How MIT revealed the secret behind Roman pozzolanic concrete

The ancient Romans excelled at construction and engineering, and their aqueducts are among the best-known examples. Many of these working feats were made possible by a distinctive building material: pozzolanic concrete, an exceptionally long-lasting concrete that helped Roman structures achieve their extraordinary strength.

One Roman building, the Pantheon, remains standing and almost 2,000 years old. It still holds the record for the largest dome in the world made from unreinforced concrete.

For a long time, the performance of this concrete has largely been linked to what went into it: pozzolana (a volcanic-ash mixture named after Pozzuoli in Italy, where major deposits occur) combined with lime. Add water, and the ingredients react to create a strong concrete.

A new look at Roman pozzolanic concrete

In 2023, however, an international research group led by the Massachusetts Institute of Technology (MIT) reported that the story is more complicated. Their work suggests the materials were not quite what many assumed-and, crucially, the way the Romans combined them was different too.

The key clues were small, pale-white pieces of lime embedded in otherwise uniform-looking concrete. These fragments had often been dismissed as evidence of inferior mixing or low-grade inputs, but that explanation did not sit comfortably with MIT materials scientist Admir Masic.

"The idea that the presence of these lime clasts was simply attributed to low-quality control always bothered me," Masic said back in January 2023.

"If the Romans put so much effort into making an outstanding construction material, following all of the detailed recipes that had been optimized over the course of many centuries, why would they put so little effort into ensuring the production of a well-mixed final product? There has to be more to this story."

How MIT studied lime clasts in Roman concrete

To investigate, Masic and colleagues-working under the leadership of MIT civil engineer Linda Seymour-analysed Roman concrete samples about 2,000 years old from the archaeological site of Privernum in Italy.

The team used several techniques to examine the lime clasts in detail, including large-area scanning electron microscopy, energy-dispersive X-ray spectroscopy, powder X-ray diffraction, and confocal Raman imaging.

A central issue was the type of lime the Romans used. The conventional account of pozzolanic concrete is that it relies on slaked lime. In this view, limestone is first fired at very high temperatures to make quicklime, a highly reactive caustic powder (calcium oxide).

When quicklime is combined with water, it becomes slaked lime (calcium hydroxide), producing a paste that is less reactive and less caustic. The standard theory has been that the Romans mixed this slaked lime with pozzolana.

Roman concrete ‘hot mixing’ with quicklime

The team’s measurements indicate the lime clasts they observed do not match that expected pathway. Instead, the researchers argue Roman concrete was likely produced by combining quicklime directly with pozzolana and water at extremely high temperatures-either on its own or alongside slaked lime. They refer to this approach as ‘hot mixing’, and it naturally produces the lime clasts seen in the material.

"The benefits of hot mixing are twofold," Masic said.

"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."

There is also an additional advantage: the lime clasts appear to give the concrete impressive self-healing behaviour.

When cracking begins, the fractures tend to propagate towards the lime clasts, which present more surface area than other particles within the concrete matrix. If water enters a crack, it reacts with the lime, generating a calcium-rich solution. As it dries, it hardens into calcium carbonate, effectively sealing the crack and stopping it from extending.

This effect has been documented in concrete from another roughly 2,000-year-old site-the Tomb of Caecilia Metella-where cracks were found to be filled with calcite. The same mechanism could help explain why Roman seawalls built 2,000 years ago have remained intact for centuries, despite relentless wave action.

To test the hypothesis, the researchers produced pozzolanic concrete using both ancient and modern-style formulations that included quicklime. They also prepared a control sample without quicklime and then carried out cracking tests. The results were clear: the quicklime concrete, once cracked, healed completely within two weeks, whereas the control concrete did not repair itself and the cracks remained.

The team is now working on commercialising their material as a more environmentally friendly alternative to today’s concretes.

"It's exciting to think about how these more durable concrete formulations could expand not only the service life of these materials, but also how it could improve the durability of 3D-printed concrete formulations," Masic said.

The research has been published in Science Advances.

A version of this article was first published in January 2023.