Why Ancient Roman Concrete Is So Strong

A rare chemical reaction strengthens it even today—and that could help threatened coastal communities

Roman Concrete
This structure in Tuscany holds clues to why Roman concrete is still so strong. J.P. Oleson

As sea levels rise and shorelines erode, the hunt is on for ways to protect the millions of people that live in seaside communities. But engineers with an eye on a wetter future might want to look to the past for inspiration. As Ben Guarino reports for the Washington Post, an innovation from ancient Rome might hold clues to creating a more durable sea wall.

Saltwater corrodes modern concrete within years. But the concrete used by ancient Romans doesn't suffer this same issue. Romans erected sea walls and piers roughly 2,000 years ago, and many still stand strong in Italian waters. Now a new study in the journal American Mineralogist explains why.

Scientists analyzed the chemical makeup of pier pieces from locations throughout Italy and assessed historical writings about ancient Roman sea structures to learn more about the tough material. This analysis suggests that the materials undergo a rare chemical reaction.

The concrete is made of quicklime, or calcium oxide, and volcanic ash. When seawater gets into its cracks, it causes a chemical reaction that actually strengthens the concrete. Minerals called Al-tobermorite and phillipsite form as the material leaches mineral-rich fluid that then solidifies, reinforcing the concrete and making the structures even stronger.

There’s just one problem: Nobody knows exactly how the concrete was made. “The recipe was completely lost,” says Marie Jackson, a geologist who led the study, in a press release. But modern-day chemical analysis could help researchers reverse-engineer the processes and materials that created the concrete long ago.

That’s good news for engineers tasked with tackling the vexing question of sea level rise. The oceans are rising more quickly than ever documented before, and even tiny changes in temperature can hasten swelling seas. As ice melts at the poles, even more water will surge into rising seas—and given predictions of more extreme weather events driven by rising atmospheric temperatures, that means there will be a continued focus on sea walls to keep communities near shores safe.

But not all sea walls are good for the shore. As Smithsonian.com reported in 2015, nearly 14 percent of American coastline is covered in concrete, and when water bounces off of sea walls it can hurt fragile coastal habitat and, ironically, make coastal areas even more susceptible to storm damage.

Perhaps ancient Roman techniques will be used in future structures—but when it comes to stopping sea level rise, even high-tech materials inspired by old-school manufacturing won’t get the world off the hook.

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