Roman concrete longevity linked to carbonation process

Researchers from the University of California, Berkeley, have identified carbonation as a critical mechanism behind the exceptional durability of Roman concrete. The findings, published in Science Advances on July 8, suggest that atmospheric carbon dioxide reacts with calcium compounds within the material to form calcite, a mineral that seals cracks and strengthens the concrete over time. This discovery challenges the long-held assumption that the pozzolanic reaction, involving volcanic ash, lime, and water, was the sole primary driver of the material’s longevity.

The study centred on a 1,900-year-old latrine at Hadrian’s Villa, a UNESCO World Heritage site located approximately 17 miles east of Rome. The specific sample was extracted from underneath a toilet seat, a location chosen because communal toilets are rarely restored, allowing the material to remain undisturbed for nearly two millennia. Paulo J. M. Monteiro, a civil engineer at UC Berkeley and study co-author, noted that the undisturbed nature of the site provided an unprecedented opportunity to examine Roman concrete in its original state.

Laboratory analysis, including high-powered microscopy and X-ray scanning, revealed that while volcanic ash was present, calcite was the primary binding agent within the concrete’s pores and fractures. The research indicates that as atmospheric carbon dioxide interacts with calcium compounds, it forms calcium carbonate, which fills small cracks and pores. This process allows ancient structures to strengthen and heal over time, a feature that modern concrete, which typically crumbles within 100 years, lacks.

The work builds on previous research, including a 2023 study by MIT materials scientist Admir Masic, which suggested Roman concrete could self-repair cracks via quicklime deposits. Masic, who was not involved in the new study, stated that the findings strengthen the idea that carbonates are more dynamic in these systems and play a fundamental role rather than a marginal one. The combined insights position carbonation as a significant, long-term contributor to the material's resilience.

With concrete production accounting for approximately 8 per cent of global carbon dioxide emissions, the research aims to inform the development of more sustainable modern infrastructure. According to the United Nations, roughly half of the buildings that will exist by 2050 have not yet been built, underscoring the urgent need for construction materials with a reduced environmental impact. Monteiro stated that unlocking these ancient secrets could help attain sustainable modern infrastructure development.