Mystery of Ancient Roman Concrete Unraveled: Discovering Applications for Modern Concrete

The Romans had far better concrete than we did. In fact, one of their breakwaters has spent the last 2,000 years submerged in the Mediterranean Sea and still exists. Now, scientists may have unraveled the chemical mystery as to how it was created.

The Romans used concrete to create some amazing structures. They constructed breakwaters, roads and even structurally complicated forms, such as the Pantheon dome. The material revolutionized the way the civilization built structures, and was first used during the late Roman Republic through the whole history of the Roman Empire. It surpassed modern concrete in durability and its manufacture was less environmentally damaging. Yet the recipe for this concrete was lost over time.

"It's not the modern concrete isn't good--it's so good we use 19 billion tons of it a year," said Paulo Monteiro, one of the researchers, in a news release. "The problem is that manufacturing Portland cement accounts for seven percent of the carbon dioxide that industry puts into the air."

In order to study Roman concrete and perhaps unravel the riddle of its creation, researchers analyzed concrete samples from Pozzuoili Bay, a northwestern region of the Bay of Naples. Like most concrete, Roman concrete was created from a mixture of materials. In this case, the researchers found that the ancient civilization mixed lime and volcanic rock. For underwater structures, the Romans mixed lime and volcanic ash for mortar and then 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.

So how does Roman concrete differ from modern concrete? There are several differences. One is the kind of glue that binds the concrete's components together. Modern concrete made with Portland cement uses a compound of calcium, silicates and hydrates (C-S-H). Roman concrete, in contrast, used aluminum and less silicon. The resulting calcium-aluminum-silicate-hydrate (C-A-S-H) was an exceptionally stable binder.

The second difference is the hydration products found in concrete. Ideal crystalline structures are nowhere to be found in conventional modern concrete. In Roman concrete, though, tobermorite can be found in combination with aluminum, creating Al-tobermorite. The two help create a crystal lattice.

So what does this mean for the future of concrete? Already, environmentally friendly concrets include volcanic ash or fly ash from coal-burning power plants as partial substitutes for Portland cement. These blended cements also product C-A-S-H, but their long-term performance could not be determined until now. By analyzing the Roman concrete, researchers found that the lime reacting with aluminum-rich pozzolan ash and seawater formed highly stable C-A-S-H and Al-tobermorite, which helped insure both strength and stability.

"For us, pozzolan is important for its practical applications," said Monteiro in a news release. "It oculd replace 40 percent of the world's demand for Portland cement. And there are sources of pozzolan all over the world. Saudi Arabia doesn't have any fly ash, but it has mountains of pozzolan."

The findings are published in the Journal of the American Ceramic Society and American Mineralogist.