What's so special about ancient Rome's concrete? Well, it just might be "the most durable building material in human history," as one engineer puts it, per the Washington Post. A new study in American Mineralogist sheds further light onto why: Romans mixed a specific volcanic ash with lime to create a concrete similar to stone, perfect for building sea walls that have lasted some 2,000 years. The key ingredient, however, was continually added by nature. Researchers at the University of Utah who studied minerals in Roman concrete samples say seawater crashing against concrete sea walls caused a chemical reaction that helped strengthen the material over time. In other words, the concrete is actually stronger today than when it was made.
Inside samples, researchers found crystals of aluminous tobermorite growing from the mineral phillipsite. Phillipsite occurs naturally in volcanic rocks, but aluminous tobermorite is "very rare" and would have formed as seawater penetrated the concrete, study author Marie Jackson tells Nature. As its crystals grew to form "interlocking plates," the concrete likely gained the strength to bend, rather than shatter, under pressure, per a release. "This rocklike concrete is behaving, in many ways, like volcanic deposits in submarine environments," says Jackson, adding that the Romans may have studied volcanoes to learn how ash could be turned to rock. She's now trying to come up with the precise concrete recipe, which could be utilized in modern sea walls as global seas rise. (These Alaskan villagers can relate to that risk.)