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Scientists reveal the secret behind the impressive durability of Roman concrete, which has stood the test of time for millennia
The inhabitants of Ancient Rome were true masters of construction and engineering, with works that still fascinate and inspire modern engineers and architects. Among these works, aqueducts stand out for their enduring functionality.
However, one of the great secrets behind these constructions is pozzolanic concrete, a material incredibly resilient that has allowed many of these structures to survive for millennia.
The discovery of the secret of ancient Roman concrete
The Pantheon in Rome, for example, remains standing intact after nearly 2.000 years, making it the largest unreinforced concrete dome in the world. But what exactly made this material so durable? Traditionally, scientists believed that the strength of ancient Roman concrete lay solely in its composition: a mixture of volcanic ash, called pozzolan, and lime, which when mixed with water, formed an incredibly strong compound.
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But in 2023, a team of researchers led by the Massachusetts Institute of Technology (MIT) made a surprising discovery. The study revealed that the composition and mixing method of the ingredients of ancient Roman concrete were different than previously thought.
These scientists observed tiny white particles, known as lime clasts, in the concrete, which had previously been seen as a sign of a bad mix. Admir Masic, a materials scientist at MIT, has always found it odd that such a detail-oriented civilization would make such a basic mistake.
“The idea that these limestone clasts were the result of a lack of quality control has always bothered me,” Masic said in January 2023. “If the Romans had put so much effort into creating an exceptional material, following detailed and optimized recipes for centuries, why would they make such a mistake? There was something more going on here.. "
Masic and his team, along with civil engineer Linda Seymour, also from MIT, studied concrete samples taken from a 2.000-year-old archaeological site in Privernum, Italy. The samples underwent a series of tests, including scanning electron microscopy and X-ray spectroscopy, which allowed for detailed analysis of the lime clasts.
Benefits of Hot Mix and Self-Curing
One of the main questions was about the nature of the lime used. Traditional theory said that pozzolanic concrete was made with slaked lime, obtained by mixing quicklime with water.
However, the team's analyses showed that the lime clasts in the samples studied did not fit this model. Instead, the researchers concluded that the Romans used a process called "hot mixing," which involved mixing quicklime directly with pozzolan and water at high temperatures.
According to Masic, this method brought a series of benefits. “Hot mixing promotes chemical reactions that are impossible to occur with slaked lime, generating compounds associated with high temperatures that reinforce the structure of the material,” he explained. Additionally, this approach accelerated the curing and setting process of the concrete, allowing for faster construction.
Another interesting aspect is the self-healing capacity of ancient Roman concrete. When cracks appear in the structure, they tend to propagate to the lime clasts, which have a larger surface area than other particles.
When water comes into contact with lime, a calcium-rich solution is formed which hardens as it dries, repairing the crack and preventing it from spreading.
This hypothesis has been confirmed in other studies, such as those carried out at the Tomb of Caecilia Metella, where calcite-filled cracks were found. This helps to explain why Roman structures such as sea walls survived so well over the centuries, even when faced with constant waves.
To test their findings, Masic's team reproduced concrete using ancient and modern recipes, with and without quicklime mixed in. In tests, the concrete that included quicklime was able to repair itself within two weeks, while the control concrete failed to recover.
Now, the team is looking to commercialize this formula, offering a more sustainable and durable alternative to modern concrete. “The idea that we can apply this knowledge to extend the lifespan of materials and improve the durability of 3D-printed concrete is incredibly exciting,” Masic said.
The full study was published in the journal Science Advances, bringing a new look at the wisdom of Roman engineering and its application in a modern world that increasingly seeks sustainable solutions.
