A 1,900-year-old Roman toilet may have finally revealed why ancient concrete lasts for thousands of years

Rome's empire crumbled centuries ago, but one of its bathrooms is still on the job. Here’s a fun fact for your next trivia night: there is a Roman public restroom that has been standing for almost 1,900 years, easily outlasting the empire that built it. A new study titled ‘Mineralized carbonates contribute to the millennial durability of Roman concrete,’ published in Science Advances and led by Xiaohong Zhu of Beijing University of Technology and Paulo Monteiro of UC Berkeley, examined a concrete toilet at Emperor Hadrian’s Villa to learn why Roman concrete doesn’t fall apart.

Why study a toilet, of all places
Many of the most famous Roman ruins, such as the Pantheon, have been patched or repaired over the centuries, making it difficult for scientists to read their original chemistry. But a toilet doesn’t have that problem since nobody spends their time trying to restore an ancient bathroom. In the report in Scientific American, Paulo Monteiro, a civil engineer at UC Berkeley and senior author of the study, says that for 19 centuries, the material sat undisturbed, quietly running a chemical experiment that nobody alive could have deliberately started. Thanks to that random time capsule, it was possible to find it.

The sample comes from the Canopus section of Hadrian’s Villa in Tivoli, Italy, roughly 17 miles east of Rome, at a site dated to the second century A.D., according to this Berkeley Engineering news release. Hadrian ruled Rome from 117 to 138 A.D., and his villa is now a UNESCO World Heritage Site.

The real MVP ingredient: calcite
For decades, the leading theory was that the strength of Roman concrete was mostly due to a reaction between volcanic ash and lime. Then, in 2023, another team suggested that something else was afoot. A 2023 study published in Science Advances, led by MIT’s Linda Seymour and Admir Masic, found that Roman concrete contains small white chunks called lime clasts that scientists had long written off as evidence of sloppy mixing. Turns out, they weren’t mistakes at all. When small cracks form, water interacts with these clasts and dissolves calcium, which re-hardens as calcium carbonate and physically seals the crack. This gives the concrete a built-in self-repair system.

This idea is taken up immediately in the new study. The team used high-resolution X-ray imaging, electron microscopy and chemical mapping down to tens of nanometers in scale, tracing a mineral called calcite that runs through the pores and cracks in the ancient sample, forming a web-like network that binds everything together. According to the Berkeley Engineering release, it’s a slow, ongoing process called carbonation, in which carbon dioxide from the air reacts with calcium compounds inside the concrete over centuries. In other words, the concrete was not just resisting damage; it may have been strengthening over time.

Why this actually matters for your city, not just ancient Rome
It’s not just a cool historical fact to throw into your group chat. As the 2018 research briefing from Chatham House points out, more than 4 billion tons of cement are produced each year, and that alone is responsible for around 8% of global CO2 emissions. It also warns that demand is still rising, with global cement output projected to top 5 billion tons annually over the next 30 years. Modern concrete, especially the steel-reinforced variety used in just about every building and bridge in the US, tends to crack and weaken within decades rather than millennia.

Here's the catch, though. The Scientific American report says that the same carbonation process that helped ancient Roman concrete get stronger with age is a slow-motion threat to modern concrete. It can lower the pH of the material and leave steel reinforcement vulnerable to rust. Fresh concrete is sufficiently alkaline to protect embedded steel, but the protection decreases as carbonation progresses. Maria Juenger, a cement materials researcher at UT Austin who was not involved in the research, has pointed out that modern buildings depend on steel in a way Roman concrete never did, so engineers cannot simply copy the ancient recipe.

That said, the report quotes Admir Masic, the MIT scientist behind the 2023 lime clast study, saying the new findings support the concept that carbonates are central, not peripheral, to how this material behaves. Researchers hope that learning how calcite forms at the nanoscale will help engineers eventually design lower-carbon concrete that heals its own cracks, rather than requiring constant repair.

The larger picture
For a generation that is genuinely anxious about crumbling infrastructure and climate change at once, this kind of research hits a sweet spot. It’s not about glorifying the past. It’s about mining engineering, nearly 2,000 years old, to find answers to a very present-day problem: how to build things that last without wrecking the planet.

So the next time someone makes a joke about the fall of Rome, just remember that its bathrooms are still winning.