by The ancient Romans were masters of construction and engineering, perhaps the most famous being aqueducts. And these still-functional wonders rely on a unique building material: pozzolanic concrete, a spectacularly durable concrete that gave Roman structures their incredible strength.
Even today, one of their structures – the Pantheon, still intact and almost 2,000 years old – holds the record for the largest unreinforced concrete dome in the world.
The properties of this concrete have generally been attributed to its ingredients: pozzolan, a mixture of volcanic ash – named after the Italian town of Pozzuoli, where there is a large deposit – and lime. When mixed with water, the two materials can react to produce strong concrete.
But it turns out that’s not the whole story. In 2023, an international team of researchers led by the Massachusetts Institute of Technology (MIT) discovered that not only are the materials slightly different than one might have thought, but the techniques used to mix them were also different.
The smoking evidence was small white chunks of lime that can be found in what otherwise appears to be well-mixed concrete. The presence of these chunks had previously been attributed to poor mixing or mismixing of the materials, but that made no sense to materials scientist Admir Masic of MIT.
“The idea that the presence of these limestone fragments was simply attributed to poor quality control has always bothered me,” Masic said in January 2023.
“If the Romans put so much effort into making an exceptional building material, following all the detailed recipes optimized over many centuries, why would they put so little effort into ensuring the production of a final product well mixed? There must be more to this story.
Masic and the team, led by MIT civil engineer Linda Seymour, carefully studied samples of 2,000-year-old Roman concrete from the Privernum archaeological site in Italy. These samples were subjected to large-area scanning electron microscopy and energy-dispersive X-ray spectroscopy, powder X-ray diffraction, and confocal Raman imaging to better understand the lime clasts.
One of the questions in mind was the nature of the lime used. The standard understanding of pozzolanic concrete is that it uses slaked lime. First, limestone is heated to high temperatures to produce a highly reactive caustic powder called quicklime or calcium oxide.
Mixing quicklime with water produces slaked lime, or calcium hydroxide: a slightly less reactive and less caustic paste. According to theory, it was this slaked lime that the ancient Romans mixed with pozzolan.
According to the team’s analysis, the lime clasts in their samples do not match this method. Roman concrete was likely made by mixing quicklime directly with pozzolan and water at extremely high temperatures, either alone or in addition to slaked lime, a process the team calls “hot mixing” which results in lime clasts.
“The benefits of hot mixing are twofold,” Masic said.
“First, when the whole concrete is heated to high temperatures, it allows for chemicals that would not be possible if you used only slaked lime, producing high temperature associated compounds that would not otherwise form. Second , this increased temperature significantly reduces curing and setting times since all reactions are accelerated, allowing for much faster construction.
And this has another advantage: the lime clasts give the concrete remarkable self-healing capabilities.
When cracks form in concrete, they propagate preferentially toward lime clasts, which have a higher surface area than other particles in the matrix. When water enters the crack, it reacts with the lime to form a calcium-rich solution that dries and hardens as calcium carbonate, sealing the crack and preventing it from spreading further.
This was observed in the concrete of another 2,000-year-old site, the tomb of Caecilia Metella, where the cracks in the concrete were filled with calcite. It could also explain why the Roman concrete of the sea walls built 2,000 years ago has survived intact for millennia despite the constant battering of the ocean.
So the team tested their findings by making pozzolanic concrete from ancient and modern recipes using quicklime. They also made a control concrete without quicklime and carried out cracking tests. Indeed, the cracked quicklime concrete was completely healed in two weeks, but the control concrete remained cracked.
The team is currently working to market its concrete as a more environmentally friendly alternative to current concretes.
“It’s exciting to think about how these more durable concrete formulations could extend not only the lifespan of these materials, but also improve the durability of 3D printed concrete formulations,” Masic said.
The research was published in Scientific advances.
A version of this article was first published in January 2023.
