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

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

The remarkable properties have generally been attributed to its ingredients: pozzolana – a mix of volcanic ash named after the Italian city of Pozzuoli, where a significant deposit is found – and lime. The two materials react with water to produce strong, long-lasting concrete.

But it turns out that’s not the whole story. In 2023, an international team led by scientists at the Massachusetts Institute of Technology (MIT) found that not only are the materials in Roman concrete slightly different from what we thought, but the techniques used to mix them were also distinct.

And since those 2023 findings, analysis of the ancient recipe’s raw materials and energy demands has revealed ways we might improve modern cement.

Related: Engineers Weigh Up Returning to Ancient Roman Concrete Recipes

Watch the video below for a summary of the MIT-led research:

The smoking guns were small, white chunks of lime in otherwise well-mixed concrete. The presence of these chunks had previously been attributed to poor mixing or ingredients.

To materials scientist Admir Masic of MIT, that did not make sense.

“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?” Masic said in January 2023.

“There has to be more to this story.”

Masic and the team, led by MIT civil engineer Linda Seymour, carefully studied 2,000-year-old samples of Roman concrete from the archaeological site of Privernum in Italy.

These samples were subjected to large-area scanning electron microscopy, energy-dispersive x-ray spectroscopy, powder X-ray diffractionand confocal Raman imaging to find out more about the lime clasts.

One major question was the nature of the lime used. The standard understanding of pozzolanic concrete is that it uses slaked lime. First, limestone is heated at high temperatures to produce a highly reactive caustic powder called quicklimeor calcium oxide.

Mixing quicklime with water produces slaked lime, or calcium hydroxide: a slightly less reactive, less caustic paste. According to theory, it was this slaked lime that ancient Romans mixed with the pozzolana.

The team’s analysis found the lime clasts in their samples are inconsistent with this method. Instead, Roman concrete was likely made by mixing the quicklime directly with pozzolana and water at extremely high temperatures – a process the team dubbed ‘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.”

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

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

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This has been observed in concrete from another 2,000-year-old site, the Tomb of Caecilia Metellawhere cracks 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.

The team conducted crack tests on pozzolanic concrete – created using both ancient and modern quicklime recipes – and on a control concrete without quicklime. Sure enough, the cracked quicklime concrete healed within two weeks, but the control concrete remained cracked.

The researchers are working on commercializing their concrete as a more environmentally friendly alternative to current mixes.

“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.

recently, in a 2025 studyengineers compared the raw material and energy requirements of Roman-style concrete to those of modern Portland cement. They found that while Roman-style mixes require more water and initial energy input, their longer lifespan could make them more sustainable over time.

The research has been published in Science Advances.

An earlier version of this article was published in January 2023.