Researchers find ancient Roman architecture’s ‘self-healing’ properties which made it survive for so long

The structures made by ancient Romans have survived millennia, the fall of an empire, witnessed wars and are standing tall to this day as a testament to the genius of their engineers and architects, but how they managed to do so remained a mystery. However, research published on Friday (January 6) in the journal Science Advances has found the answer and it is in the material used to create these majestic structures.

The team, which included researchers from the United States, Italy and Switzerland, found that the technique used to make concrete durable might be due to its “self-healing” properties. Take Pantheon for example, which nearly 2,000 years later, still stands as the world’s largest dome of unreinforced concrete, which “would not have existed without the concrete as it was in the Roman time”, said Admir Masic, lead author of the paper and an Massachusetts Institute of Technology (MIT) professor of civil and environmental engineering.

The researchers analysed 2,000-year-old concrete samples collected from a wall at the archaeological site of Privernum, Italy, the structure which is said to have a similar composition to that of concrete found through the Roman Empire and found pozzolanic concrete. It is a durable building material which contributes to the strength of Roman architecture and the main ingredient of which is named after the Italian city of Pozzuoli, on the Bay of Naples.

The study led by the Massachusetts Institute of Technology (MIT) found that the durability and “self-healing” properties of the concrete came from its ingredients, pozzolana from Pozzuoli which is a mix of volcanic ash found in the city and lime. According to the research, when mixed together with water they combined to form a strong concrete. “They knew that was a great material, but they probably didn’t know that it would last thousands of years,” said Masic.

Additionally, the team also found that the white chunks on the surface of Roman architecture which are not found in modern concrete formulations, originate from lime and are called “lime clasts” formed due to the composition of the concrete. Notably, what was previously dismissed as a consequence of sloppy mixing practices or low-quality raw materials is actually what contributes to the previously unknown self-healing capability of this type of concrete.

“The idea that the presence of these lime clasts was simply attributed to low-quality control always bothered me,” said Masic. However, upon further inspection with high-resolution multiscale imaging and chemical mapping techniques, the team discovered different forms of calcium carbonate in these lime clasts which happen in places where water is not readily available.

They concluded that they arose because of the use of quicklime, which is the most reactive form of limestone and was created by limestone being heated at high temperatures. This would also be the most dangerous form of limestone, however, once mixed with water it produces slaked lime or calcium hydroxide which is a slightly less reactive form which the researchers first thought was mixed with pozzolana and then used by ancient Romans.

However, the team’s analysis later found that this was not the case and the method was inconsistent with their findings. Therefore, the research suggests that the Romans opted for a process called “hot mixing” which would use quicklime directly with the pozzolana and water at extremely high temperatures. “The benefits of hot mixing are twofold,” said Masic.

He added, “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. Second, this increased temperature significantly reduces curing and setting times since all the reactions are accelerated, allowing for much faster construction.”

This would be in addition to the lime clasts’ self-healing abilities. According to the study, when a crack in the structure occurs it travels through these lime clasts whose reaction with water leads them to form a calcium-rich solution which seals the damage and prevents it from spreading any further.