MIT Scientists Discover 'Self-Healing' Properties of Roman Concrete

Researchers at the Massachusetts Institute of Technology have made a discovery regarding Roman concrete — concrete that has withstood millennia.

Roman concrete has a unique ability to "self-heal," according to MIT researchers. That self-healing process could reduce the impact of present-day cement production, which accounts for 8% of global greenhouse gas emissions.

The ancient Romans made great strides in engineering, creating a vast system of aqueducts, ports and temples. Many of those structures that are still around today were built with this unique Roman-style concrete, which has proven itself through harsh conditions and aging.

Researchers initially believed the secret to be pozzolanic material like volcanic ash. Historians say this type of ash was shipped throughout the Roman empire.

However, after a closer examination, small bright white mineral features were also found in the Roman concrete. The white chunks are commonly referred to as "lime clasts." They were derived from lime.

"The idea that the presence of these lime clasts was simply attributed to low quality control always bothered me," said MIT professor of civil and environmental engineering Admir Masic. "If the Romans put so much effort into making an outstanding construction material ... why would they put so little effort into ensuring the production of a well-mixed final product? There has to be more to this story."

MIT researchers found the white substance to be various forms of calcium carbonate created through extreme temperatures.

The researchers believe "hot mixing" to have been the key to concrete's durability.

"The benefits of hot mixing are twofold," Masic said. "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."

According to Masic, during the hot mixing, lime clasts create brittle nanoparticulate structures. They create reactive calcium sources, "critical" to the self-healing traits of building materials. When it reacts with water, it recrystallizes as calcium carbonate and fills the cracks to strengthen the material.

The researchers produced samples of hot-mixed concrete with both ancient and modern formulas. Then they cracked them and allowed water to move within the cracks. Two weeks later, the cracks were completely "healed." Their report was published by Science Advances.

Masic and the researchers are now looking to commercialize the modified concrete process.

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