What did the Romans do for us is a common question that has spawned dozens of books and TV documentaries – but one of their simplest discoveries may unlock the secret of protecting vulnerable cities against the sea.
Scientists were puzzled why modern concrete and steel sea walls crumbled into the sea after a few decades while Roman concrete walls have withstood a battering from the sea for more than 2,000 years.
The same forgotten construction technique has seen Roman buildings defy the elements while flimsy modern-day structures have a much shorter lifespan.
Researchers from the University of Utah believe they may have stumbled across the answer – the ingredients the Romans mixed to form their concrete.
Mystery of how to mix ingredients
Ancient engineers blended volcanic ash with lime, seawater and rock to construct their breakwaters.
But although the ingredients have been identified, no one knows the quantities involved for a perfect mix.
When combined, the ingredients undergo a ‘pozzolanic reaction’ that generates crystal growth in the mix that gives the strength for the concrete to last thousands of years.
“We’re looking at a system that’s contrary to everything one would want in cement-based concrete,” said Marie Jackson, who is a geology and geophysics research professor at the University of Utah.
“We’re looking at a system that thrives in open chemical exchange with seawater.”
Rediscovering ancient recipe
The team studied sections of concrete from a Roman jetty at Orbetello, Italy, with X-rays that revealed the reaction – and discovered that the crystals continue to develop as the concrete ages in seawater.
Modern materials do not chemically react with seawater in the same way.
Now, the university team is investigating how to rediscover the recipe for Roman concrete that will allow engineers to protect a coastline for centuries rather than decades.
“Romans were fortunate in the type of rock they had to work with,” said Jackson. “They saw that volcanic ash grew cements to make the mix stronger. We don’t have those rocks in a lot of the world, so there would have to be substitutions made.”