Landscape Architect Articles

Secrets of Ancient Roman Concrete Revealed07-25-13 | News

Secrets of Ancient Roman Concrete Revealed

A team of researchers sampled a concrete breakwater built by ancient Romans in Pozzuoli Bay, near Naples, Italy, that dates back to around 37 B.C. The team found that the long-term strength of the structure comes from the use of volcanic ash in the mortar mix, which creates a more stable bonding compound within the cement. Modern cement, by contrast, is designed to last about 100 years at best.
Credit: UC Berkeley/D. Bartoli, courtesy of J.P. Oleson

In a quest to make concrete more durable and sustainable, an international team of geologists and engineers is taking inspiration from a concrete breakwater constructed by the ancient Romans, which has endured in the Mediterranean Sea for more than 2,000 years.

"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, professor of civil and environmental engineering at the University of California, Berkeley, and research team leader. "The problem is that manufacturing Portland cement accounts for seven percent of the carbon dioxide that industry puts into the air."

Portland cement provides the "glue" that holds most modern concrete together. But making it releases carbon, both from the fuel that mixes the component clays and limestone at 2,642˚ Fahrenheit and the heated limestone itself. Monteiro's team found that the Romans, by contrast, used less lime, and made their concrete from limestone baked at 1,652˚F or lower, requiring far less fuel than Portland cement.

The discovery and use of these methods could cut greenhouse gas emissions during concrete production, while improving the durability of modern concrete, which often shows signs of degradation within 50 years, particularly in ocean environments.

The Romans made their concrete by mixing lime and volcanic rock into mortar, which was packed into wooden forms with more volcanic ash and submerged. The seawater instantly triggered a hot chemical reaction, hydrating the lime – incorporating water molecules into its structure – and reacting with the ash to cement the mixture together. Ash with similar mineral characteristics to that used by the Romans, called pozzolan, is found in many parts of the world.

Some environmentally friendly modern concretes already include volcanic ash, rice husk ash or fly ash from coal-burning power plants as partial substitutes for Portland cement. These blended cements produce a calcium-aluminum-silicate-hydrate (C-A-S-H) bonding agent that was found in the Roman concrete but is not present in modern blends. The discovery of C-A-S-H in the ancient breakwater is a testament production method's long-term durability, which had been considered too new to prove conclusively.

Further analyses showed that the Roman recipe needed less than 10 percent lime by weight, made at two-thirds or less the temperature required by Portland cement. Lime reacting with aluminum-rich pozzolan ash and seawater formed highly stable C A-S-H and Al-tobermorite, ensuring strength and longevity. While modern construction is unlikely to adopt these methods immediately – Roman concrete takes longer to cure, which could slow build times and increase costs – the materials and the way the Romans used them hold lessons for the future.

"For us, pozzolan is important for its practical applications," says Monteiro. "It could replace 40 percent of the world's demand for Portland cement. And there are sources of pozzolan all over the world."

The researchers' findings are described in two papers, one posted online May 28 in the Journal of the American Ceramic Society, and the other scheduled to appear in the October issue of American Mineralogist.