The secret to making sustainable, strong concrete may have been at the bottom the Mediterranean Sea for the past 2,000 years: Researchers believe that the ancient Romans created concrete that is more environmentally friendly and durable than modern cement.
Concrete is one of the most commonly used building materials in the modern world, but its role in creating carbon emissions is often overlooked. Worldwide, some 19 billion tons of concrete is used annually, and the high temperatures necessary to produce it are responsible for up to 7 percent of all human carbon dioxide emissions.
"It's a beautiful material, it's used all over the world, but the weakness is we use too much of it. We cannot continue with business as usual," says Paulo Monteiro, a researcher at the U.S. Department of Energy's Lawrence Berkeley National Laboratory. His findings were published in the Journal of the American Ceramic Society. "We have to find alternative ways to make concrete."
Monteiro and his colleagues may have found an alternative: "sea" concrete used by Romans for harbor installations in the Mediterranean is made with a different concentration of materials than today's mix of limestone and clay, which allowed it to be baked at a much lower temperature (about 1,650 degrees, compared to 2,640 degrees for modern "Portland" concrete). The result is a strong concrete that is less harmful to the environment.
Engineers have had some success using the byproducts of coal power plants, known as "fly ash" to create a type of "green" concrete, but there is only so much fly ash to go around, Monteiro says. So engineers may soon do as the Romans did and use a mix of volcanic ash and limestone.
"Volcanic ash is available in a good deal of the world, usually there are entire mountains of it following a volcanic eruption," he says. "The Romans were unbelievably good at using it as a building material."
Archaeologists have long known the Roman formula for creating this type of concrete, but it wasn't until recently that Monteiro and his team discovered that ancient concrete is about as durable as present mixtures. The Romans' mixture does have several drawbacks: It takes several weeks of setting to reach its full strength potential, which makes it difficult to use for projects such as dams or bridges. But its longevity, Monteiro says, is obvious.
"They wanted a concrete that could last forever," he says. "They were practical and good engineers, and from the ruins you can see it has long-term durability."