Call it the ultimate cold case.
No crime was committed. But a mystery was waiting to be solved. The critical clues had been buried at sea -- for more than two thousand years. And yes, the final answer had much to do with cold, hard C-A-S-H.
In a sense, the mystery began with a man named Marcus Vitruvius Pollio. He was a man of some prominence, an engineer for the Emperor Augustus. About 15 BCE, Marcus also put down the recipe for Roman concrete, whose durability has been an enduring mystery.
Modern concrete buildings -- most made with Portland cement -- are built to last for a hundred or so years. But Roman concrete has withstood the rise and fall of empires. The Pantheon is proof of its resilience, as are many other ancient cement structures around the Mediterranean. The best Roman concrete has withstood two millennia of attacks by wave and water, since it was also used to create breakwaters and artificial harbors.
So what makes Roman concrete so durable? That mystery was recently solved by an international team of researchers led by Paulo Monteiro of the Energy Department’s Lawrence Berkeley National Laboratory (Berkeley Lab), a professor of civil and environmental engineering at the University of California, Berkeley.
The team first took evidence from the "crime" scene, an underwater sample of Roman concrete from an ancient breakwater in Pozzuoli Bay, near Naples, Italy. They then gave it a close examination with their "magnifying glass," Berkeley Lab’s Advanced Light Source (ALS).
Through the ALS -- and a variety of other high-tech tools -- researchers found a number of clues. For instance, the "glue" that holds modern concrete together is composed of calcium, silicates and hydrates (C-S-H). However, Berkeley Lab researchers discovered that Roman concrete uses less of the ‘S’ and adds an ‘A,’ in the form of aluminum, for C-A-S-H. The addition of aluminum (Al) from the specific kind of volcanic ash the Romans used for seawater concrete, called pozzolan, leads to the formation of a rare mineral called Al-tobermorite, which adds significantly to the strength and stability of the concrete. Together, those factors led to a concrete for the ages. See their full analysis in the Journal of the American Ceramics Society.
For all its durability, Roman concrete has the drawback of hardening too slowly for some modern construction projects. But it could be useful for many other applications, such as containing hazardous wastes for generations. The concrete could also be a solid construction substitute for countries without easy access to fly ash, a key ingredient in some environmentally friendly modern concretes. And since fly ash is created from coal-burning power plants, Roman concrete could also prove more environmentally friendly than its Portland-cement counterpart.
By solving a two-thousand year old mystery, researchers using the Energy Department’s world-class facilities are showing the way to building a better future. That’s a pretty solid reward for solving the ultimate cold case.