Contemporary concrete is designed to last for about 100 years. Yet at the bottom of the Mediterranean Sea lie the remains of Roman harbors, buildings and other structures that have remained surprisingly intact for almost 2000 years. What's the secret to their phenomenally durable concrete?
"The Romans developed a huge harbor infrastructure in the first century B.C. and first century A.D., and they built the harbors so well that they didn't need to keep repairing them," explained Marie Jackson, a researcher at the University of California, Berkeley's civil and environmental engineering department.
Jackson and several of her Berkeley colleagues, lead by civil engineering professor Paulo Monteiro, recently examined the underwater structures at Pozzuoli, an Italian seaside town on the Bay of Naples. These harbors were in use for centuries, long after the method of their construction was lost with the fall of Rome. The harbors are still in relatively good shape today, considering they've been subject to centuries of salt water and erosion. [See also: Rome Can Be Built in a Day ]
But apparently, the salt water is part of the reason why Roman concrete lasts so long.
Like modern chemists, Roman builders took limestone and burned it to create lime, a key component of mortar. But then, Roman builders mixed the lime with volcanic ash from the Gulf of Naples in the Mediterranean Sea, and saturated the mixture with salt water.
The reaction between the volcanic ash, called pozzolan, and the salt water naturally produces a bonding material called calcium aluminum silicate hydrate, or CASH.
"In a sense, what the Romans were doing is almost working with the salt water environment to create a material that actually remains durable for about the same time frame as rock," Jackson told TechNewsDaily.
This structure of CASH differs significantly from modern industrial concrete, which lacks aluminum and relies more heavily on silicates. [See also: Are Wiry Solar Cells an Alternative to Silicon?]
Furthermore, an X-ray analysis of the structures at Pozzuoli revealed that the concrete contains tobermorite, a crystalline material whose structure is considered "ideal"—meaning that the way its molecules arrange themselves is highly organized and therefore very strong–and which is nowhere to be found in industrial concrete.
What's more, the researchers' findings suggest that the Roman method is far 'greener' than current techniques, releasing far less carbon dioxide (CO2) into the atmosphere.
Most modern concrete is made from Portland cement, a substance derived from lime that has been baked at approximately 2,600 degrees Fahrenheit. It is because it needs to achieve these high temperatures that the production of Portland cement creates 7% of industry CO2 emissions.
[See also: New Rocket Fuel Helps NASA 'Go Green' ]
The Romans weren't thinking of CO2 when making their cement, but the strength of their concrete is no accident. Several Roman historians mention the recipe in their writings, including Pliny the Elder and Seneca.
Some makers of modern concrete have experimented with using volcanic ash as a substitute for Portland cement, but until now, no one knew how long such mixtures could be expected to last.
"The first thing we need to be looking at is how can we use volcanic rock or volcanic ash to recreate some of the durability and cohesion of the ancient Roman concrete [for modern purposes]," said Jackson.
The research, which was published today (June 4) in the Journal of the American Ceramic Society, was partially funded by the U.S. Department of Energy's Office of Science, and the Roman Maritime Concrete Study, or ROMACONS.
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