Why is ancient Roman concrete still standing?
In October I wrote a story about how rising global temperatures may lead to shorter lifespans for concrete structures. The story noted that while modern concrete buildings and bridges have very short lifespans — sometimes fewer than 50 years — a number of concrete structures from Ancient Rome still stand today.
What makes Roman concrete so durable? A new study published this month in the Proceedings of the National Academy of Sciences finds that its remarkable strength owes to neat properties that make it behave more like a geological substance than a manmade material.
“At some point early in its history, [Roman concrete] starts to behave like a rock, and what we know is that rocks last a very long time,” says Marie Jackson, lead author of the study and a geologist at the University of California, Berkeley.
Modern concrete mixes are engineered to harden quickly — within a year (and often much sooner than that), they’re as strong as they’ll ever be. After that, Jackson says, “only destructive reactions take place.”
Roman concrete operates on a much longer scale. In this new study, Jackson’s team recreated the concrete mix used to build the Great Hall of Trajan’s Market, which was built in 110 CE and is still standing nearly 2,000 years later. They combined pieces of volcanic rock (the “aggregate” in concrete, usually sand today) with a mortar made from volcanic ash and quicklime, mixed with water — a formula the Romans came to early in the first millennium, after 150 years of experimentation.
The researchers analyzed the Roman concrete as it hardened. Within 90 days, they observed that the lime had been used up in chemical reactions, replaced by a mineral called stratlingite that begins to slowly grow in a crystal structure. This is the point when, in a precise technical sense, Roman concrete really does begin to behave like a rock.
All concrete cracks over time along the microscopic gaps between the coarse aggregate (the sand or volcanic rocks) and the mortar. These cracks are fine as long as they’re small and isolated, but they threaten the structure when they link up. In modern concrete, the sand and the mortar never really join together, creating “interfacial zones” along which cracks can propagate. In Roman concrete, these stratlingite crystals grow along the interfacial zones, binding the aggregate and the mortar together. Even 2,000 years later, the process continues.
“[Roman concrete] can keep growing new crystals,” says Jackson. “There are self-healing properties in this material.”
Roman concrete has its drawbacks — it has low “compressive” strength compared to modern concrete, which means you can’t pile a lot of weight atop a small space, as we do with skyscrapers. But most buildings aren’t skyscrapers. Jackson is working with industry experts to develop a Roman-style concrete that uses volcanic material, and could allow us to build houses, dams, and bridges with far-longer lifespans than current versions.