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.
Closer examination of the lime clasts prompted questions, too, about the methods used to manufacture Roman concrete. The process began by extracting the lime required from limestone or other calcium-rich rocks, which were processed to produce quicklime (calcium oxide), a highly reactive caustic powder. It was previously thought that this was then mixed with water to produce a less reactive paste of calcium hydroxide – a process known as ‘slaking’ – and it was this slaked lime that was combined with the pozzolana and coarse aggregate material to make concrete. However, this process does not explain the presence of the lime clasts. Instead, the researchers suggest, the quicklime itself could have been added directly to the concrete mix, either instead of or in addition to slaked lime. This process is called ‘hot mixing’ because of the exothermic reaction it creates, and is therefore more volatile than slaking, but has multiple benefits. In addition to preserving the lime clasts that help the concrete independently heal cracks from weathering and seismic events, the hot mixing process speeds up the curing and setting time of the concrete, since all of the reactions involved are accelerated, so construction can happen faster.
In order to test their ideas, the team created samples of experimental concrete using quicklime, as well as a control sample that lacked it. These concretes were then deliberately cracked, and water was flowed through them to see how quickly they could heal themselves. While the control sample remained broken, the concrete containing quicklime sealed up the crack and stopped water flow completely within two weeks, suggesting that the use of quicklime does indeed produce a more durable building material.
In addition to revolutionising our understanding of Roman construction, it is hoped that this discovery can be used to help develop longer-lasting concrete for modern construction, thus reducing the environmental impact of the industry.
The research has been published in Science Advances (https://doi.org/10.1126/sciadv.add1602).