Archaeologists excavating a long-buried construction site in Pompeii have uncovered a preserved workshop that sheds light on how Romans produced their durable concrete. The findings challenge previous understandings of Roman building techniques and highlight the material's self-repairing properties. The site, frozen by the eruption of Mount Vesuvius in AD 79, offers a rare snapshot of ancient engineering in action.
In the shadow of Mount Vesuvius, the ancient city of Pompeii near modern Naples, Italy, lay preserved under layers of volcanic ash since its destruction in AD 79. Excavations at a construction site there, which began in the 1880s and resumed in early 2023, have unearthed a remarkably intact workshop dedicated to concrete production. This discovery includes tools for mixing, heaps of quicklime, recycled roof tiles, and even wall inscriptions noting work schedules and material amounts, providing direct evidence of Roman building practices.
Admir Masic, a researcher at the Massachusetts Institute of Technology, described the site's condition as "exceptionally well preserved," likening the experience to stepping back into 79 CE alongside the workers. Chemical analysis of the untouched material piles reveals that Romans employed a hot-mixing technique using quicklime, or calcium oxide, rather than the slaked lime—calcium hydroxide—mentioned in historical texts. In this method, quicklime was combined with aggregates like volcanic ash and pozzolana before water was added, triggering a chemical reaction that generated heat during setting.
This approach positioned quicklime as the key component for structural concrete, while slaked lime served mainly for plasters and mortars requiring smooth finishes. A standout feature is the formation of lime clasts, small fragments that enable ongoing self-healing. As Masic explains, "These lime clasts act as calcium reservoirs, dissolving and recrystallising in pores and cracks or reacting with volcanic ash to strengthen concrete microstructure."
The technique's enduring relevance is evident: a contemporary builder could reportedly adapt quickly to the Roman process, underscoring the timeless craft behind the empire's lasting architecture. These insights, detailed in a study published in Nature Communications (DOI: 10.1038/s41467-025-66634-7), revise long-held views and affirm the ingenuity that contributed to Roman structures' remarkable longevity.