What a Roman Latrine Taught Engineers About Lasting Concrete

The best experiment ever run on Roman concrete had no scientists. It ran itself, under the wooden seats of a communal toilet at Hadrian’s Villa outside Rome, for nineteen centuries. When UC Berkeley researchers extracted a sample from that latrine and put it under a scanning electron microscope, what they found inside changed what scientists thought they knew about why ancient concrete still stands while modern concrete cracks apart in decades.

The study, published in Science Advances and led by Xiaohong Zhu with senior author Paulo J. M. Monteiro of UC Berkeley, identifies a mechanism behind Roman concrete’s extraordinary durability that had never been properly evaluated before: a slow, continuous self-healing process operating inside the material itself for nearly two millennia.

The Experiment Nobody Started

Hadrian’s Villa sits in Tivoli, 27 kilometres east of Rome. Built in the early second century AD, it was one of the most elaborate residential complexes in the Roman world. Many of its structures have been excavated, studied, and partially restored over the centuries. The communal latrine in the western substructures of the Canopus section was not among them.

That neglect was precisely what made it scientifically valuable. Undisturbed materials preserve their chemical history in ways that restored structures cannot. The latrine concrete had reacted only with what nature gave it: atmospheric carbon dioxide, moisture, and time.

Nobody restores a latrine, Monteiro said. So the material sat undisturbed for 19 centuries, quietly running an experiment no one alive could start.

What Was Already Known, and What Was Missing

For decades, researchers had attributed Roman concrete’s longevity to the pozzolanic reaction: when volcanic ash, water, and highly reactive lime combine, they produce powerful binding agents that harden and densify the concrete. The new study does not replace that explanation. While the pozzolanic reaction is of fundamental importance, Monteiro said, the findings suggest that carbonation over a long period of time also enhances durability and can help concrete seal cracks as it ages.

Carbonation is the process by which carbon dioxide from the air slowly diffuses into concrete and reacts with calcium-rich compounds, producing calcite, a crystalline mineral. In modern steel-reinforced concrete, carbonation is a problem: it lowers pH and can strip the protection that keeps steel rebar from corroding. In Roman concrete, which contained no steel, the consequences were entirely different.

How Calcite Sealed Two Thousand Years of Damage

The team used high-resolution X-ray imaging, three-dimensional tomography, multi-scale spectroscopy, and electron microscopy at scales down to tens of nanometres. What they found was a specific crystal formation called radiaxial fibrous calcite: fan-shaped crystals growing outward from volcanic rock fragments embedded in the mix, extending across small cracks and building continuous mineral bridges through the material.

These bridges improved how loads transferred through the concrete matrix and significantly reduced the material’s permeability to water and to the chemical agents that degrade ordinary concrete. As cracks formed under structural stress or thermal change, the ongoing growth of calcite gradually closed them. The material was not simply enduring. It was responding to damage by filling it.

Calcite helps fill small cracks, pores, and voids in the concrete, Monteiro explained, creating a dense and cohesive structure that improves load transfer and limits water infiltration, contributing to its long-term stability.

A Lesson With Two Edges

The same process that strengthened Roman concrete poses a specific threat to modern infrastructure. Contemporary concrete relies on steel reinforcement, and its high alkalinity shields that steel from corrosion. Carbonation, by lowering pH over time, gradually removes that protection. The same reaction that quietly strengthened Roman concrete is a slow threat to ours, Monteiro said.

But the findings also point toward an opportunity. Engineers are increasingly interested in using controlled carbonation to lock carbon dioxide permanently into mineral form inside concrete during curing. Cement manufacture currently releases roughly 0.83 tonnes of carbon dioxide for every tonne of clinker produced and accounts for around eight percent of global carbon emissions. Concrete that absorbs and mineralizes carbon dioxide as it matures would address both durability and emissions at once.

Monteiro cautioned against expecting rapid results. The calcite accumulation observed at Hadrian’s Villa took centuries to reach its present density. Modern construction cannot wait that long. Modern engineers face a delicate balancing act between durability and sustainability, he said. But the mechanisms identified could guide the development of low-clinker systems that incorporate carbonation dynamics intentionally from the outset.

What Neglect Preserved

Most surviving Roman structures have been subject to centuries of repair and restoration. The chemistry inside a carefully maintained wall reflects later interventions as much as original Roman practice. The latrine at Hadrian’s Villa, because nobody thought to preserve it, holds something that restored monuments cannot: an unbroken nineteen-century record of what Roman concrete does when left entirely alone.

The wooden seats above the ancient drain rotted long ago. The concrete beneath them did not. It accumulated a crystalline record of its own gradual self-repair, one that twenty-first century materials science can now read. A latrine survived because nobody considered it worth restoring. What it preserved may inform how the next generation of concrete is built.

Sources

Zhu, X., Chae, S. R., McElhany, S., Liang, C., Zheng, Q., Li, J., and Monteiro, P. J. M. (2026). Mineralized carbonates contribute to the millennial durability of Roman concrete. Science Advances, 12(28), eaeb0754. DOI: 10.1126/sciadv.aeb0754. UC Berkeley Engineering press release