How ancient Romans made seawater-proof concrete
BERKELEY, California: A new look inside 2,000-year-old concrete – made from volcanic ash, lime (the product of baked limestone), and seawater – has provided new clues to the evolving chemistry and mineral cements that allow ancient harbour structures to withstand the test of time.
The research has also inspired a hunt for the original recipe so that modern concrete manufacturers can do as the Romans did.
A team of researchers working at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) used X-rays to study samples of Roman concrete – from an ancient pier and breakwater sites – at microscopic scales to learn more about the make-up of their mineral cements.
The team’s earlier work at Berkeley Lab’s Advanced Light Source (ALS), an X-ray research centre known as a synchrotron, found that crystals of aluminous tobermorite, a layered mineral, played a key role in strengthening the concrete as they grew in relict lime particles. The new study, published in American Mineralogist, is helping researchers to piece together how and where this mineral formed during the long history of the concrete structures.
The work ultimately could lead to a wider adoption of concrete manufacturing techniques with less environmental impact than modern Portland cement manufacturing processes, which require high-temperature kilns. These are a significant contributor to industrial carbon dioxide emissions, which add to the buildup of greenhouse gases in Earth’s atmosphere.
Marie Jackson, a geology and geophysics research professor at the University of Utah had led the study.
Jackson said that lime (also known as calcium oxide, or CaO) – exposed to seawater in the Roman concrete mixture – probably thoroughly reacted with volcanic ash early in the history of the massive harbour structures. Previous studies showed how the aluminous tobermorite crystallised in the lime remnants during a period of elevated temperature.
The new findings suggest that after the lime was consumed via these pozzolanic chemical reactions, a new period of mineral growth began.
They may explain an ancient observation by the Roman scientist Pliny the Elder, who opined that the concrete, “as soon as it comes into contact with the waves of the sea and is submerged, becomes a single stone mass, impregnable to the waves and every day stronger.”
In fact, the Romans relied on the reaction of a volcanic rock mixture with seawater to produce the new mineral cements. In rare instances, underwater volcanoes, such as the Surtsey Volcano in Iceland, produce the same minerals found in Roman concrete.
“Contrary to the principles of modern cement-based concrete,” Jackson said, “The Romans created a rock-like concrete that thrives in open chemical exchange with seawater.” — Newswise