Leave modern concrete structures in the ocean, and a few decades later they'll be in need of replacing, or at least a serious patch job. Meanwhile, ancient Roman concrete is still standing strong after thousands of years, and not only does it resist damage, but the salt water actually makes it stronger. X-ray examinations have found the key to the mixture's incredible longevity, which could help improve modern recipes.

In recent years, we've electrified concrete to melt snow, made it more fire-resistant, added bacteria to make it self-heal any cracks that form, and have found ways to "program" it to be stronger. But it seems that the Romans were way ahead of their time, with a superior method for making concrete that could stand the test of time.

Although the complete recipe has been lost over the millennia, studies of samples have shown that volcanic ash, lime and seawater are the main ingredients. But the real magic seems to happen when those ingredients interact with the environment – specifically the saltwater incessantly pounding on the surface.

Researchers from Berkeley Lab and the University of Utah took samples of ancient concrete from 2,000-year-old harbor structures in Orbetello, Italy, and studied them in the X-ray research center at Berkeley Lab's Advanced Light Source (ALS) to try to find clues to its durability.

"At the ALS we map the mineral cement microstructures," says Marie Jackson, lead researcher on the study. "We can identify the various minerals and the intriguingly complex sequences of crystallization at the micron scale."

The team found that when seawater seeps into the concrete, it dissolves the lime inside. Normally, this kind of corrosion would destroy modern concrete in a matter of years, but it actually strengthens the Roman stuff by allowing crystals of Al-tobermorite and phillipsite to grow, plugging the holes.

A microscope image of a mix of volcanic ash, lime and seawater, which has resulted in the growth of Al-tobermorite crystals(Credit: Marie Jackson)

"We're looking at a system that's contrary to everything one would not want in cement-based concrete," says Jackson. "We're looking at a system that thrives in open chemical exchange with seawater."

Reviving this long-lost, ingenious technique would certainly be handy today, but the exact formula is still unknown. The researchers are experimenting with different combinations of seawater and volcanic rock to try to unlock its secrets, which could be useful for building longer lasting seawalls, dams and piers.

The research was published in the journal American Mineralogist and the researchers describe their work in the video below.

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