Pompeii building site reveals how the Romans made concrete

A recent excavation at Pompeii, a city frozen in time by the eruption of Mount Vesuvius, has provided archaeologists with valuable insights into the ancient Roman concrete-making techniques.

Located near Naples, Italy, Pompeii was covered in volcanic ash in AD 79, preserving its structures and artifacts. Excavations of a construction site in Pompeii, which began in the 1880s and resumed in early 2023, revealed a remarkably well-preserved concrete workshop.

Archaeologists uncovered detailed work schedules, material quantities, concreting tools, quicklime piles, and even recycled roof tiles at the site. The level of preservation amazed the researchers, offering a unique opportunity to understand Roman concrete production firsthand.

Admir Masic from the Massachusetts Institute of Technology expressed astonishment at the site’s condition, remarking that it provided a window into ancient concreting practices that no laboratory simulation could replicate.

Chemical analysis of the materials in the Pompeii workshop challenged the conventional wisdom about Roman concrete production. While historical texts suggested the use of slaked lime, the excavated evidence revealed a hot mixing method involving quicklime and volcanic ash or a mineral.

This innovative approach involved mixing all ingredients, including the quicklime, before adding water, leading to a chemical reaction that heated the concrete as it solidified. The study indicated that quicklime played a crucial role in structural concrete, while slaked lime was typically reserved for finishing touches.

One of the key advantages of the hot mix method was the presence of lime clasts in the concrete, enabling continuous self-repair of minor cracks and faults. These lime fragments acted as calcium reservoirs, enhancing the concrete’s strength and durability over time.

According to Masic, a modern concrete worker would find the Roman concreting process familiar and could seamlessly integrate into the workshop in Pompeii. The ancient chemistry, combined with the craft of concrete making, showcased a timeless tradition of construction techniques.

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