Roman Concrete’s Secret May Explain Its Extraordinary Durability

The ancient Romans were exceptional builders and engineers, with their still-working aqueducts perhaps their best-known achievement. Those feats of architecture depended on a distinctive building material: pozzolanic concrete, an extraordinarily long-lasting substance that gave Roman buildings their remarkable resilience.

Even now, the Pantheon – preserved almost 2,000 years after it was built – remains the world record-holder for the largest dome made from unreinforced concrete.

Pozzolanic concrete and Roman construction

The durability of this concrete has usually been explained by its components: pozzolana, a volcanic-ash mixture named after the Italian city of Pozzuoli, home to an important deposit, and lime. Combined with water, these materials can react to create robust concrete.

However, that explanation is incomplete. An international research group headed by the Massachusetts Institute of Technology (MIT) discovered that both the ingredients and the mixing methods differed somewhat from previous assumptions.

The key evidence was small white pieces of lime visible within otherwise thoroughly blended concrete. Earlier explanations had blamed these fragments on inferior materials or inadequate mixing, an interpretation that 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 in a January 2023 statement.

"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 Romans may have made concrete

Masic and his colleagues, led by MIT civil engineer Linda Seymour, closely examined 2,000-year-old Roman concrete samples recovered from Privernum, an archaeological site in Italy. To understand the lime clasts more clearly, 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 contained slaked lime. Limestone is first heated to very high temperatures, producing a highly reactive, caustic powder known as quicklime, or calcium oxide.

When quicklime is mixed with water, it becomes slaked lime, or calcium hydroxide, a paste that is less caustic and slightly less reactive. The prevailing theory was that ancient Romans combined this slaked lime with pozzolana.

The team's examination suggested that the lime clasts in its samples did not fit that process. Instead, Romans probably combined quicklime directly with pozzolana and water at extremely high temperatures, either alone or alongside slaked lime. The researchers call this process "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."

Lime clasts give Roman concrete self-healing properties

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

As cracks develop, they tend to pass through 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 that dries and solidifies into calcium carbonate. This seals the fracture and stops it spreading.

Researchers have seen this effect in concrete from the Tomb of Caecilia Metella, another site dating back 2,000 years, where calcite has filled cracks in the material. It may also account for the survival of Roman concrete seawalls, built 2,000 years ago and still intact despite millennia of relentless wave action.

The team then put its conclusions to the test, creating pozzolanic concrete from both ancient and modern recipes containing quicklime. It also produced a control concrete without quicklime before carrying out crack tests. As expected, the cracked quicklime concrete healed completely within two weeks, whereas the control concrete remained fractured.

The researchers are now seeking to commercialise their concrete as a more environmentally friendly alternative to modern concrete.

"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 study was published in Science Advances.

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