How Has Roman Concrete Lasted for Millennia? 1,900-Year-Old Latrine Offers Clues

The Canopus, a pool at Hadrian's Villa in Tivoli, Italy Carole Raddato via Flickr under CC BY-SA 2.0 Ancient Roman infrastructure has stood the test of time. Today, you can walk through Italy and see concrete buildings, roads and aqueducts that have survived for about two millennia. Modern concrete, on the other hand, usually crumbles within roughly 100 years. Scientists have long tried to uncover the secrets of Roman concrete’s durability. For years, they assumed that its longevity was thanks to one key chemical process: the pozzolanic reaction, which occurs when volcanic ash reacts with the chemical lime and water. While that still holds, there seems to be more to the story. It turns out that another chemical reaction, known as carbonation, might also contribute to Roman concrete’s longevity. The findings, published in the journal Science Advances on July 8, could help researchers develop more sustainable and resilient concrete materials. For the new work, researchers traveled to the 1,900-year-old Hadrian’s Villa, a UNESCO World Heritage site that sits about 17 miles east of Rome. The sprawling estate is an architectural marvel, but one of its scientific gems are the communal toilets. They offer an unprecedented opportunity to study Roman concrete in its original state, unaltered . “Nobody restores a latrine,” says Paulo J. Monteiro, a study co-author and civil engineer at the University of California, Berkeley, to Sam Macdonald at Scientific American. “So, the material sat undisturbed for 19 centuries, quietly running an experiment no one alive could start.” Need to know: Who was Hadrian? Hadrian was the emperor of Rome from 117 to 138 C. He’s well known for having a wall, called Hadrian’s Wall, built in northern England to protect the Roman province of Britannia from neighbors in what’s now Scotland. Monteiro and his colleagues took a concrete sample from underneath a toilet seat. Back in the lab, they examined it under a high-powered microscope, scanned it with X-rays and analyzed its chemical composition. As expected, the specimen contained evidence that volcanic ash, lime and water had been combined to form the material. However, a closer look at the concrete’s pores and fractures revealed that calcite, a mineral with calcium, carbon and oxygen, was the primary binding agent. When atmospheric carbon dioxide reacts with the calcium compounds in the concrete, it forms the hard mineral calcite, which contains a lot of the compound calcium carbonate. The mineral fills small cracks and pores in the concrete, allowing ancient structures to strengthen and heal over time. “While the pozzolanic reaction is of fundamental importance, our findings suggest that carbonation over a long period of time also enhances the durability of concrete and can help it seal cracks as it ages,” Monteiro says in a statement.