We Finally Know Why Ancient Roman Concrete Lasts Thousands of Years

The ancient Romans were masters of building and engineering, perhaps most famously represented by the aqueducts. And those still functional marvels rely on a unique construction material: pozzolanic concrete, a spectacularly durable concrete that gave Roman structures their incredible strength.

Even today, one of their structures – the Pantheon, still intact and nearly 2,000 years old – holds the record for the world's largest dome of unreinforced concrete.

The properties of this concrete have generally been attributed to its ingredients: pozzolana, a mix of volcanic ash – named after the Italian city of Pozzuoli, where a significant deposit of it can be found – and lime. When mixed with water, the two materials can react to produce strong concrete.

But that, as it turns out, is not the whole story. In 2023, an international team of researchers led by the Massachusetts Institute of Technology (MIT) found that not only are the materials slightly different from what we may have thought, but the techniques used to mix them were also different.

Roman concrete was probably made by mixing the quicklime directly with the pozzolana and water at extremely high temperatures, by itself or in addition to slaked lime, a process the team calls 'hot mixing' that results in 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."

And it has another benefit: The lime clasts give the concrete remarkable self-healing abilities.

When cracks form in the concrete, they preferentially travel to the lime clasts, which have a higher surface area than other particles in the matrix. When water gets into the crack, it reacts with the lime to form a solution rich in calcium that dries and hardens as calcium carbonate, gluing the crack back together and preventing it from spreading further.

This has been observed in concrete from another 2,000-year-old site, the Tomb of Caecilia Metella, where cracks in the concrete have been filled with calcite. It could also explain why Roman concrete from seawalls built 2,000 years ago has survived intact for millennia despite the ocean's constant battering.

So, the team tested their findings by making pozzolanic concrete from ancient and modern recipes using quicklime. They also made a control concrete without quicklime and performed crack tests. Sure enough, the cracked quicklime concrete was fully healed within two weeks, but the control concrete stayed cracked.