Pompeii Reveals the Secret of Ancient Rome’s Ultra-Durable Concrete

Pompeii construction site reveals Roman concrete recipe

A building site from almost 2,000 years ago, dating to the presumed destruction of Pompeii in 79 CE, has provided fresh evidence of the secret behind Ancient Rome’s exceptionally durable concrete.

Archaeologists last year uncovered a completely preserved construction site beneath the volcanic ash that engulfed Pompeii: an unusual glimpse of Roman construction work halted in an instant.

The site contained carefully arranged heaps of materials, including the components used to produce the renowned long-lasting concrete found in structures such as the Pantheon. Its immense unreinforced dome has remained standing for thousands of years.

A new study suggests that the key was a method materials scientist Admir Masic, of the Massachusetts Institute of Technology (MIT), calls "hot-mixing".

This process directly combines concrete’s ingredients: a volcanic-ash blend known as pozzolan and quicklime, which produces considerable heat within the mixture when it comes into contact with water.

"The benefits of hot mixing are twofold," Masic said back in 2023 when he first discovered the technique through experimentation.

"First, when the overall concrete is heated to high temperatures, it allows chemistries that are not possible if you only used slaked lime, producing high-temperature-associated compounds that would not otherwise form. Second, this increased temperature significantly reduces curing and setting times since all the reactions are accelerated, allowing for much faster construction."

A third and especially important advantage is the extraordinary self-repairing capacity supplied by the remaining lime fragments, known as clasts. This may help explain why Roman monuments endure while those of other civilisations have fallen apart.

As concrete cracks, the fractures tend to extend towards lime clasts, which have more surface area than other particles in the matrix. If water reaches a crack, it reacts with the lime and creates a calcium-rich solution. As this dries, it hardens into calcium carbonate, sealing the crack and stopping it from expanding.

"There is the historic importance of this material, and then there is the scientific and technological importance of understanding it," Masic says.

"This material can heal itself over thousands of years, it is reactive, and it is highly dynamic. It has survived earthquakes and volcanoes. It has endured under the sea and survived degradation from the elements."

Hot-mixing and the Vitruvian puzzle

Although hot-mixing addressed long-standing questions about Roman concrete, it also created another: the formula did not correspond with the account of producing the building material in De architectura, a 1 BCE treatise by the architect Vitruvius.

Under the Vitruvian approach, lime was first combined with water through a process called slaking. The slaked lime was then mixed with pozzolan. Yet this technique does not create the lime clasts found in actual samples of Roman concrete.

Scientists have been perplexed by this discrepancy for many years. Vitruvius’ texts are the most comprehensive surviving records of Roman architecture and building methods. He outlined a wall-building technique called opus caementicium, but material specimens from ancient structures did not match his directions.

The Pompeii materials resolve the question. Masic and his colleagues carried out isotope analysis on five dry material piles, identifying pozzolan containing pumice and lithic ash, quicklime, and even lime clasts.

Most importantly, the dry components had already been mixed together – an archaeological smoking gun.

Evidence from Pompeii’s Roman concrete materials

Microscopic examination of mortar from the walls showed clear signs of hot-mixing: broken lime clasts, calcium-rich reaction rims extending into volcanic-ash particles, and minute calcite and aragonite crystals developing inside pumice vesicles.

Raman spectroscopy verified the mineral changes, while isotope analysis traced the chemical routes of carbonation through time.

"Through these stable isotope studies, we could follow these critical carbonation reactions over time, allowing us to distinguish hot-mixed lime from the slaked lime originally described by Vitruvius," Masic says.

"These results revealed that the Romans prepared their binding material by taking calcined limestone (quicklime), grinding [it] to a certain size, mixing it dry with volcanic ash, and then eventually adding water to create a cementing matrix."

This does not automatically mean that Vitruvius was mistaken. He may have documented a different way to manufacture concrete, or his text could have been misunderstood. However, the findings suggest the material’s most durable version emerged through hot-mixing.

The researchers believe this knowledge could inform concrete production today, centuries after the Roman Empire collapsed and left its surviving monuments not only as evidence of its splendour, but also of its people’s ingenuity.

Modern concrete is among the most commonly used construction materials worldwide. It is also notably short-lived, frequently deteriorating within decades when exposed to environmental pressures. Its manufacture also carries a severe environmental burden, consuming vast resources and adding to greenhouse-gas emissions.

Making concrete more durable alone could substantially improve its sustainability.

"We don't want to completely copy Roman concrete today. We just want to translate a few sentences from this book of knowledge into our modern construction practices," says Masic, who has started a company called DMAT to do just that.

"The way these pores in volcanic ingredients can be filled through recrystallization is a dream process we want to translate into our modern materials. We want materials that regenerate themselves."

The research has been published in Nature Communications.