Roman concrete is famous for its ability to stand the test of time, from the sea walls battered by waves for millennia to the Pantheon, which was built in the 2nd century AD and remains the largest unreinforced concrete dome in the world. The secret of the ancient material’s durability has intrigued researchers for decades, but a new study led by a team from the Massachusetts Institute of Technology may have found the key.
It has previously been assumed that the strength of Roman concrete came primarily from pozzolana, volcanic ash from the area of Pozzuoli in the Bay of Naples, which is known to have been used in construction across the Roman Empire. However, in the latest study, researchers instead focused on a different component: small, white inclusions referred to as ‘lime clasts’, which are found throughout Roman concrete. In the past, these little features – which originate from lime, another of the key ingredients of Roman concrete – have been dismissed as the result of poor mixing practices or low-quality raw materials, but Admir Masic, who led the study, was not convinced. If the Romans put so much effort into perfecting detailed recipes and shipping the best material hundreds of miles to create the ideal concrete, why would they put so little care into manufacturing the final product?
In order to answer this question, the researchers studied samples of 2,000-year-old concrete from the ancient city walls of Privernum, near Rome, which have a similar composition to other concretes found throughout the Roman Empire. They determined that the lime clasts were extremely rich in calcium, prompting the suggestion that these ‘imperfections’ may in fact have been a deliberate feature, one that contributed to the concrete’s vital self-healing capability. When cracks form due to environmental stresses, they commonly pass through the lime clasts, so when water enters these cracks, it dissolves the lime clasts, thus providing a calcium-rich solution that recrystalises as calcium carbonate, effectively gluing the crack back together. This hypothesis is supported by studies on Roman concrete from other sites, such as the Tomb of Caecilia Metella, just outside Rome, which shows evidence of cracks that have indeed been filled with calcite. Another reaction pathway is for the calcium-rich solution to react with volcanic ash-related ingredients, creating new products that further reinforce the concrete.
