The ancient Romans were no slouch when it came to constructioncreating entire cities from concrete, packed with huge temples, arenas and aqueducts that still stand today. Did Pantheon or The Coliseum, these buildings are built to survive. But what was the secret of their power?

According to a study in Scientific progress, the techniques the ancient Romans used to mix their concrete resulted in tiny mineral pieces throughout the material. And although these pieces were previously interpreted as imperfections in the concrete, the study authors say they are actually the secret ingredient that makes Roman structures so strong, allowing them to repair themselves over time.

Much more than solving an age-old architectural mystery, the authors add that their work could contribute to stronger, more sustainable forms of concrete in the future, reducing the overall road tax of the material for our climate.

Cracking of ancient concrete

Scholars have spent many years trying to answer why so many Roman structures, such as buildings, docks, sewers and sea walls, are still standing after thousands of years, while so many modern structures collapse after only 20 or 30 years. And they’ve thrown in a few theories — the most common claim that the establishment of volcanic ash makes the building material of time much safer.

But scientists have never given much thought to the tiny millimeter specks calcium carbonate common throughout Roman concrete. That is, until now.

“Since I first started working with ancient Roman concrete, I’ve always been fascinated by these features,” said Admir Masic, study author and associate professor of civil and environmental engineering at MIT, according to a press release. “They are not found in modern concrete formulations, so why are they present in these ancient materials?”

Read more: If Rome wasn’t built in a day, how long did it take?

Produced in the mixing process, these widespread mineral flecks, sometimes called “limestone debris,” were once seen as a sign of sloppy stirring. However, the study suggests that they actually play a role in maintaining and strengthening the ancient concrete over time, automatically filling any cracks that form on its surface.

“The idea that the presence of these limestone debris is simply attributed to poor quality control has always bothered me,” adds Masich. “If the Romans had put so much effort into making an exceptional building material, following all the detailed recipes that had been optimized over many centuries, why would they put so little effort into ensuring the production of a well-mixed end product?”

Limestone slabs for durable concrete

At its core, concrete is created from the combination of a calcium-containing substance called var and water as well as an array finely and coarsely chopped aggregatesuch as volcanic ash and rubble.

Traditionally, scholars believed that the ancient Romans included slaked lime, a type of lime that had already been added to water to make a sticky, wet paste, in their concrete. But the authors of the study say that this ingredient cannot explain the creation of the clasts that are so completely characteristic of ancient construction.

Instead, after examining samples of 2,000-year-old concrete from the Italian archaeological site of Privernumthe study authors theorized that the Romans added quicklime, a purer form of lime without any water, which caused the concrete debris to form.

And it is these adhesives that make concrete so stable, providing the material with an automatic ability to fix and strengthen.

In fact, the study authors say that whenever cracks form in ancient concrete, they target the calcium clasts, which invariably break due to their inherently brittle structure. But they won’t stay broken forever.

Instead, when water penetrates these cracks, the crushed calcium is then transformed into a calcium-saturated solution that spontaneously hardens and fills the cracks, preventing them from spreading further.

Ultimately, to support their theory that the strength of Roman structures derives from this debris, the authors created and cracked concrete samples with and without quicklime and running water through the cracks. Within two weeks, the weaknesses in the quicklime concrete were completely filled, while the weaknesses in the non-slaked lime concrete were not.

The authors see this discovery as “paving the way for the development of more durable, resilient and sustainable concrete formulations” in the future.

“Whether damage occurred within years of construction or centuries afterward, as long as the limestone remains, these self-healing functions can continue,” the authors concluded in their study.

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