Using the Advanced Light Source at Lawrence Berkeley National Laboratory, in California, engineers and geologists have examined the structure of Roman concrete. Their findings revealed how the extraordinarily stable compound, calcium-aluminum-silicate-hydrate (C-A-S-H) in the concrete binds the material. This discovery could improve the durability of modern concrete, which within 50 years often shows signs of degradation, particularly in ocean environments.
The manufacturing of Roman concrete also leaves a smaller carbon footprint than its modern counterpart. The process for creating Portland cement, a key ingredient in modern concrete, requires fossil fuels to burn calcium carbonate (limestone) and clays at about 1450 degrees Celsius. That process is responsible for seven per cent of global carbon dioxide emissions every year.
Cleaner lime production
The production of lime for Roman concrete is much cleaner, requiring temperatures that are two-third less of that required for Portland cement.
?Roman concrete has remained coherent and well consolidated for 2000 years in aggressive maritime environments,? Marie Jackson, a research engineer in civil and environmental engineering at the University of California, Berkeley, said. ?It is one of the most durable construction materials on the planet, and that was no accident. Shipping was the lifeline of political, economic and military stability for the Roman Empire, so constructing harbours that would last was critical.?
For the study, published in the Journal of the American Ceramic Society, Jackson and colleagues characterised samples of Roman concrete taken from a breakwater in Pozzuoli Bay, near Naples, Italy. Of particular interest was how the Romans? underwater concrete endured the unforgiving saltwater environment.
Marcus Vitruvius Pollio, an engineer for Octavian, who became Emperor Augustus, described the recipe for Roman concrete around 30 BC. The not-so-secret ingredient is volcanic ash, which the Romans combined with lime to form mortar. They packed this mortar and rock chunks into wooden moulds immersed in seawater. Rather than battle the marine elements, Romans harnessed saltwater and made it an integral part of the concrete.
Also key was a very rare hydrothermal mineral called aluminum tobermorite (Al-tobermorite) that formed in the concrete.
The use of Roman concrete decreased because ?as the Roman Empire declined, and shipping declined, the need for the seawater concrete declined,? Jackson explained. ?You could also argue that the original structures were built so well that, once they were in place, they didn?t need to be replaced.?
Modern day alternatives
While Roman concrete is durable, it is unlikely to replace modern concrete because it is not ideal for construction where faster hardening is needed. But researchers are examining ways to apply their discoveries about Roman concrete to the development of more earth-friendly and durable modern concrete.
They are investigating whether volcanic ash would be a good, large volume substitute in countries without easy access to fly ash, an industrial waste product from the burning of coal that is commonly used to produce modern, green concrete.
?There is not enough fly ash in this world to replace half of the Portland cement being used,? said Paulo Monteiro, professor of civil and environmental engineering. ?Many countries don?t have fly ash, so the idea is to find alternative, local materials that will work, including the kind of volcanic ash that the Romans used. Using these alternatives could replace 40 per cent of the world?s demand for Portland cement.?