Walk through Rome today and you see concrete structures — the Pantheon, aqueducts, market halls — that have stood for nearly two millennia. Modern concrete, by contrast, typically begins to crumble within a century. For decades, researchers attributed Roman concrete's longevity to the pozzolanic reaction, in which volcanic ash, lime and water bind into a durable matrix. But a new study reveals that carbonation plays a larger role than previously understood.
A team led by Paulo J. M. Monteiro at UC Berkeley extracted a concrete sample from underneath a toilet seat in a communal latrine at Hadrian's Villa — a spot that had been left entirely undisturbed for 1,900 years. "Nobody restores a latrine," Monteiro noted. The sample was thus a pristine time capsule of Roman construction chemistry. Back in the laboratory, high-powered microscopes, X-ray scans and chemical analysis revealed something unexpected: calcite — a calcium carbonate mineral — was the primary binding agent filling the concrete's pores and cracks.
The process works like this. Atmospheric carbon dioxide slowly seeps into the concrete and reacts with calcium compounds to form calcite. Over centuries, this mineral grows inside every tiny fissure and void, sealing the material from within. The result is a self-healing structure that actually becomes stronger with age rather than degrading. While the pozzolanic reaction remains important, the study shows that carbonation "substantially enhances the durability and potential self-healing properties of concrete."
The implications extend beyond archaeology. Concrete production is responsible for roughly 8 percent of global carbon dioxide emissions, and the United Nations estimates that half of the buildings that will exist by 2050 have not yet been built. Understanding how Roman concrete naturally reinforced itself could inspire modern formulations that last longer, require less maintenance and emit less carbon during production.
Knowledge takeaway: Roman concrete gains strength over time through carbonation, where atmospheric CO₂ reacts with calcium compounds to form calcite that fills cracks; a 1,900-year-old latrine at Hadrian's Villa provided the undisturbed sample that proved this mechanism; the discovery could inform more sustainable, self-healing concrete for modern construction.