In many cases, Roman concrete has proven to be more durable than its modern equivalents, which can deteriorate within a few decades. Scientists involved in a new study now claim to have uncovered a mysterious ingredient that allowed the Romans to make their building materials durable and to build sophisticated structures in challenging locations such as harbors, canals and earthquake zones.

The study team, which included scientists from the United States, Italy and Switzerland, analyzed two thousand-year-old concrete samples taken from a city wall at the archaeological site of Privernum in central Italy, which are similar in composition to other concrete found throughout the Roman Empire .

They found that the white bits in the concrete, referred to as lime clasts, give the concrete the ability to heal cracks that have formed over time. At the same time, these white pieces were previously overlooked as evidence of careless mixing or poor quality raw materials.

“It was really hard for me to believe that the ancient Roman engineers wouldn’t have done a good job because they really took care in choosing and processing the materials,” said study author Admir Masic, an associate professor of civil and environmental engineering at the Massachusetts Institute of Technology. “Scholars wrote down exact recipes and spread them throughout the Roman Empire,” added Masic.

The new insight could help make today’s concrete production more sustainable. “Concrete enabled the Romans to revolutionize architecture. They were able to create and transform cities into something extraordinary and beautiful for life. And this revolution basically completely changed the way people live,” explained Masic.

Lime clasts and durability of concrete

Concrete is essentially a man-made stone or rock that is created by mixing cement, a binder usually made from limestone, water, fine aggregate (sand or finely crushed rock), and coarse aggregate (gravel or crushed rock).

Roman texts indicated the use of slaked lime (where the lime is first mixed with water before mixing) as a binder. “That’s also why scientists assumed that this is how Roman concrete was actually produced,” said Masic.

However, with further study, experts concluded that the lime clasts were caused by the use of quicklime (calcium oxide) – the most reactive and dangerous dry form of limestone – in the mixing of concrete, not slaked lime.

Additional analysis of the concrete showed that the lime clasts formed at extreme temperatures consistent with the use of quicklime. Such hot mixing was the key to the concrete’s durability.

“The benefits of hot mixing are twofold,” Masic said in a press release. “First, when the aggregate concrete is heated to high temperatures, it allows for the formation of chemicals that would not occur if you were to use only slaked lime, and creates compounds associated with high temperatures that would not otherwise form. Second, this increased temperature significantly reduces curing and setting times because all reactions are accelerated, allowing for much faster construction.”

To find out if lime clasts were really responsible for the longevity of Roman concrete, the team conducted an experiment.

They made two samples of concrete, one according to the Roman formula and the other made according to modern standards, and deliberately cracked them. After two weeks, water could not pass through the concrete made according to the Roman formula, while the piece of concrete made without slaked lime passed without any problems.

The findings suggest that lime clasts can dissolve in cracks and recrystallize when exposed to water, sealing weathering cracks before they widen. According to the scientists, this self-regenerating potential could open the way to the production of more durable, and thus more sustainable, modern concrete. Such a move would reduce concrete’s carbon footprint, which accounts for up to 8% of global greenhouse gas emissions, according to the study.

For years, scientists believed that Roman concrete was so strong thanks to the volcanic ash from the Pozzuoli area on the Gulf of Naples. This type of ash was transported throughout the vast Roman Empire for use in construction, and was described as a key ingredient in concrete in the accounts of contemporary architects and historians. Masic explained that both components are important, however, lime has been overlooked in the past.