A latrine at one of ancient Rome’s grandest estates is helping scientists understand why Roman concrete has lasted for almost 2,000 years. A new study, published in the journal Science Advances, points to carbonation as a major reason behind the long life of Roman concrete, adding to the long-held belief that volcanic ash alone explains its strength.
Researchers studied a concrete sample taken from a latrine in the western substructures of the Canopus, part of Hadrian’s Villa in Tivoli, Italy.
The villa served as the retreat of Emperor Hadrian in the second century A.D. and is now a UNESCO World Heritage site. The sample came from a toilet collector connected to the site’s old sewage system.
How carbonation shaped the strength of Roman concrete
The team, which included Xiaohong Zhu of the University of California, Berkeley’s Department of Civil and Environmental Engineering, used advanced imaging tools to study the concrete’s inner structure.
These included micro and nano computed tomography scans, along with electron microscopy and spectroscopy. The scans let researchers see details as small as 43 nanometers, revealing how the concrete formed and held together over centuries.
The findings show Roman builders mixed volcanic rock containing minerals such as leucite and analcime with lime, using a water-to-binder ratio of about 0.4 to 0.45. Over time, unreacted lime in the mix absorbed carbon dioxide and moisture from the air.
This slow reaction produced large amounts of calcite, a natural form of calcium carbonate, which became the main substance holding the concrete together.
The study also found smaller amounts of calcium-aluminum-silicate-hydrate near the volcanic rock fragments. This compound added extra strength but played a smaller role than calcite overall.
Self-healing calcite offers clues for modern builders
Researchers explain that the calcite formed in a fibrous pattern, growing outward and filling small cracks and gaps in the material. This process appears to have given the ancient concrete a kind of self-healing ability, allowing it to repair minor damage on its own across long stretches of time.
The study notes that this natural aging process took centuries to unfold and cannot be copied quickly in modern construction. Still, the researchers say the findings could guide the development of longer-lasting, lower-carbon concrete for use today.
Earlier research on Roman concrete had focused heavily on volcanic ash and its chemical reaction with lime. This new work argues that carbonation deserves equal attention as a driver of durability, since it creates the bulk of the material binding the concrete together.
The study offers a rare, detailed look at how an unreinforced, lime-based building material can remain structurally sound for nearly two millennia without steel reinforcement.
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