Why does Roman concrete last so much longer than ours?

Roman concrete has mostly stood the test of time. The Pantheon for example was dedicated in 128 CE and has the world’s largest unreinforced concrete dome. Today, it’s still intact.

Even some ancient Roman aqueducts still deliver water to Rome.

On the other hand – In your town or city you probably have at least one piece of brutalist architecture. Big in the 50s and 60s, these now controversial concrete structures were considered utilitarian and long lasting. Yet today, without restoration, some of these reinforced concrete buildings have begun to crumble.

Researchers from MIT think they have the answer to Roman concrete’s millennia long success – white specks called lime clasts.

Lime clasts are small white chunks which originate from lime, found ubiquitously through Roman concrete but not in modern day concrete.

“Ever since I first began working with ancient Roman concrete, I’ve always been fascinated by these features,” says Professor Admir Masic, an MIT civil and environmental engineer.

“These are not found in modern concrete formulations, so why are they present in these ancient materials?”

We understand quite a bit about how the Romans used to make concrete from ancient scholars Vitruvius and Pliny, who wrote about the strict specifications for the materials.

However, Masic wasn’t sure which type of lime the Romans would have been using.

It was assumed that when lime was incorporated into Roman concrete, it was first slacked –combined with water to form a highly reactive paste-like material. But this process alone could not account for the presence of the lime clasts.

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“Was it possible that the Romans might have actually directly used lime in its more reactive form, known as quicklime?” he says.

Researchers used spectroscopic examination to provide clues that the calcium carbonate had been formed at extreme temperatures. This would be expected from the exothermic reaction produced by using quicklime instead of, or in addition to, the slaked lime in the mixture.

Hot mixing, the team has now concluded, was actually the key to the super-durable nature.

“The benefits of hot mixing are twofold,” Masic says.

“First, when the overall concrete is heated to high temperatures, it allows chemistries that are not possible if you only used slaked lime, producing high-temperature-associated compounds that would not otherwise form. Second, this increased temperature significantly reduces curing and setting times since all the reactions are accelerated, allowing for much faster construction.”

The team believe that these lime clasts could be ‘self-healing’ when cracks begin to form.

To prove that this was the mechanism responsible for the durability of the Roman concrete, the team produced samples of hot-mixed concrete that incorporated both ancient and modern formulations, deliberately cracked them, and then ran water through the cracks.

Within two weeks the cracks in the ancient formula had completely healed and the water could no longer flow. An identical chunk of concrete made without quicklime never healed, and the water just kept flowing through the sample.

However, this isn’t the first time that we thought we’d uncovered the secret to Roman concrete’s longevity. Researchers back in 2017 discovered that seawater in the manufacturing process also strengthened Roman concrete.

Concrete is one of the most used substances on earth – second only to water. With tens of billion tonnes of concrete used every year, finding ways to make it last longer or use less is an important goal.

Perhaps Roman concrete still has more secrets deep inside its long-lasting shell.

The research has been published in Science Advances.

Correction: 11/01/2023. The original post used an image of the Parthenon, which was corrected to Pantheon.

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