Researchers determine the durability of Roman concrete

The Pantheon in Rome, with the world’s largest unreinforced concrete dome, dates back to A.D. 128 and is still intact, and some ancient Roman aqueducts still deliver water to Rome.

Researchers have spent decades trying to understand the secret of this ultradurable construction material, particularly in structures that endured harsh conditions.

Now, a team of investigators from MIT, Harvard University, and laboratories in Italy and Switzerland, has discovered ancient concrete-manufacturing strategies that incorporated several key self-healing functionalities. The findings are published in Science Advances, in a paper by MIT professor of civil and environmental engineering Admir Masic, former doctoral student Linda Seymour, and four others.

Researchers have previously assumed that ancient concrete’s durability was based on pozzolanic material such as volcanic ash. These ancient samples also contain millimetre-scale bright white mineral features, which are recognized as a ubiquitous component of Roman concretes. These white chunks - lime clasts - originate from lime, another component of Roman concrete mix.

“Ever since I first began working with ancient Roman concrete, I’ve always been fascinated by these features,” Masic said in a statement. “These are not found in modern concrete formulations, so why are they present in these ancient materials?”

Previously disregarded as evidence of careless mixing practices, or poor-quality raw materials, the new study suggests that lime clasts gave the concrete a previously unrecognized self-healing capability. “The idea that the presence of these lime clasts was simply attributed to low quality control always bothered me,” said Masic. “If the Romans put so much effort into making an outstanding construction material, following all of the detailed recipes that had been optimized over the course of many centuries, 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.”

Using high-resolution multiscale imaging and chemical mapping techniques pioneered in Masic’s research lab, the researchers gained new insights into the potential functionality of these lime clasts.

According to MIT, it had been assumed that when lime was incorporated into Roman concrete, it was first combined with water to form a highly reactive paste-like material, a process called slaking. According to the team, 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?” said Masic.

Studying samples of ancient concrete, he and his team determined that the white inclusions were made from various forms of calcium formed at extreme temperatures, as would be expected from the exothermic reaction produced by using quicklime instead of, or in addition to, slaked lime. The team concluded that hot mixing is key to the durability of Roman concrete.

“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.”

MIT said that during the hot mixing process, the lime clasts develop a brittle nanoparticulate architecture, creating an easily fractured and reactive calcium source, which could provide self-healing functionality. As soon as cracks start to form within the concrete, they can preferentially travel through the high-surface-area lime clasts. This material can then react with water, creating a calcium-saturated solution, which can recrystallize as calcium carbonate to fill the crack, or react with pozzolanic materials to further strengthen the composite material. These reactions take place spontaneously and heal the cracks before they spread. Previous support for this hypothesis was found through the examination of other Roman concrete samples that exhibited calcite-filled cracks.

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 had completely healed. An identical piece of concrete made without quicklime did not heal and the water continued to flow through the sample. The team is now working to commercialize this modified cement material.