A recently uncovered ancient building site in Pompeii, preserved since the eruption of Mount Vesuvius, has enabled archaeologists to finally understand the process behind Roman concrete production.

Located near Naples, Pompeii was entombed under layers of volcanic ash in 79 AD. Excavation efforts commenced in the 1880s but were halted until early 2023, when extensive new diggings began.

The latest excavations revealed a nearly intact concrete workshop, featuring inscriptions on the walls detailing work schedules and material quantities. Numerous concrete tools, heaps of quicklime, and salvaged roof tiles were also discovered.

Admija Masic of the MIT research team expressed amazement at the site’s remarkable preservation, stating it provides insights into Roman concrete techniques that “laboratory recreations simply cannot achieve.”

“The material was just as it was at the moment the eruption halted the city,” Masic mentioned. “Studying it felt as if I were transported back to 79 A.D., standing beside the workers mixing and pouring concrete.”

The team’s revelations indicate that established assumptions about Roman concrete production should be reconsidered.

Historical records suggest that Romans utilized slaked lime (calcium hydroxide) for concrete, combining it with water before mixing it with other components like volcanic ash.

Contrarily, chemical analyses of dry, premixed materials found in Pompeian workshops show that ancient builders actually employed a hot mixing technique using quicklime, or calcium oxide. This method involved combining the lime with all other ingredients including volcanic ash and minerals, before adding water, which was heated as a result of the chemical reaction, causing some concrete to set.

“Our findings demonstrate that quicklime was crucial to structural concrete,” Masic states. “In contrast, slaked lime was typically used for finish mortar and plaster, where a smooth surface and workability were paramount.”

The hot-mixing method offers the advantage of lime remnants, known as crusts, remaining in the concrete post-hardening, which can autonomously heal small cracks and defects.

“These lime fragments serve as reservoirs of calcium that can dissolve and recrystallize in crevices and fractures, or interact with volcanic ash to enhance the microstructure of the concrete,” Masić explains.

A modern concrete worker would easily understand the Roman concrete-making technique at Pompeii and could seamlessly transition into the workshop to start working, he states. “The chemistry is ancient, yet the technology is readily recognizable.”

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