Powerful X-ray beams have helped explain the extraordinary longevity of Roman monuments. The discovery, published in the Proceedings of the National Academy of Sciences, offers hope for eliminating one of the most intractable sources of greenhouse gas emissions while also helping buildings endure.
Ancient Roman monuments such as the Colosseum are vital to the tourism economy of many a city. Yet scientists have long been puzzled by how they are still standing. These objects have survived earthquakes, human invasion and thousands of years of storms that, in recent years, have been compounded by often highly acidic rain with industrial facilities nearby.
Modern buildings may survive rather less well, for all our advanced engineering techniques. However, there is another important reason to learn the Romans' secret.
Portland cement, the most commonly used ingredient in modern concretes and mortars, requires the release of large amounts of carbon dioxide in its creation. The main ingredient is lime, usually obtained through energy intensive mining of limestone, and the high temperature of manufacturing releases carbon dioxide—along with some of the very pollutants that cause acid rain which then destroys buildings and nature alike.
With 19 billion tonnes of concrete used each year, Portland cement accounts for 5% of anthropogenic global carbon emissions.
The Romans used a different method. Roman concrete, also called Opus caementicium, literally built the infrastructure of the Roman empire, including the famously straight and enduring roads. They also used lime, but much less of it than we do, and their furnaces were less than two-thirds as hot, drastically slashing fuel consumption.
While parts of their formula have come down to us from writers such as Pliny the Elder, for years no one could replicate their methods or produce anything that promised the same endurance.
Recent studies have provided pieces of the puzzle, and experiments have been conducted to replicate Roman methods in order to slash concrete's environmental footprint.
Now another piece has been added using the Advanced Light Source (ALS) synchrotron at the Lawrence Berkeley National Laboratory. The ALS's powerful X-rays revealed the essential role of crystalline binding hydrate in the concrete from Trajan's Market.
"The mortar resists microcracking through in situ crystallization of platy strätlingite, a durable calcium-alumino-silicate mineral,” says Dr. Marie Jackson of the University of California, Berkeley, and first author of the study. The pattern is very different from what is observed in concrete made with Portland cement, which allows cracks to combine and grow.
The minerals prevent small cracks from growing into larger ones. This, in Jackson's words, “Enables the concrete to maintain its chemical resilience and structural integrity in a seismically active environment at the millennial scale."
The mortar in Trajan's Market concrete was made from 88% volcanic rock. A reproduction made by Jackson's team replicated the resistance to fracturing observed in the 1,900-year-old original, suggesting the team is closing in on their goal of cheap and durable concrete at a fraction of the environmental consequences of that in use today.
5