Roman Concrete’s Hidden Self-Healing Secret Revealed

The ancient Romans excelled at construction and engineering, with their aqueducts perhaps their best-known achievement. Those enduring, still-working structures depend on an unusual building material: pozzolanic concrete, an exceptionally hard-wearing substance that gave Roman buildings their remarkable resilience.

Even now, the Pantheon – almost 2,000 years old and still standing – retains the record for the world’s largest unreinforced concrete dome.

Roman concrete and its key ingredients

The qualities of this concrete have usually been linked to its components: pozzolana, a blend of volcanic ash named for the Italian city of Pozzuoli, home to a substantial deposit, and lime. Combined with water, these materials can react to create robust concrete.

However, that explanation is apparently incomplete. In 2023, an international research group headed by the Massachusetts Institute of Technology (MIT) discovered that both the materials and the way they were combined differed somewhat from previous assumptions.

The crucial evidence came from small white pieces of lime visible within concrete that otherwise appears thoroughly blended. These fragments had formerly been explained as the result of inferior mixing or poor-quality materials, but this interpretation did not convince 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."

Masic and his colleagues, led by MIT civil engineer Linda Seymour, closely examined 2,000-year-old Roman concrete specimens from Privernum, an archaeological site in Italy. To better understand the lime clasts, they used large-area scanning electron microscopy and energy-dispersive X-ray spectroscopy, powder X-ray diffraction, and confocal Raman imaging.

A central question concerned the type of lime involved. The conventional account of pozzolanic concrete holds that it was made with slaked lime. Limestone is first heated to very high temperatures, creating a highly reactive, caustic powder known as quicklime, or calcium oxide.

Adding water to quicklime produces slaked lime, or calcium hydroxide, a paste that is less reactive and less caustic. The theory had been that ancient Romans then blended this slaked lime with pozzolana.

How hot mixing strengthened pozzolanic concrete

The team’s analysis indicated that the lime clasts in its samples do not fit that process. Instead, Roman concrete was likely produced by directly combining quicklime with pozzolana and water at extremely high temperatures, either alone or alongside slaked lime. The researchers call this approach "hot mixing", and it produces the 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."

The process offers a further advantage: lime clasts give the concrete striking self-healing properties.

When cracks develop, they tend to pass through the lime clasts, which have a greater surface area than other particles in the concrete matrix. Water entering a crack reacts with the lime, creating a calcium-rich solution. This then dries and solidifies into calcium carbonate, sealing the crack and stopping it from extending.

Evidence of this effect has appeared in concrete at another 2,000-year-old location, the Tomb of Caecilia Metella, where cracks have been filled with calcite. It may also account for the survival of Roman concrete seawalls built 2,000 years ago, which have remained intact for millennia despite continual punishment from the sea.

The researchers tested their conclusions by producing pozzolanic concrete according to both ancient and modern recipes containing quicklime. They also made a control concrete without quicklime before carrying out crack tests. As expected, cracks in the quicklime concrete had completely healed after two weeks, whereas the control concrete remained cracked.

The team is now seeking to commercialise its concrete 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 was published in Science Advances.

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