These magnificent structures from ancient Rome have been preserved for many millennia, a testimony to the genius of Roman engineers who invented concrete.
How did they keep such colossal buildings as the Pantheon, which has the largest unreinforced dome in the world, and the Colosseum standing for over 2,000 years?
The history behind Italy’s most popular cultural site
Roman concrete has been proven to last longer than modern concrete in many instances. Modern concrete can be susceptible to deterioration within a matter of decades. Scientists behind a new study have discovered the secret ingredient that allowed Romans to build complex structures in difficult places like docks, sewers, and earthquake zones.
Researchers from Italy, Switzerland and the United States participated in the study. They analysed concrete samples dating back to 2000 years old that were found in a city wall in central Italy. These concrete samples are very similar to concrete from other parts of the Roman Empire.
The concrete’s ability to heal cracks caused by time, as well as white chunks (also known to be lime clasts), was discovered by the researchers. These white chunks were previously thought to be evidence of poor-quality raw materials or sloppy mixing.
Admir Masic, an associate professor of civil engineering at the Massachusetts Institute of Technology, said, “It was really hard to believe that ancient Roman engineers would not do a great job because they made careful choices when choosing and processing material.”
Masic said that “Scholars wrote down exact recipes and imposed these on construction sites (across all the Roman Empire).”
This new discovery could make concrete manufacturing more sustainable and potentially shake up society like the Romans did.
Masic stated that concrete allowed the Romans an architectural revolution. “The Romans were able create and transform cities into extraordinary places that are beautiful and enjoyable to live in. This revolution changed the way we live.
Tourists visit Rome’s Colosseum during June 2019. Credit to Eyes Wide Open/Getty Images
Concrete’s durability and lime clasts
Concrete is basically artificial stone or rock. It is made by mixing cement and a binding agent, typically made of limestone, water, fine aggregate, (sand or finely broken rock) and coarseaggregate (gravel or crushed rocks).
Roman texts suggested that slaked lime was used in binding agents. This is why scholars assumed that this was how Roman concrete was created, Masic stated.
Further research revealed that lime clasts were caused by the addition of quicklime (calcium dioxide) to concrete.
Further analysis revealed that the concrete was formed by lime clasts at extremely high temperatures. Hot mixing was the key to concrete’s durability.
Masic stated in a press release that hot mixing has two benefits. “First, when the overall concrete is heated to high temperatures, it allows chemistries that are not possible if you only used slaked lime, producing high-temperature-associated compounds that would not otherwise form. The second benefit is that the reaction times are significantly reduced at higher temperatures, which allows for faster construction. “
The team carried out an experiment to determine if the lime clasts are responsible for Roman concrete’s apparent ability of repair itself.
They created two concrete samples, one according to Roman formulas, the other according to modern standards. Then they deliberately cracked them. Two weeks later, water couldn’t flow through concrete with a Roman recipe. However, it was able to flow through concrete without quicklime.
The researchers found that lime clasts could dissolve into cracks, and then recrystallize when exposed to water. This can heal cracks caused by weathering. This self-healing ability could lead to more durable and sustainable concrete. According to the study, concrete’s carbon footprint could be reduced by such a move. Concrete is responsible for up to 8 percent of global greenhouse gas emissions.
Researchers believed that the strength of Roman concrete was due to volcanic ash from Pozzuoli (on the Bay of Naples) for many years. This type of ash was used to construct Roman concrete and was described by historians and architects as a key ingredient in concrete.