The talent of the Roman builders no longer needs to be demonstrated. Vast road networks, unsinkable ports, majestic palaces: two thousand years later, the vestiges of these achievements bear witness to a mastery that leaves one speechless. But who knows that many of these wonders were built in concrete? The Pantheon in Rome, inaugurated in 128 AD, remains even the largest concrete dome ever made to date. As for the ancient aqueducts, they remain essential pieces in the water distribution network of the Italian capital.
How did they do it? To measure the feat, it suffices to know that modern concrete (designed according to the so-called “Portland” recipe) is guaranteed for one hundred years. With a few suspensive clauses, such as earthquakes, which are quite frequent in the south of La Botte. So, what is the secret of the ancient builders? Asterix readers obviously have the answer: those Romans were crazy. A somewhat thin argument for scientists who have been trying to unravel the mystery for decades. In recent years, their quest has taken on a new dimension. Because, on its own, the concrete industry is considered responsible for 7% to 8% of all greenhouse gas emissions.
In an article published on Friday, January 6, in the journal Science Advances, Admir Masic’s team at the Massachusetts Institute of Technology (MIT, USA), accompanied by Italian and Swiss researchers, has just added an essential element to the reconstruction of the ancient recipe. According to these researchers, the exceptional resistance of the material is due to the presence of a well-known element: lime. But not in just any form: quicklime, the most reactive, the most difficult to handle.
On the trail of lime clasts
For a long time, however, the properties of Roman concrete were exclusively attributed to pozzolan, this volcanic rock which Italy abounds with. The American archaeologist Marie Jackson was thus the first to demonstrate that by replacing the sand, used today, by this crushed eruptive stone and by mixing it with volcanic ash and lime, a crystallization reaction gave rise to an ore called “stratlingite”, the lamellae of which might fill any faults in the mortar and add a certain elasticity.
But, for Admir Masic, something was still missing. He and his colleagues focused their research on lime clasts, those little white flakes always present in Roman concrete. And their spectroscopic analysis showed that these inclusions of calcium carbonate had formed at high temperature. “Such a reaction is not compatible with the use of inactivated limehe explains. Conversely, using quicklime in the mixture and then adding water causes a reaction that releases intense heat and causes the clasts to be incorporated into the mortar. And it is they who will serve as a source of calcium for the process of self-repair. »
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