How MIT explained Roman concrete’s durability in 2023 and 2025

The ancient Romans were exceptional builders and engineers, and their aqueducts remain the most famous showcase of that skill. Many of these still-working structures depend on a distinctive building material: pozzolanic concrete, an unusually hard-wearing blend that helped Roman construction endure.

A standout example is the Pantheon in Rome. Nearly 2,000 years after it was built, it remains in remarkably good condition-and it still holds the record for the largest dome in the world made from unreinforced concrete.

Roman pozzolanic concrete and enduring structures

For a long time, the concrete’s performance has been credited mainly to what went into it: pozzolana-volcanic ash (named after Pozzuoli in Italy, where a major deposit exists)-combined with lime. When these ingredients react with water, they form a strong, long-lasting concrete.

However, that explanation is incomplete. In 2023, an international research group led by scientists at the Massachusetts Institute of Technology (MIT) reported that Roman concrete not only used materials that differ slightly from what researchers had assumed, but was also prepared using mixing methods that were not the same as modern expectations.

Since those 2023 results, further study of the ancient recipe-looking specifically at its raw materials and energy needs-has suggested potential routes to improving contemporary cement.

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

The MIT 2023 discovery: 'hot mixing'

A key clue was the presence of small white fragments of lime scattered through otherwise well-blended concrete. Previously, these pieces had often been explained away as signs of sloppy mixing or substandard ingredients.

MIT materials scientist Admir Masic found that interpretation unconvincing.

"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 colleagues, led by MIT civil engineer Linda Seymour, closely examined 2,000-year-old Roman concrete taken from Privernum, an archaeological site in Italy.

To learn what the lime fragments (lime clasts) actually were, the team used large-area scanning electron microscopy, energy-dispersive X-ray spectroscopy, powder X-ray diffraction, and confocal Raman imaging.

A central issue was identifying the form of lime used. The usual account of pozzolanic concrete assumes slaked lime was involved. In that standard sequence, limestone is first heated to very high temperatures, producing quicklime (calcium oxide), a highly reactive, caustic powder.

When quicklime is combined with water, it becomes slaked lime (calcium hydroxide): a paste that is somewhat less reactive and less caustic. According to conventional theory, ancient Romans then blended this slaked lime with pozzolana.

The new analyses showed that the lime clasts in the samples do not fit that approach. Instead, the researchers concluded Roman concrete was probably produced by combining quicklime directly with pozzolana and water at extremely high temperatures. The team referred to this technique as 'hot mixing', and it naturally 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."

How lime clasts enable self-healing

There is an additional advantage: lime clasts appear to give the concrete impressive self-healing behaviour.

When the material cracks, the fractures tend to run into the lime clasts because they offer more surface area than other particles in the surrounding matrix. If water enters the crack, it can react with the lime to create a calcium-rich solution. As this dries, it hardens into calcium carbonate, effectively sealing the crack and stopping it from growing.

Evidence of this kind of repair has been seen in concrete at another 2,000-year-old location, the Tomb of Caecilia Metella, where cracks have been infilled with calcite. The same mechanism may also help explain why Roman concrete seawalls, built around 2,000 years ago, have remained intact for millennia despite constant pounding from the sea.

To test the effect directly, the researchers carried out crack experiments on pozzolanic concrete made with both ancient and modern quicklime recipes, alongside a control concrete made without quicklime. As expected, the quicklime-based concrete repaired its cracks within two weeks, while the control sample stayed cracked.

The team is now working to commercialise this concrete as a greener alternative to today’s common 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.

From ancient recipe to modern cement (2025 analysis)

In a more recent 2025 study, engineers assessed the raw materials and energy required for Roman-style concrete compared with modern Portland cement. They reported that Roman-style blends need more water and a higher initial energy input, but that their extended service life could make them more sustainable over the long term.

The research has been published in Science Advances.

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