Aerial view taken in May 2020 of the Pantheon in Rome, which includes the largest unreinforced concrete cupola in the worldFilippo MONTEFORTE
How did Roman constructions, for example the aqueducts or the Pantheon in Rome, stand the test of time so well? This question has long plagued experts.
American and European researchers believe they have finally discovered the secret of longevity of these architectural marvels, approximately 2,000 years old: a concrete capable of repairing itself.
While some modern buildings fall into disrepair after only a few decades, these scientists hope that their discovery can help reduce the environmental and climate impact of concrete production, which generates significant greenhouse gas emissions.
Until now, the strength of Roman concrete was attributed to one ingredient: volcanic ash from the Bay of Naples region of Italy, which was sent all over the Roman Empire for use in construction.
But this time the researchers focused their attention on the presence of another characteristic: tiny shiny white pieces, coming from lime, another ingredient used for the design of concrete.
“Since I started working on Roman concrete, I have always been fascinated” by the presence of these pieces, said in a press release Admir Masic, co-author of this study published in the journal Science Advances and professor at the prestigious Massachusetts Institute of Technology (MIT), in the United States. “They are not present in modern concrete, so why were they in the old?”
Experts previously thought that these tiny pieces were the result of poor mixing of the mixture, or poor quality raw materials.
But by examining, using advanced imaging techniques, the concrete of a wall of the city of Privernum in Italy, the researchers discovered that these small white pieces were in fact calcium carbonate, formed at very high temperatures.
They concluded that the lime was not (or not only) incorporated by being mixed with water, as previously thought, but in the form of quicklime.
According to the researchers, it is this “hot mix” that gives this concrete its astonishing solidity.
Indeed, when cracks appear, rainwater coming into contact with the concrete produces a solution saturated with calcium, which then recrystallizes into calcium carbonate, thus making it possible to fill the cracks.
To verify this hypothesis, the team of scientists made samples of concrete using the same process, which they then deliberately cracked and on which they poured water. Result: After two weeks, the concrete was completely repaired. Another sample produced without quicklime remained cracked.
In the future, the researchers want to try to market this concrete with the modified composition.