Concrete has been integral to buildings and architecture since the Romans used it to construct monuments like the Pantheon and the Coliseum. The Romans also used the material to build marine structures in breakwaters to protect their harbors, which have endured some 2,000 years despite the constant, erosive presence of saltwater. Even today, they fare far better against the sea than any modern concrete could. Now, science is explaining Roman concrete’s mysterious resilience–and how we could benefit from it today.
Marie Jackson, a geologist and research associate professor at the University of Utah who has studied the properties of Roman concrete for years, has discovered one of the primary reasons why Roman concrete is so resilient while its modern counterparts crumble within decades: the influence of seawater. In a paper published in American Mineralogist, she shows that rather than eroding the concrete, seawater actually strengthens Roman concrete’s mineralogical structure by enabling the growth of rare minerals that act like interlocking plates that reinforce the concrete’s binding structure. That means that over time, the seawater reinforces the molecular structure of the material, making it more resilient as time goes by.
Today’s conventional Portland cement concrete doesn’t respond well to any kind of environmental stress, including seawater, in comparison with the Roman sort. But why? It has to do with the recipe. The Romans used a mixture of volcanic ash, lime, and seawater, combined with pieces of volcanic rock. And in Roman marine concrete, Jackson found traces of aluminous tobermorite, a very rare mineral that’s difficult to create even in small quantities in a lab setting. Roman concrete that had been exposed to seawater had it in spades, which Jackson says made it more resilient across the centuries.
“The actual compressive strength of the material is not very high—it’s mainly the resilience of the material and the ability to respond to environmental impacts over time,” Jackson says. “The Romans actually spent several hundred years developing these materials.”
So why don’t all modern structures built in the ocean use Roman concrete? The problem is that the exact method and recipe for creating the material has been lost, leaving scientists with the daunting challenge of reverse engineering ancient concrete without any guidance. The Romans also were mostly working with volcanic rock, due to the geological makeup of Italy, so any modern attempts at a similar solution would likely require substitutes. To create an alternative using the discoveries from her research, Jackson is now working with a geological engineer to create a recipe that might function like its Roman ancestors.
In particular, she recently recommended it for a proposed tidal lagoon in the U.K., where officials are proposing a U-shaped breakwater to harness the movement of the tides to generate electricity. Jackson says that it would take 120 years to recoup the investment of the lagoon, and modern concrete would have corroded by the time the lagoon was just beginning to make money. Instead, she recommended engineering the lagoon using Roman-style concrete, since it has been shown to last for millennia.
Next, Jackson is heading to Iceland to study the geology of the volcano Surtsey, which last erupted in the 1960s. Scientists have found the same mineral cements as Roman concrete on Surtsey, and Jackson will continue her research into the material by drilling down into the mountain. Perhaps there she’ll come closer to the long-lost recipe for Roman concrete–far from Italy, hidden in the rock of an Icelandic volcano.