The dome of the Pantheon. (Ed Freeman/Stone/Getty Images)
The ancient Romans were masters of building and engineering, perhaps most famously represented by aqueducts. And those marvels that are still functional rely on a unique construction material: pozzolanic concrete, a spectacularly durable concrete that gave Roman structures their incredible strength.
Even today, one of their structures – the infamous 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 have generally been attributed to its ingredients: pozzolana, a mixture of volcanic ash – named after the Italian city of Pozzuoli, where a significant deposit 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. An international team of researchers led by the Massachusetts Institute of Technology (MIT) found that not only are the materials slightly different from what we might have thought, but the techniques used to mix them were also different.
The smoking guns were small, white pieces of lime that could be found in what otherwise appeared to be well-mixed concrete. The presence of these pieces had previously been attributed to poor mixing or materials, but this did not make sense to materials scientist Admir Masic of MIT.
“The idea that the presence of these lime clasts was simply attributed to low quality control has always bothered me,” says Masic.
“If the Romans put so much effort into making an excellent construction material, following all the detailed recipes that had been optimized over many centuries, why would they put so much 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 Roman concrete samples from the archaeological site of Privernum 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 clasts of lime.
One of the questions on her mind was the nature of the lime used. The standard understanding of pozzolanic concrete is that it uses ground lime. First, the 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 pozzolana.
Based on the team’s analysis, the lime clasts in their samples are not consistent with this method. Rather, Roman concrete was probably made by quickly mixing lime directly with pozzolana and water at extremely high temperatures, alone or in combination with slaked lime, a process the team calls “hot mixing” that results in – lime clasts.
“The benefits of hot mixing are twofold,” says Masic.
“First, when the general concrete is heated to high temperatures, it allows chemicals that are not possible if you only used slaked lime, which produces compounds associated with high temperature that would not otherwise form. Second, this increased temperature reduces b “significantly curing and setting. times as all reactions are accelerated, allowing much faster construction.”
And it has another benefit: The lime clasts give the concrete remarkable self-healing abilities.
When cracks form in concrete, they preferentially travel to the lime clasts, which have a higher surface area than other particles in the matrix. When water gets into the cracks, it reacts with the lime to form a calcium-rich solution that dries and hardens as calcium carbonate, sealing the cracks back together and preventing them from spreading further.
This was observed in the concrete from another 2,000-year-old site, the Tomb of Caecilia Metella, where cracks in the concrete were filled with calcite. It may also explain why Roman concrete from sea walls built 2,000 years ago has survived intact for millennia despite the constant battering of the ocean.
Therefore, 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. Sure enough, the cracked quicklime concrete was fully healed within two weeks, but the control concrete remained cracked.
The team is now working on marketing their concrete as a more environmentally friendly alternative to current concrete.
“It is exciting to think about how these more durable concrete formulations can expand not only the service life of these materials, but also how you can improve the durability of stamped concrete formulations 3D,” says Masic.
The research was published in Science Advances.
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