Mineralized carbonates contribute to the millennial durability of Roman concrete

Roman concrete structures have remained serviceable for nearly two millennia and are widely regarded as outstanding examples of durable ancient engineering. Existing literature attributes Roman concrete longevity to the pozzolanic reaction that occurs between reactive volcanic ashes and lime. While the pozzolanic reaction is of fundamental importance, we argue that carbonation over a long period of time also substantially enhances the durability and potential self-healing properties of concrete. To validate this claim, a comprehensive analysis of the concrete used in a latrine included in the Canopus western substructures, in Hadrian's Villa (Roman imperial residence, Tivoli, Italy, 2nd century AD), is performed using multiscale spectroscopic and tomographic approaches to uncover the mechanisms behind the resilience of these enduring structures. Microstructural evidence reveals that volcanic lava composed of leucite, analcime, and ferrian diopside was combined by the Romans with lime at a water-to-binder ratio of ~0.4 to 0.45 to mix the concrete. Calcite cementation in pore spaces and fractures served as the primary binding mineral phase, while the formation of a relatively minor amount of calcium-aluminum-silicate-hydrate precipitated around and between lava fragments to further enhance the integrity of concrete. Conversely, unhydrated calcium oxide reacted with atmospheric carbon dioxide and moisture to form volumetrically dominant calcite cements, creating the primary driver to reinforce structural strength and occlude porosity. This radiaxial fibrous calcite has the potential to mitigate environmental and mechanical stresses in modern concrete infrastructure and advance the development of sustainable and resilient construction materials in the future.