Carbonation May Explain Roman Concrete’s Longevity

Having survived nearly two millennia, Roman concrete structures are widely regarded as outstanding examples of durable ancient engineering. Researchers have often attributed this longevity to the pozzolanic reaction, a chemical process that occurs when volcanic ash, water and highly reactive lime are combined to create powerful binding agents that harden and strengthen the concrete. But new research reveals that there might be more to the story.

Now, a team led by Paulo Monteiro of UC Berkeley and Xiaohong Zhu, currently at Beijing University of Technology, has shown that another process - carbonation - may also contribute in important ways to the durability of Roman concrete. Carbonation is a slow, ongoing chemical reaction between atmospheric carbon dioxide (CO₂) and calcium compounds in the concrete. Prior to this study, researchers had no way to evaluate its effects over thousands of years.

As reported today in Science Advances, researchers performed a comprehensive analysis of a concrete latrine in Hadrian’s Villa - a site in Tivoli, Italy, that dates to the second century A.D. - to uncover the mechanisms behind the resilience of these structures. Using advanced 3D imaging techniques, including multi-scale spectroscopy and tomography, they showed how the calcium carbonate networks formed through mineralized carbonation helped bind and densify ancient Roman concrete over time.

“While the pozzolanic reaction is of fundamental importance,” said Monteiro, Professor of the Graduate School in civil and environmental engineering, “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 explained that calcite - a mineral form of calcium carbonate found in limestone and formed through hydration and carbonation processes - helps fill small cracks, pores and voids in the concrete. This cementing mechanism creates a dense and cohesive structure that improves load transfer within the matrix and limits water infiltration, contributing to its long-term stability. Over time, the continual growth of calcite can help close fine cracks, limiting further damage.

These findings could guide the development of next-gen concrete materials. “Calcite has the potential to help mitigate environmental and mechanical stresses in modern concrete infrastructure,” said Monteiro. “This would go a long way toward advancing the development of sustainable and resilient construction materials in the future.”

Researchers also hope that a deeper understanding of Roman concrete will lead to the development of low-carbon cements. Clinker, an ingredient used to manufacture standard cement, currently exacts a heavy environmental toll, with 0.83 tons of carbon dioxide released for every ton of clinker produced.

“Understanding how calcium carbonate crystallization dynamics bind concrete together and contribute to its long-term durability could provide new insights into the long-term mineralogical evolution and natural carbonation of lime-based binders,” said Monteiro. “This could potentially inform new strategies for durable low-clinker systems.”

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