Researchers at the Lawrence Berkeley Laboratory had a question: How could underwater concrete structures built by the ancient Romans endure through two millenia, when concrete structures built in twentieth-century North America have trouble making it to their fiftieth birthday?
Now, a research team led by Paulo Monteiro, a professor of civil and environmental engineering at UC Berkeley, is finding some answers. The chemical structure of hardened Roman cement is different from the structure of modern cement — because the Roman recipe used slightly different natural and man-made ingredients. Those ingredients are still around on the planet — and the Roman formulas from the past may point to a future where concrete lasts longer, and requires less energy to make.
"Analysis of samples provided by team member Marie Jackson pinpointed why the best Roman concrete was superior to most modern concrete in durability, why its manufacture was less environmentally damaging – and how these improvements could be adopted in the modern world," says a news release on the Berkeley Lab website ("Roman Seawater Concrete Holds the Secret to Cutting Carbon Emissions").
"Concrete was the Roman Empire's construction material of choice," explains another Berkeley report ("To improve today's concrete, do as the Romans did," by Sarah Yang. "It was used in monuments such as the Pantheon in Rome as well as in wharves, breakwaters, and other harbor structures. Of particular interest to the research team was how Roman's underwater concrete endured the unforgiving saltwater environment."
It appears the Romans had a special trick for producing concrete in seaport structures: Their formula's effectiveness actually depended on contact with seawater. Says the Berkeley report: "The Romans made concrete by mixing lime and volcanic rock. For underwater structures, lime and volcanic ash were mixed to form mortar, and this mortar and volcanic tuff were packed into wooden forms. The seawater instantly triggered a hot chemical reaction. The lime was hydrated – incorporating water molecules into its structure – and reacted with the ash to cement the whole mixture together."
"Roman concrete has remained coherent and well-consolidated for 2,000 years in aggressive maritime environments," said Marie Jackson. "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 harbors that would last was critical."
The Roman formula's strength and durability, researchers learned, result from the way the process incorporates aluminum into the semi-crystalline concrete matrix. "Roman concrete differs from the modern kind in several essential ways," the Berkeley report says. "One is the kind of glue that binds the concrete's components together. In concrete made with Portland cement this is a compound of calcium, silicates, and hydrates (C-S-H). Roman concrete produces a significantly different compound, with added aluminum and less silicon. The resulting calcium-aluminum-silicate-hydrate (C-A-S-H) is an exceptionally stable binder."
Analysis of core samples recovered from ancient Roman structures on the seabed (which were still strong after 20 centuries) showed that the concrete contained "less than 10 percent lime by weight, made at two-thirds or less the temperature required by Portland cement," the researchers reported. "Lime reacting with aluminum-rich pozzolan ash and seawater formed highly stable C-A-S-H and Al-tobermorite, ensuring strength and longevity. Both the materials and the way the Romans used them hold lessons for the future."
Researcher Monteiro says that in theory, naturally occurring minerals used in ancient concrete could reduce the world's reliance on energy-intensive manufactured cement ingredients. Says Monteiro: "Pozzolan is important for its practical applications. It could replace 40 percent of the world's demand for Portland cement. And there are sources of pozzolan all over the world."