Mystery Solved: Why Roman Concrete Seems Immortal

Posted by Richard on April 13, 2023

When in Rome, do as the Romans do. That’s also pretty good advice if you’re paving a driveway or building something out of concrete. Turns out, Roman concrete actually has a self-healing process, which helps explain why Roman structures have proven to be so durable over the centuries.

Concrete was a favored construction material for the Romans, and to this day, ancient aqueducts continue to carry water across Italy. The longevity of Roman mixes has long vexed scientists. Even with modern innovations, matching the durability of Roman concrete is quite the feat.

Early on, scientists hypothesized that volcanic ash from Pozzuoli was the secret sauce, and at least in part, the ash seems to have played an important role. Of course, the ash itself is a unique composition, and many scientists figured that it was specifically the ash from the Pozzuoli region, not volcanic ash in general, that held the key. Yet the Pozzuoli ash doesn’t explain the whole story. Scientists have long observed tiny white mineral quicklime specks in Roman concrete. However, most researchers wrote those specks off as impurities resulting from shoddy mixing and materials.

Not so. These quicklime specks are actually one of the key ingredients that make Roman concrete so long-lasting. The Romans cooked up concrete with a hot mixing process and added quicklime to their blend — an ingredient that is often avoided even to this day because it can be dangerous and difficult to work with. But once quicklime is exposed to water, it dissolves, seeps into cracks, and then recrystallizes. The result is that whenever Roman concrete cracks, more of it will be exposed to water during the next rainstorm. The quicklime will dissolve and seep into the cracks, then recrystallize, thus filling in the cracks.

With these insights in hand, scientists believe they can craft stronger concrete formulas, which in the long run could reduce waste and perhaps even reduce the amount of greenhouse gases released into the air.