The ancient Romans were renowned for their engineering prowess, particularly evident in structures like the Pantheon, which still stands today after nearly 2,000 years. More amazing is the durability in hostile environments such as seawater
The secret to their durable concrete, known as pozzolanic concrete, was thought to lie in a mix of volcanic ash (pozzolana) and lime. However, recent research led by MIT in 2023 revealed surprising new insights.
The conventional belief was that Romans used slaked lime, produced by heating limestone to create quicklime and then mixing it with water. The study, examining 2,000-year-old concrete samples from Privernum in Italy, challenges this understanding. The team discovered small lime clasts in the concrete, suggesting a process termed "hot mixing." This involved combining quicklime directly with pozzolana and water at high temperatures, accelerating reactions and reducing curing times, allowing for faster construction.
A large-area elemental map (Calcium: red, Silicon: blue, Aluminum: green) of a 2 cm fragment of ancient Roman concrete (right) collected from the archaeological site of Privernum, Italy (left). A calcium-rich lime clast (in red), which is responsible for the unique self-healing properties in this ancient material, is clearly visible in the lower region of the image. (Credits: MIT researchers)
One remarkable benefit of this hot mixing process is the concrete's self-healing ability. Lime clasts, with a higher surface area, attract cracks. When water enters, it reacts with the lime, forming a solution rich in calcium that solidifies into calcium carbonate, effectively repairing and preventing further crack propagation. This self-healing mechanism has been observed in Roman concrete from sites like the Tomb of Caecilia Metella, explaining its longevity despite exposure to the elements.
To validate their findings, the team recreated ancient and modern pozzolanic concrete using quicklime and conducted crack tests. The results were compelling—cracks in the quicklime concrete healed within two weeks, highlighting its potential as a more durable alternative to modern concretes.
Excitingly, researchers are exploring the commercialization of this ancient Roman concrete as an environmentally friendly option, aiming to extend the lifespan of structures and enhance the durability of 3D-printed concrete formulations. The study not only sheds light on the sophistication of ancient Roman engineering but also holds promise for sustainable construction practices in the future.
References
Michelle Starr's excellent report in Science Alert
Hot mixing: Mechanistic insights into the durability of ancient Roman concrete
https://news.mit.edu/2023/roman-concrete-durability-lime-casts-0106
