The ancient Romans were masters of construction and engineering, perhaps most notably represented by aqueducts. These still-functioning marvels rely on a unique building material: pozzolanic concrete, an amazingly durable concrete that gave Roman structures their incredible strength.
Even today, one of their buildings – the Pantheon, 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 town of Pozzuoli, where large deposits of it can be found – and Lime. When mixed with water, the two materials can react to produce strong concrete.
But, as it turns out, that's not the whole story. In 2023, an international team of researchers led by the Massachusetts Institute of Technology (MIT) found that not only were the materials slightly different than we thought, but the techniques used to mix them were different too.
The smoking guns were small white pieces 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 Masek of MIT.
“The idea that the presence of these limestone blocks is simply due to poor quality control has always bothered me.” Macek said Back in January 2023.
“If the Romans put so much effort into making a premium building material, following all the detailed recipes that were refined over many centuries, why did they put so little effort into ensuring a well-mixed final product was produced? There has to be more to this story.”
Macek and the team, led by MIT civil engineer Linda Seymour, studied 2,000-year-old Roman concrete samples from the archaeological site of Perevernum 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 on my mind was the nature of the lime used. The standard understanding of pozzolanic concrete is that it is used Hydrated lime. First, limestone is heated at high temperatures to produce a highly reactive caustic powder called limestone 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 hydrated lime that the ancient Romans mixed with pozzolana.
Based on the team's analysis, the lime blocks found in their samples were not consistent with this method. Alternatively, Roman concrete may have been made by mixing quicklime directly with pozzolana and water at extremely high temperatures, alone or in addition to hydrated lime, a process the team calls “hot mixing” that produces lime crumbs.
“The benefits of hot mixing are two-fold.” Macek said.
“First, when aggregate concrete is heated to high temperatures, it allows chemistries to take place that are not possible if you use only hydrated lime, creating high-temperature bonded compounds that cannot form otherwise. Second, this increased temperature dramatically reduces of curing and hardening times since all reactions are accelerated, allowing for much faster construction.”
It has another benefit: lime crumb gives concrete a remarkable ability to self-heal.
When cracks form in concrete, they preferentially move into lime blocks, 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 like calcium carbonate, sticking the crack back together and preventing it from spreading further.
this It has been noticed In concrete from another 2,000-year-old site, the tomb of Cecilia Metella, where cracks in the concrete were 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 constant pounding of the oceans.
So, the team tested their findings by making pozzolanic concrete from ancient and modern recipes using quicklime. They also made control concrete without quicklime and performed cracking tests. Sure enough, the cracked limestone concrete was fully cured within two weeks, but the control concrete remained cracked.
The team is now working to market their concrete as a more environmentally friendly alternative to current concrete.
“It is exciting to think about how these more durable concrete compositions can increase not only the service life of these materials, but also how they can improve the durability of 3D printed concrete compositions.” Macek said.
The research was published in Advancement of science.
A version of this article was first published in January 2023.
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