How did Roman buildings like the aqueducts or the Pantheon in Rome last so long? This question has been worrying experts for a long time.

American and European researchers believe they have finally discovered the secret to the longevity of these 2,000-year-old architectural marvels: concrete that can be regenerated, AFP reports.

While some modern buildings collapse after only a few decades, the researchers hope that their discovery can help reduce the environmental and climate impacts of concrete production, which produces significant greenhouse gas emissions.

Until now, the strength of Roman concrete was attributed to a single ingredient: volcanic ash from the Bay of Naples region of Italy, which was transported throughout the Roman Empire for use in construction.

This time, however, the researchers focused their attention on the presence of another feature: small, shiny white pieces of lime, another ingredient used in concrete design.

“Ever since I started working on Roman concrete, I’ve always been fascinated” by the presence of these pieces, Admir Masic, co-author of the study published in the journal Science Advances and a professor at the prestigious Massachusetts Institute of Technology (MIT) in the US, said in a statement. “They are not in modern concrete, so why were they present in the old one?”.

Until now, experts believed that small pieces were the result of a bad mixture or low-quality raw materials.

But when researchers used advanced imaging techniques to examine the concrete of a city wall in Privernum, Italy, they discovered that the small white pieces were actually calcium carbonate formed at very high temperatures.

They concluded that the lime was incorporated (or not only) by mixing with water, as previously thought, but in the form of quicklime.

According to the researchers, it is this “hot mix” that gives this concrete its amazing strength.

Indeed, when cracks appear, rainwater in contact with concrete forms a saturated solution of calcium, which then recrystallizes into calcium carbonate, thus filling the cracks.

To test this hypothesis, a team of scientists produced concrete samples using the same process, which they intentionally fractured and poured water over. The result: after two weeks, the concrete was completely repaired. Another sample made without quicklime remained cracked.

In the future, the researchers want to try to commercialize this concrete with a modified composition.