Researchers may have uncovered a big reason behind the remarkable durability of concrete from ancient Rome. Writing in the journal Science Advances, scientists say they took a closer look at white chunks in the concrete known as lime clasts, reports Ars Technica. These have been spotted before and largely chalked up to a sloppy mixing process, an explanation that didn't make sense to lead author Admir Masic of MIT. "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?" he says, per a news release.
His team concluded that these lime clasts were not only there on purpose, they gave the concrete what amounts to self-healing properties. As CNN explains: The findings "suggest that the lime clasts can dissolve into cracks and recrystallize after exposure to water, healing cracks created by weathering before they spread." They came about through the addition to the concrete mix of what we commonly call quicklime (calcium oxide). In a process known as "hot mixing" because of the temperatures involved, the quicklime would have been mixed with volcanic ash and other aggregates first, before water was added, per the Guardian.
"The Pantheon would not exist without the concrete as it was in the Roman time," says Masic. However, he says he doubts the Romans were aware of the precise chemistry involved with their mix. "They knew that was a great material, but they probably didn't know that it would last thousands of years." (Seawater may have played a role in making ancient Rome's seawalls endure.)