report that the Romans used slaked lime—calcium hydroxide—to make concrete and mixed the lime with water before adding it to other ingredients, such as volcanic ash.

However, chemical analysis of dry pre-mixed piles found in a workshop at Pompeii indicates that ancient concrete workers actually used a hot-mix method using quicklime or calcium oxide. This involved mixing the lime with all the other ingredients, including volcanic ash or mineral obtained before adding water, and a chemical reaction that heats a portion of the concrete as it sets.

“Our data shows that quicklime played a major role in structural concrete,” Masik says. “Slaked lime, on the other hand, was commonly used for finishing mortars and plasters where workability and a smooth surface were important.”

One advantage of the hot mix method is that pieces of lime, called flakes, remain in the concrete after it has set, allowing Permanent self-repair of cracks and other minor structural deficiencies.

“These limestone debris act as calcium reservoirs, dissolving and recrystallizing in pores and cracks or reacting with volcanic ash, strengthening the microstructure of the concrete,” Masik says.

According to him, a modern concrete worker would understand the Roman process of concreting in Pompeii, could easily enter the workshop and begin working almost immediately. “The chemistry is ancient, but the craft is recognizable.”

Historic Herculaneum – discovery of Vesuvius, Pompeii and ancient Naples

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