Ancient Roman Concrete Recipes Surprisingly Different, Study Finds
The secrets behind the incredible strength and durability of ancient Roman concrete have been uncovered by an international team of researchers led by the Massachusetts Institute of Technology (MIT). By studying samples of 2,000-year-old Roman concrete from the archaeological site of Privernum in Italy, the team discovered that the ingredients and mixing techniques were different than previously believed.
The key ingredient in Roman concrete is pozzolana, a mix of volcanic ash and lime. It was thought that the lime used was slaked lime, made by mixing quicklime with water. However, the team’s analysis revealed the presence of small lime chunks that did not align with this method. Instead, the researchers believe that the Romans used a process called “hot mixing,” which involved combining quicklime directly with the pozzolana and water at extremely high temperatures. This method allowed for the formation of compounds that would not have been possible with slaked lime alone and significantly reduced curing and setting times.
“The benefits of hot mixing are twofold,” explained materials scientist Admir Masic of MIT. “First, it allows for the formation of high-temperature-associated compounds that enhance the concrete’s properties. Second, it speeds up the construction process due to accelerated reactions.”
One major advantage of Roman concrete’s unique composition is its self-healing abilities. When cracks form in the concrete, they preferentially travel to the lime clasts – the white chunks – which have a higher surface area than other particles in the concrete. When water enters the cracks, it reacts with the lime to form a solution rich in calcium that dries and hardens as calcium carbonate. This process effectively glues the crack back together, preventing further spreading. The team observed this self-healing ability in concrete samples from the Tomb of Caecilia Metella, where cracks were filled with calcite.
To test their findings, the researchers recreated ancient and modern recipes of pozzolanic concrete using quicklime. In crack tests, the Roman concrete with quicklime fully healed within two weeks, while the control concrete without quicklime remained cracked. These results confirmed the importance of the lime clasts in the self-healing process.
The implications of this research extend beyond understanding ancient Roman engineering techniques. The team is now working on commercializing their concrete formulation, which they believe could be a more environmentally friendly alternative to current concretes. With its improved durability and self-healing abilities, the concrete could potentially extend the service life of structures and enhance the durability of 3D-printed concrete formulations.
“It’s exciting to think about how these more durable concrete formulations could revolutionize the construction industry,” said Masic.
The research, published in Science Advances, sheds new light on the remarkable engineering achievements of the ancient Romans and opens up possibilities for innovative construction materials of the future.
Note: This article was originally published in January 2023.