Ancient Roman infrastructure has stood the test of time. Today, you can walk through Italy and see concrete buildings, roads and aqueducts that have survived for about two millennia. Modern concrete, on the other hand, usually crumbles within roughly 100 years.

Scientists have long tried to uncover the secrets of Roman concrete’s durability. For years, they assumed that its longevity was thanks to one key chemical process: the pozzolanic reaction, which occurs when volcanic ash reacts with the chemical lime and water. While that still holds, there seems to be more to the story.

It turns out that another chemical reaction, known as carbonation, might also contribute to Roman concrete’s longevity. The findings, published in the journal Science Advances on July 8, could help researchers develop more sustainable and resilient concrete materials.

For the new work, researchers traveled to the 1,900-year-old Hadrian’s Villa, a UNESCO World Heritage site that sits about 17 miles east of Rome. The sprawling estate is an architectural marvel, but one of its scientific gems are the communal toilets. They offer an unprecedented opportunity to study Roman concrete in its original state, unaltered by modern hands.

“Nobody restores a latrine,” says Paulo J. M. Monteiro, a study co-author and civil engineer at the University of California, Berkeley, to Sam Macdonald at Scientific American. “So, the material sat undisturbed for 19 centuries, quietly running an experiment no one alive could start.”

Monteiro and his colleagues took a concrete sample from underneath a toilet seat. Back in the lab, they examined it under a high-powered microscope, scanned it with X-rays and analyzed its chemical composition.

As expected, the specimen contained evidence that volcanic ash, lime and water had been combined to form the material. However, a closer look at the concrete’s pores and fractures revealed that calcite, a mineral with calcium, carbon and oxygen, was the primary binding agent.

When atmospheric carbon dioxide reacts with the calcium compounds in the concrete, it forms the hard mineral calcite, which contains a lot of the compound calcium carbonate. The mineral fills small cracks and pores in the concrete, allowing ancient structures to strengthen and heal over time.

“While the pozzolanic reaction is of fundamental importance, our findings suggest that carbonation over a long period of time also enhances the durability of concrete and can help it seal cracks as it ages,” Monteiro says in a statement.

The work builds on a study published in 2023 that suggested that Roman concrete could repair cracks on its own because it was created with chemical reactions involving quicklime, a form of limestone, which left behind calcium-rich deposits in the material. The deposits could react with water, such as rain, and recrystallize to fill in any gaps.

With the new study, carbonates have entered the limelight. The research “strengthens the idea that carbonates are more dynamic in these systems and play a fundamental role, not a marginal one,” says Admir Masic, a materials scientist at MIT who co-authored the 2023 study but was not involved in the new work, to Scientific American.

Monteiro and his colleagues hope that by understanding how Roman concrete worked, modern-day experts can build concrete that has less of an environmental impact. Concrete is one of the world’s most consumed materials, but its production emits an enormous amount of heat-trapping carbon dioxide—about 8 percent of emissions worldwide. According to the United Nations, roughly half of the buildings that will exist by 2050 have not yet been built, which is why it’s important to develop construction materials that have a reduced carbon footprint.

“This study shows how exploring ancient engineering techniques can lead to important revelations,” Monteiro says in the statement. “We hope that by unlocking Roman secrets for enhancing concrete durability, we can someday attain sustainable modern infrastructure development.”