The ancient Romans were master builders and engineers, perhaps the most famous of which is a representation of the still-functioning aqueducts. And those architectural marvels are based on a unique building material: pozzolan concrete, an amazingly durable material that gave Roman structures their incredible strength.
Even today, one of their structures – the Pantheon, which is still intact and nearly 2,000 years old – holds the record for the world’s largest unreinforced concrete dome.
The properties of this concrete are generally attributed to its ingredients: pozzolana, a mixture of volcanic ash – named after the Italian city of Pozzuoli, where large deposits of it can be found – and gear. When mixed with water, the two materials can react to produce strong concrete.
But that, as it turns out, isn’t the whole story. An international team of researchers led by the Massachusetts Institute of Technology (MIT) found that not only were the materials a little different than we might think, but the techniques used to mix them were also different.
The smoke guns were small white bits of lime that could be found in what appeared to be well-mixed concrete. The presence of these pieces had previously been attributed to poor mixing or materials, but that didn’t make sense to materials scientist Admir Masic of MIT.
“The notion that the presence of these limestone lumps was simply attributable to poor quality control has always bothered me,” he says. Masic said in a statement released in January 2023.
“If the Romans put so much effort into making a premium building material, following all the detailed recipes that were perfected over many centuries, why take so little effort to ensure a well-mixed final product? There must be more to this story.”
Musk and the team, led by MIT civil engineer Linda Seymour, carefully studied 2,000-year-old samples of Roman concrete from the archaeological site of Privernum in Italy. These samples were subjected to large-area scanning electron microscopy, energy-dispersive X-ray spectroscopy, powder X-ray diffraction, and confocal Raman imaging to gain a better understanding of the calcareous masses.
One of the questions to consider was the nature of the lime used. The standard understanding of pozzolanic concrete is that it is used slaked lime. First, limestone is heated at high temperatures to produce a highly reactive caustic powder called Quicklimeor calcium oxide.
Mixing quicklime with water produces slaked lime, or calcium hydroxide: a less reactive and less caustic paste. According to the theory, it was this slaked lime that the ancient Romans mixed with pozzolana.
Based on the team’s analysis, the calcareous fractions in their samples are not compatible with this method. Alternatively, Roman concrete may have been made by mixing quicklime directly with pozzolana and water at very high temperatures, alone or in addition to slaked lime, a process the team calls “hot mixing” that leads to the formation of lime.
“The benefits of hot mixing are twofold,” he says. Musk said.
“First, when aggregate concrete is heated to high temperatures, it allows chemistry that is not possible if only hydrated lime was used, resulting in high-temperature bound compounds that would not otherwise form. Second, this increased temperature greatly reduces curing and setting times as All reactions are speeded up, allowing for much faster construction.”
Another benefit: Lime has a remarkable ability to self-heal.
When cracks form in concrete, they preferentially pass into calcareous blocks, which have a higher surface area than other particles in the matrix. When water gets into the crack, it reacts with the lime to form a calcium-rich solution that dries and hardens as calcium carbonate, gluing the cracks together and preventing them from spreading further.
this It has been noticed In concrete from another 2,000-year-old site, the tomb of Caecilia Metella, where cracks in the concrete have been filled with calcite. It could also explain why Roman concrete from sea walls built 2,000 years ago has remained intact for thousands of years despite the ocean’s relentless beating.
So, the team tested their findings by making pozzolan concrete from both ancient and modern recipes using quicklime. They also made control concrete without quicklime and conducted crack tests. Sure enough, the cracked lime concrete was completely cured within two weeks, but the concrete remained cracked.
The team is now working to market their concrete as a more environmentally friendly alternative to existing concrete.
“It is exciting to consider how these more durable concrete structures can increase not only the service life of these materials but also how they can improve the durability of 3D printed concrete structures,” Musk said.
Research published in Science advances.
A version of this article was first published in January 2023.