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A bathroom at Emperor Hadrian’s villa has kept an engineering secret for nearly 2,000 years.
Researchers studying concrete from a communal latrine at Hadrian’s Villa in Tivoli, Italy, found that the material kept reacting throughout the centuries. Beneath the toilet seats, carbon dioxide, moisture, and leftover lime slowly formed calcite crystals that filled pores and sealed tiny cracks, leaving a bathroom floor that could help explain how Roman concrete strengthened with age.
A Toilet Time Capsule
The sample came from a waste collector beneath the toilet seats of a latrine in the western structures of the Canopus, part of Hadrian’s sprawling second-century estate outside Rome. Hadrian ruled from 117 to 138 C.E., and his villa is now a UNESCO World Heritage site.
The latrine ironically helped preserve a cleaner record of the original concrete. Temples, palaces and aqueducts are often repaired over centuries, which can alter the material scientists want to study. The concrete beneath these toilet seats appears to have been left largely undisturbed.
“Nobody restores a latrine,” Paulo J. M. Monteiro, a civil engineer at the University of California, Berkeley, told Scientific American.
Monteiro and colleagues, led by Xiaohong Zhu, examined the sample with 3D X-ray imaging, electron microscopy, and chemical tests. These tools let them map pores, cracks, volcanic fragments, and mineral crusts at nanometer scale.
The concrete was made with the familiar Roman recipe: a mix of volcanic rock, volcanic ash, and lime. Black volcanic fragments sat inside a lime-based mortar. The mix also had a water-to-binder ratio close to that of many modern concretes.
But inside the ancient material, the researchers found something striking. Calcite, a hard mineral made of calcium carbonate, had spread through pores and fractures that acted like a passive repair system.
The Old Explanation Was Incomplete
For years, scientists pointed to the pozzolanic reaction as the secret to Roman concrete’s sturdiness. In that process, volcanic ash reacts with lime and water to form cement-like minerals that strengthen the material.
That explanation still holds. The new study found cement-like compounds around volcanic rock fragments. These minerals helped the mortar grip the stone, reinforcing a zone that often becomes weak in concrete.
But the study argues that another reaction also played a major role: carbonation.
Carbonation happens when carbon dioxide from the air reacts with calcium-rich material. In the Hadrian’s Villa sample, that process produced calcite. Over centuries, calcite filled empty spaces, coated particles and sealed hairline cracks.
“Calcite had been suspected as an important binding phase in inland Roman concrete before,” Monteiro explained. “What is new is that we can now see how it binds.”
The team saw calcite crystals growing outward from pore walls. Some formed fibrous, beard-like structures that narrowed cavities. In cracks, new calcite deposits helped close the gaps.
The finding also builds on a 2023 MIT-led study that focused on white lime chunks in Roman concrete. Those chunks were once dismissed as evidence of sloppy mixing. The MIT team proposed that they could instead act as calcium reservoirs. When water entered a crack, calcium could dissolve, move into the gap and recrystallize.
The new study gives researchers a clearer overview of that process.
Ancient Chemistry, Modern Limits
The obvious modern question is whether engineers can design concrete that repairs small cracks before they spread.
But copying ancient concrete will not be simple. Most modern concrete contains steel reinforcement. Fresh concrete protects steel because it is highly alkaline. Carbonation can lower that protection, allowing steel to rust and crack the surrounding material.
So while carbonation strengthened Roman concrete, it can damage modern reinforced concrete. A Roman wall and a highway bridge face different engineering goals and problems.
The authors also caution against treating Roman-style carbonation as a fast climate solution. The reaction in Hadrian’s latrine unfolded over centuries or millennia. It cannot capture large amounts of carbon dioxide within the normal lifetime of most infrastructure.
Still, the study points toward some useful ideas. Engineers could design materials that use controlled carbonation to seal microcracks, especially in massive unreinforced structures, repairs, or conservation work.
The study was published in the journal Science Advances.
This story originally appeared on ZME Science. Want to get smarter every day? Subscribe to our newsletter and stay ahead with the latest science news.
