Roman Concrete is Still Standing Tall – and Now We Know Why

Despite their empire falling more than 1,500 years ago, the Roman recipe for concrete remains stronger than anything around today. And it’s all thanks to seawater.

Roman concrete combined volcanic ash, lime, volcanic rock, and seawater. That final ingredient, it turns out, is key. The unusual combination of ingredients actually gets stronger as new minerals form over millennia.

To make the breakthrough, researchers at the Department of Energy’s Lawrence Berkeley National Laboratory made micron-level X-ray studies of samples of Roman concrete from ancient piers and breakwaters.

Those scans found the concrete contained crystals of aluminous tobermorite, a layered mineral that was crucial to strengthening the concrete over time. The crystals were formed when seawater, lime and volcanic ash mixed together to generate heat. The findings were published in the journal American Mineralogist.

“The Romans created a rock-like concrete that thrives in open chemical exchange with seawater,” said geologist Marie Jackson from the University of Utah, who led the study. The scans also revealed that a second period of mineral growth occurred, strengthening the concrete throughout as seawater sloshed around its structure.

By analysing individual mineral grains, the researchers were able to see tobermorite growing throughout the concrete, often alongside another mineral known as phillipsite. As the seawater dissolved the volcanic ash, these new minerals were formed. Over time, the seawater dissolved more of the ash, making the concrete stronger and stronger.

Modern concrete, by comparison, isn’t designed to change structure after it hardens. So while mineral growth hardens Roman concrete, the substance we use today cracks and breaks if such reactions occur. Seawater, in particular, is the nemesis of modern sea defences, with the steel reinforcements rusting as the concrete around them erodes.

While the Romans were lucky to have the perfect rocks to work with, Jackson hopes that modern science can help recreate their concrete mix.

Roman Concrete
Roman piers and other harbour structures built using an ancient cement mix have lasted for millennia. Picture: Alamy
Roman Concrete
Ruins of a concrete Roman bath in Caesarea Maritima in Israel. Image credits: James Cocks via Wikimedia Commons

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