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Scientists reveal how Roman buildings have survived so long

(CNN) — The majestic structures of ancient Rome have survived for millennia, testament to the skill of Roman engineers, who perfected the use of concrete.

But how did its construction materials help keep colossal buildings like the Pantheon (which has the largest unreinforced dome in the world) and the Colosseum standing for more than 2,000 years?

In many cases, Roman concrete has proven to be more durable than its modern equivalent, which can deteriorate in a matter of decades. Now, the scientists responsible for a new study claim to have discovered the mysterious ingredient that allowed the Romans to make their building material so durable and erect elaborate structures in difficult places like docks, sewers and seismic zones.

The study team, made up of researchers from the United States, Italy and Switzerland, analyzed 2,000-year-old concrete samples taken from a wall at the Privernum archaeological site in central Italy, and similar in composition to other samples. of concrete found throughout the Roman Empire.

The researchers found that the white chunks of the concrete, called lime clasts, gave it the ability to heal cracks that formed over time. White bits had previously been overlooked as evidence of sloppy mixing or poor quality raw material.

“It was very hard for me to believe that the ancient Roman (engineers) didn’t do a good job, because they put a lot of effort into choosing and processing the materials,” says Admir Masic, study author and associate professor of civil and environmental engineering. at the Massachusetts Institute of Technology.

“Scholars wrote precise recipes and imposed them on works (from all over the Roman Empire),” added Masic.

The new finding could help make today’s concrete manufacturing more sustainable, potentially revolutionizing society just as the Romans did in their day.

“Concrete allowed the Romans an architectural revolution,” explains Masic. “The Romans were able to create and turn cities into something extraordinary and beautiful to live in. And that revolution basically completely changed the way humans live.”

ROMAN CONCRETE

Tourists visit the Colosseum in Rome in June 2019. Credit: EyesWideOpen/Getty Images

Lime clasts and the durability of concrete

Concrete is essentially artificial stone or rock, formed by mixing cement, a binding agent typically made of limestone, water, fine aggregate (sand or finely crushed rock), and coarse aggregate (gravel or crushed rock).

Roman texts had suggested the use of slaked lime (when lime is first combined with water before mixing) as a binder, and so scholars had assumed this was how Roman concrete was made, Masic said.

Upon further study, the researchers concluded that the lime clasts were due to the use of quicklime (calcium oxide) — the most reactive and dangerous dry form of limestone — in mixing the concrete, in place of use of dead lime, or in addition to it.

Further analysis of the concrete demonstrated that the lime clasts formed at the extreme temperatures predicted by the use of quicklime, and that “hot mix” was key to the durable nature of the concrete.

“The benefits of hot mixing are twofold,” Masic says in a press release. “First, when the concrete as a whole is heated to high temperatures, it allows for chemistry that would not be possible if only dead lime was used, producing high-temperature associated compounds that would not otherwise form.

Second, this increase in temperature significantly reduces curing and setting times, as all reactions are speeded up, allowing for much faster construction.”

To investigate whether lime clasts were responsible for the apparent ability of Roman concrete to repair itself, the team conducted an experiment.

They made two concrete samples, one following Roman formulas and the other according to modern standards, and deliberately cracked them. After two weeks, the water could not flow through the concrete made with the Roman recipe, while it passed without problems through the piece of concrete prepared without quicklime.

Their findings suggest that lime clasts can dissolve in cracks and recrystallise upon exposure to water, healing weathered cracks before they spread. According to the researchers, this self-healing potential could pave the way towards the production of a more durable and therefore more sustainable modern concrete. This would reduce the carbon footprint of concrete, which accounts for up to 8% of global greenhouse gas emissions, according to the study.

For many years, researchers thought that volcanic ash from the Pozzuoli area, in the Bay of Naples, was what made Roman concrete so strong. This type of ash was transported throughout the vast Roman Empire for use in construction, and was described as a key ingredient in concrete in the accounts of architects and historians of the time.

According to Masic, both components are important, but lime has been overlooked in the past.

The research was published in the academic journal Science Advances.

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