It is a mystery that continues to baffle modern engineers. Why do 2,000-year-old Roman piers survive to this day, yet modern concrete seawalls embedded with steel crumble within decades?
Even Pliny the Elder, writing in Naturalis Historia in 79AD, noted that concrete structures in ancient harbours, ‘become a single stone mass, impregnable to the waves, and every day stronger,’ despite being constantly battered by seawater.
Now scientists in the US think they have found the answer, and it could finally lead to modern sea defences which withstand time and tide.
They discovered that when saltwater mixes with the volcanic ash and lime used by Roman builders, it leads to the growth of interlocking minerals, which bring a virtually impenetrable cohesion to concrete.
"We're looking at a system that's contrary to everything one would want in cement-based concrete," said Prof Marie Jackson, a geology and geophysics research professor at the University of Utah who led the study.
"We're looking at a system that thrives in open chemical exchange with seawater."
Roman engineers made concrete by mixing volcanic ash with lime and seawater to make a mortar, and then added chunks of volcanic rock. The combination of ash, water, and lime produces what is called a pozzolanic reaction, named after the city of Pozzuoli in the Bay of Naples, triggering the formation of crystals in the gaps of the mixture as it sets.
The same reaction happens in nature, and clumps of natural cement called ‘tuffs’ can be found scattered around volcanic areas, which is probably what gave the Romans the idea.
Structures such as the Pantheon and Trajan's Markets are testament to the strength that the mineralisation brings to the concrete, but it was a mystery how structures in the sea survive the constant onslaught of waves.
For the new study researchers studied cores of concrete from the ancient Roman pier, Portus Cosanus in Orbetello, Italy using high-powered light beam x-rays and discovered the minerals had grown into the cracks caused by tidal erosion, proving that the saltwater reaction continues even after the concrete has set.
In contrast most modern concrete is a mix of Portland cement - limestone, sandstone, ash, chalk, iron, and clay, among other ingredients, heated to form a glassy material that is finely ground - mixed with sand or crushed stone that are not intended to chemically react, and so do not cause mineralisation when mixed with saltwater.
The exact recipe for Roman concrete has been lost, but the team is now working with geological engineers to find a replacement using seawater from San Francisco Bay and volcanic rock from the Western United States.
If they succeed, it will allow builders to construct sea defences which last for centuries while also being beneficial for the planet. Portland cement requires high temperature kilns which are known to be a significant contributor to carbon dioxide emissions and global warming.
“Romans were fortunate in the type of rock they had to work with," added Prof Jackson. "They observed that volcanic ash grew cements to produce the tuff. We don't have those rocks in a lot of the world, so there would have to be substitutions made."
The research was published in the journal American Mineralogist.