Concrete was the foundation of the ancient Roman empire. It enabled Rome’s storied architectural revolution as well as the construction of buildings, bridges, and aqueducts, many of which are still used some 2,000 years after their creation.
In 2023, MIT Associate Professor Admir Masic and his collaborators published a paper describing the manufacturing process that gave Roman concrete its longevity: Lime fragments were mixed with volcanic ash and other dry ingredients before the addition of water. Once water is added to this dry mix, heat is produced. As the concrete sets, this “hot-mixing” process traps and preserves the highly reactive lime as small, white, gravel-like features. When cracks form in the concrete, the lime clasts redissolve and fill the cracks, giving the concrete self-healing properties.
There was only one problem: The process Masic’s team described was different from the one described by the famed ancient Roman architect Vitruvius. Vitruvius literally wrote the book on ancient architecture. His highly influential work, “De architectura,” written in the 1st century B.C.E., is the first known book on architectural theory. In it, Vitruvius says that Romans added water to lime to create a paste-like material before mixing it with other ingredients.
“Having a lot of respect for Vitruvius, it was difficult to suggest that his description may be inaccurate,” Masic says. “The writings of Vitruvius played a critical role in stimulating my interest in ancient Roman architecture, and the results from my research contradicted these important historical texts.”
Now, Masic and his collaborators have confirmed that hot-mixing was indeed used by the Romans, a conclusion he reached by studying a newly discovered ancient construction site in Pompeii that was exquisitely preserved by the volcanic eruption of Mount Vesuvius in the year 79 C.E. They also characterized the volcanic ash material the Romans mixed with the lime, finding a surprisingly diverse array of reactive minerals that further added to the concrete’s ability to repair itself many years after these monumental structures were built.
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