We finally know why ancient Roman concrete could last thousands of years: ScienceAlert

The ancient Romans were masters of construction and engineering, perhaps best known for their aqueducts. And these still-functional marvels rely on a unique construction material: pozzolana 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 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 a significant amount of it can be found – and lime. When mixed with water, the two materials can react to produce strong concrete.

But that, as it turns out, is not the whole story. In 2023, an international team of researchers led by the Massachusetts Institute of Technology (MIT) discovered that not only were the materials slightly different than we might have thought, but the techniques used to mix them were also different.

The smoking guns were small, white pieces of chalk found in what otherwise appears to be well-mixed concrete. The presence of these chunks was previously attributed to poor mixing of materials, but that was not true for MIT materials scientist Admir Masic.

“The idea that the presence of these limes was simply attributed to low quality control has always bothered me,” Masic said in January 2023.

“If the Romans put so much effort into creating an excellent building material, following all the detailed recipes optimized over many centuries, why would they put so little effort into ensuring the production of a well-mixed final product ? ? There must be more to this story.’

Masic and the team, led by MIT civil engineer Linda Seymour, carefully studied 2,000-year-old samples of 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 gain a better understanding of the lime clasts.

One of the questions in the back of my mind was the nature of the lime used. The standard understanding of pozzolana concrete is that it uses slaked lime. First, limestone is heated at 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, less caustic paste. According to the theory, it was this slaked lime that the ancient Romans mixed with the pozzolana.

Based on the team’s analysis, the limestone types in their samples are not consistent with this method. Instead, Roman concrete was likely made by mixing the quicklime directly with the pozzolana and water at extremely high temperatures, on its own or in addition to slaked lime, a process the team calls “hot mixing” that results in the lime clasts .

“The benefits of hot mixing are twofold,” Masic said.

“First, when the entire concrete is heated to high temperatures, it allows chemicals to form that are not possible if you use hydrated lime alone, producing high-temperature-associated compounds that would not otherwise form. Second, this elevated temperature reduces the setting and hardening times significantly because all reactions are accelerated, allowing for much faster construction.”

And it has another advantage: the types of lime give the concrete a remarkable self-healing capacity.

When cracks develop in the concrete, they preferentially move to the lime clasts, which have a larger 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. This will glue the crack back together and prevent it from spreading further.

This has been observed 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 seawalls built 2,000 years ago has remained intact for millennia, despite continued ocean destruction.

So the team tested their findings by making pozzolana concrete based on ancient and modern recipes using quicklime. They also made a control concrete without quicklime and conducted crack tests. Sure enough, the cracked quicklime concrete was completely healed within two weeks, but the control concrete remained cracked.

The team is now working on commercializing their concrete as a more environmentally friendly alternative to current concrete.

“It is exciting to think about how these more sustainable concrete formulations could not only extend the life of these materials, but also how it could improve the durability of 3D printed concrete formulations,” said Masic.

The research was published in Scientific progress.

A version of this article was first published in January 2023.