Using the Advanced Light Source at the U.S. Department of Energy's Lawrence Berkeley National Laboratory, researchers from the University of California, Berkeley studied a Roman breakwater that has spent the last 2,000 years submerged in the Mediterranean Sea.
Their study found that the best Roman concrete was superior to most modern concrete in durability, and its manufacture was less environmentally damaging.
"It's not that modern concrete isn't good -- it's so good we use 19 billion tons of it a year," said Paulo Monteiro of the Berkeley Lab, who led the study. "The problem is that manufacturing Portland cement accounts for seven percent of the carbon dioxide that industry puts into the air."
Portland cement holds most modern concrete together, but requires heating limestone and clays to 1,450 degrees Celsius. Carbon is released by burning fuel and from the heated limestone (calcium carbonate) itself.
The Romans, by contrast, used less than ten percent lime by weight, mixing it with volcanic rock and baking it at just 900 degrees Celsius or lower, requiring far less fuel that Portland cement.
For underwater structures, lime and volcanic ash were mixed to form mortar, which was packed with volcanic tuff. The seawater triggered a hot chemical reaction, hydrating the lime and reacting with the ash to cement the mixture together.
The Romans incorporated aluminum and used less silicon, and the result contained a rare, crystalline hydrothermal mineral called aluminum tobermorite (Al-tobermorite). The ideal crystalline structure is what gave Roman seawater concrete its superior strength.
"In the middle 20th century, concrete structures were designed to last 50 years" Monteiro said. "Now we design buildings to last 100 to 120 years." Yet the Roman concrete has survived 2,000 years of waves and chemical attack.
Environmentally friendly modern concretes include volcanic ash or fly ash from coal-burning power plants as partial substitutes for Portland cement. An alternative ash with similar mineral characteristics to volcanic ash, called pozzolan, is found in many parts of the world.
Monteiro says pozzolan "could replace 40 percent of the world's demand for Portland cement," improving the durability and sustainability of modern concrete.