The talent of Roman builders requires no further proof. Vast networks of roads, unsinkable harbors, majestic palaces: two thousand years later, the remains of those accomplishments bear witness to a breathtaking mastery. But who knew that many of these marvels were built with concrete? The Pantheon in Rome, inaugurated in 128 A.D., is still the largest concrete dome ever built. As for the ancient aqueducts, they remain essential parts of the Italian capital's water distribution network.
How did they do it? To appreciate the feat, you only need to consider that modern concrete (using the so-called "Portland recipe") is guaranteed for 100 years; with a few provisos, such as earthquakes, which are quite common in the south of Italy. So, what is the ancient builders' secret? For decades, scientists have been trying to unravel the mystery. In recent years, their quest has acquired a new dimension. The concrete industry alone is considered responsible for 7% to 8% of all greenhouse gas emissions.
In an article published Friday, January 6, in the journal Science Advances, Admir Masic's team at the Massachusetts Institute of Technology (MIT, USA), together with Italian and Swiss researchers, has just added an essential element to the recreation of the ancient formula. According to the researchers, the exceptional strength of the material is due to the presence of a well-known element: lime. But not just in any form: quicklime, the most reactive and the most delicate to handle.
Tracking lime clasts
For a long time, the properties of Roman concrete were attributed exclusively to pozzolan, a volcanic rock abundant in Italy. The American archaeologist Marie Jackson was the first to demonstrate that by replacing sand, which is used today, with that ground eruptive stone and by mixing it with volcanic ash and lime, a crystallization reaction produces a mineral called "stratlingite," whose lamellae could fill potential cracks in the mortar and add a certain elasticity.
For Admir Masic, something was still missing. He and his colleagues focused their research on lime clasts, the small white flakes still present in Roman concrete. Their spectroscopic analysis indicated that those calcium carbonate inclusions were formed at high temperatures. "Such a reaction is incompatible with the use of inactivated lime," he said. "Conversely, the use of quicklime in the mixture and the subsequent addition of water causes a reaction that generates intense heat and leads to the incorporation of clasts in the mortar. And they are the ones that are going to act as a source of calcium for the self-repair process."
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