How MIT uncovered the secret of self-healing Roman concrete with hot mixing

The ancient Romans became renowned for construction and engineering, and their aqueducts remain the most celebrated examples. Many of these structures still operate today, largely thanks to a distinctive building material: pozzolanic concrete, an exceptionally long-lasting concrete that helped Roman works achieve extraordinary strength.

Nearly 2,000 years on, one Roman landmark still demonstrates what this material could do. The Pantheon remains standing and continues to hold the record for the world’s largest dome made from unreinforced concrete.

Roman pozzolanic concrete and its legacy

For a long time, the performance of this concrete was mainly explained by what went into it: pozzolana (a volcanic ash mixture named after Pozzuoli in Italy, where major deposits exist) combined with lime. When these ingredients are mixed with water, they can react to form a robust concrete.

However, that explanation turned out to be incomplete. In 2023, an international research group led by the Massachusetts Institute of Technology (MIT) reported that the components were not quite what many had assumed-and that the way the Romans combined them also differed from the standard picture.

MIT study of lime clasts at Privernum

The key clue was the presence of small, pale-white pieces of lime embedded in otherwise apparently well-blended concrete. These fragments had often been dismissed as the result of sloppy mixing or inferior materials, but that interpretation 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."

To investigate, Masic and colleagues-under the leadership of MIT civil engineer Linda Seymour-analysed 2,000-year-old Roman concrete taken from the archaeological site of Privernum in Italy. The team examined the material using large-area scanning electron microscopy and energy-dispersive X-ray spectroscopy, powder X-ray diffraction, and confocal Raman imaging, focusing in particular on the lime clasts.

A central issue was which form of lime the Romans actually used. The conventional account of pozzolanic concrete assumes slaked lime. In that model, limestone is first heated to very high temperatures to create a highly reactive, caustic powder known as quicklime (calcium oxide).

When quicklime is then combined with water, it forms slaked lime (calcium hydroxide), a paste that is less reactive and less caustic. According to the prevailing theory, it was this slaked lime that Roman builders blended with pozzolana.

"Hot mixing" and why it matters

The team’s measurements indicated that the lime clasts they observed did not fit with the standard slaked-lime-only approach. Instead, the researchers concluded that Roman concrete was likely produced by mixing 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 leads to the formation of lime clasts.

"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."

Self-healing Roman concrete

The lime clasts also appear to provide a further advantage: striking self-healing behaviour.

As cracks develop in the concrete, they tend to propagate towards the lime clasts, which present a higher surface area than other particles in the surrounding matrix. Once water penetrates the fracture, it reacts with the lime to create a calcium-rich solution. This then dries and solidifies into calcium carbonate, effectively bonding the crack shut and helping to stop it spreading.

A similar effect has been documented in concrete from the Tomb of Caecilia Metella, another site around 2,000 years old, where fractures have been found filled with calcite. The same mechanism could also help explain why Roman concrete seawalls built roughly 2,000 years ago have endured for millennia, despite relentless wave action.

Testing ancient and modern recipes

To check their interpretation, the researchers produced pozzolanic concrete using quicklime, following both ancient and modern formulations. They also prepared a control concrete that did not include quicklime and then ran cracking experiments. As anticipated, samples containing quicklime healed completely within two weeks, while the control concrete remained cracked.

The group is now pursuing commercialisation of their material, positioning it 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.