Secret of how Roman concrete survived tidal battering for 2,000 years revealed
It is a mystery that continues to baffle modern engineers. Why the 2000 year old battery that still survives today, but the modern concrete walls and built in steel crumble in the decades?
Even Pliny the Elder, Naturalis History written in 79AD, pointed out that the concrete structures in the old port “became a mass of stone, impenetrable to the waves and stronger every day,” despite being constantly hit by water from sea.
Now, American scientists believe they have found the answer, and this could eventually lead to modern sea defenses that are resistant to weather and tide.
They found that when salt water is mixed with volcanic ash and lime used by Roman builders, it leads to the growth of intertwined minerals, which provide concrete cohesion virtually impenetrable.
“We look at a system that is contrary to anything one might want specifically based on concrete,” said Professor Marie Jackson, a professor of geology and geophysics research at the University of Utah who led the study.
“We look at a system that is developed in an open chemical exchange with sea water.”
The Roman engineers concreted by mixing volcanic ash with lime and sea water to form a mortar, and then added pieces of volcanic rock.
The combination of ash, water and lime product called a pozzolanic reaction, named the city of Pozzuoli in the Bay of Naples, which caused the formation of crystals in the interstices of the mixture as and when established.
The same reaction occurs in nature, and piles of natural cement called “tuff” are scattered around the volcanic areas, which is probably what gave the Romans the idea
Structures such as the Pantheon Market and Trajan reflect the strength that mineralization brings to the concrete, but it was a mystery how the structures in the sea survive the constant attack of the waves.
For the new study, researchers studied the concrete core of the ancient Roman pier Portus Cosanus Orbetello, Italy, using a beam of high-power X-ray light and discovered that minerals had grown cracks caused by tidal erosion, Which shows that the reaction of the salt water continues even after the application of the concrete.
However, the most modern concrete is a mixture of Portland cement – limestone, sandstone, ash, chalk, iron and clay, among other ingredients, heated to form a vitreous material finely mixed with crushed sand or stone which are not intended to React chemically, and do not cause mineralization when mixed with sea water.
The exact recipe for Roman concrete was lost, but the team is now working with geological engineers to find a replacement with seawater from the San Francisco Bay and volcanic rocks of the western United States.
If they succeed, it will allow manufacturers to build marine defenses that last for centuries while enjoying the planet. Portland cement requires high temperature furnaces, which are known to contribute significantly to carbon dioxide emissions and global warming.
“The Romans were sort of rock-hard guys they worked with,” said Professor Jackson. “They found that volcanic ash cements developed to produce tuff. We do not have such rocks in many people, so it would have to be substitutions.”
The research was published in the American Mineralogist magazine.