For centuries, engineers and scientists have wondered how the Roman structures such as the Pantheon and aqueducts still stand up to date after almost 2,000 years. The secret is in their unusual building material: pozzolanic concrete, a combination of volcanic ash and lime. Although this formula has been known for centuries to give Roman concrete its durability, recent studies have unveiled an even deeper secret lying behind the extraordinary resilience of Roman concrete.
In 2023, a group of scholars, headed by MIT researchers, discovered that Roman builders did not only use slaked lime (calcium hydroxide), as it was believed earlier. Instead, they added quicklime (calcium oxide) directly to the mixture with volcanic ash and water, a process they referred to as “hot mixing.” This process made the concrete mixture undergo high temperatures, which triggered chemical reactions that created heat-related compounds, which decreased the curing time and increased durability.
Importantly, this approach generated the small white lime clasts in the concrete. Previously regarded as evidence of bad mixing, these lime chunks are now known to be key to Roman concrete’s self-healing properties. When cracks develop, water interacts with the lime clasts, forming calcium carbonate that fills and closes the cracks—effectively enabling the concrete to heal itself.
Ancient techniques in tests verified these findings: concrete based on quicklime healed itself within two weeks, but modern concrete was still damaged.
The implications are vast. Not only have the scientists uncovered the long-standing secret of the Roman concrete’s longevity, but they might have also discovered an environmentally friendly, long-lasting substitute for modern building. The team is currently in the process of commercializing this ancient technology to be used in future infrastructure—perhaps changing the way we build and maintain structures for generations to come.
