The Romans were not the pioneers in the invention of concrete. But they were the first to employ the material on a large scale. By 200 BC, it was appearing everywhere: on extensive road networks, aqueducts, bathhouses, harbors, and the great sea walls of the Roman Empire. And you can still find many of these structures around – take a look at the Colosseum and the Pantheon in Rome, very well preserved for two millennia old grandparents.
Roman concrete resisted well even in buildings that were in direct contact with sewage flows or were hit by sea waves and earthquakes. In several cases, it has proven to be a more durable material than its modern counterparts. And this is a question that has puzzled researchers for decades.
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Now, engineers at the Massachusetts Institute of Technology (MIT) and Harvard claim to have discovered one of Roman concrete’s secret ingredients – one that could give it a self-repairing superpower.
They investigated the preparation of the material, tested some versions in the laboratory and published their results in the journal Science.
The study team started by analyzing concrete samples from the town of Priverno in central Italy. They came from a 2,000-year-old wall and look like concrete found in other buildings from the Roman Empire. And what was in these samples? Small white fragments, called lime clasts, which have already appeared in previous analyzes – and have already been related to the lack of care in the concrete mix or the possible low quality of the raw material.
But that idea never convinced Admir Masic, an MIT researcher who participated in the latest study. “If the Romans put so much effort into making an excellent building material, following all the detailed recipes that were optimized over many centuries [e espalhadas pelo Império]why would they put so little effort into ensuring a well-blended final product?”
By analyzing these fragments, the team found that they formed at the extreme temperatures expected in mixes that use quicklime (quicklime or calcium oxide) in concrete production – rather than hydrated lime (calcium hydroxide). This white powder, when reacting with the water in the mixture, would start a chemical reaction that produces significant amounts of heat, explain the researchers. And in that scenario, it couldn’t completely dissolve – hence the white lumps in the concrete.
The team even ran experiments to see if quicklime and clasts were behind Roman concrete’s durability—and its apparent ability to repair itself. They made two samples of concrete: one following this possible Roman recipe and another by different modern standards. Then they cracked them open and let a trickle of water run through the samples.
Within two weeks, water no longer passed through the cracks in the Roman concrete – because they were no longer there, unlike the modern specimen. The researchers suggest that lime clasts can dissolve and recrystallize after exposure to water, repairing cracks that, in real life, are created by weathering.
Quicklime, which forms lumps in concrete, would then be the mysterious ingredient in Roman concrete.
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