The ancient Romans were masters of architecture and engineering, perhaps most famous for their aqueducts. And these functional wonders rely on a unique building material: pozzolanic concrete, an incredibly durable concrete that gave Roman architecture incredible strength.
Today, one of its structures, the Pantheon, is almost 2,000 years old and intact and holds the record for the world’s largest unreinforced concrete dome.
This property of concrete is generally attributed to its composition. Pozzolana is a mixture of volcanic ash and is named after the Italian city of Pozzuoli, which has large deposits of volcanic ash. lime. When mixed with water, the two materials react to create strong concrete.
But as it turns out, there’s more to it than that. In 2023, an international research team led by the Massachusetts Institute of Technology (MIT) discovered that not only are the materials slightly different from what we think, but the techniques used to mix them are also different.
The smoke bomb was a small lump of white lime contained within what appeared to be well-mixed concrete. The presence of these clumps was previously thought to be due to improper mixing or materials, but that didn’t make sense to MIT materials scientist Admir Masic.
“The idea that the presence of these lumps of lime was simply due to poor quality control has always bothered me.” Majik said Fast forward to January 2023.
“If the Romans went to great lengths to create superior building materials, following all the detailed recipes that have been optimized over centuries, why did they ensure a well-mixed final product?” Didn’t it take a lot of effort to produce it?” There’s probably more to this story. ”
Macic and a team led by MIT civil engineer Linda Seymour carefully studied samples of 2,000-year-old Roman concrete taken from the Privernum ruins 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 better understand the limestone mass.
One of the questions in mind was the nature of the lime used. The standard understanding of pozzolanic concrete is slaked lime. First, limestone is heated to high temperatures to produce a highly reactive caustic powder called . quicklimeor calcium oxide.
When quicklime is mixed with water, it produces slaked lime, or calcium hydroxide, which is a slightly less reactive and less caustic paste. According to the theory, it was this slaked lime that the ancient Romans mixed with pozzolana.
According to the research team’s analysis, the limestone chunks in the samples are not consistent with this method. Rather, Roman concrete was probably made by mixing quicklime, either alone or in addition to slaked lime, directly with pozzolana and water at extremely high temperatures, a process the researchers call “hot mixing.” In the process, lumps of lime are formed.
“Hot mixing has two benefits.” Majik said.
“First, when the entire concrete is heated to high temperatures, chemical reactions occur that would not be possible if slaked lime were used alone, creating high-temperature-related compounds that would not otherwise form. Second, this The increased temperature significantly reduces curing and condensation, as all reactions are accelerated, allowing for much faster construction.”
And this has another advantage as well. A lump of lime gives the concrete remarkable self-healing abilities.
When a crack forms in concrete, the crack propagates preferentially to the limestone mass, which has 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, re-bonding the crack and preventing it from spreading further.
this observed Concrete from the tomb of Caecilia Metella, another 2,000-year-old site, with cracks in the concrete filled in with calcite. It may also explain why the Roman concrete of the sea wall, built 2,000 years ago, has remained intact for thousands of years despite the constant impact of the ocean.
So the team created pozzolanic concrete from ancient and modern recipes using quicklime and tested the results. A control concrete containing no quicklime was also prepared and tested for cracking. Sure enough, the cracked quicklime concrete was completely repaired within two weeks, but the control concrete remained cracked.
The team is currently working on commercializing the concrete as a more environmentally friendly alternative to current concrete.
“Not only how can these more durable concrete mixes extend the service life of these materials, but also how can we improve the durability of 3D printed concrete mixes? It’s really fun to think about it.” Majik said.
This research scientific progress.
A version of this article was first published in January 2023.