The secret ingredient in Roman concrete that means buildings can last for millennia

The Pantheon in Rome, which has stood for over 2,000 years, has long puzzled experts.

Scientists think they've found a secret ingredient in the ancient recipe that makes quicklime self-healing.

Adding quicklime to the mix causes a chemical reaction that leaves calcium deposits in the concrete

When cracks appear at a later stage, they cause calcium deposits to recrystallise into calcium carbonate, which fills in the gaps. The cracks are healed before they spread and compromise the integrity of the structure.

The world's largest unreinforced concrete dome in the Pantheon, which was dedicated in 128AD, is still intact despite many modern concrete structures crumbling over time.

The core of Hadrian's Wall is made of ancient concrete and still supplies Rome with water.

According to Pliny the Elder, concrete structures in harbours become a single stone mass, impregnable to the waves, and every day stronger.

Modern engineers may be able to build structures that can last a long time. Lime clasts are calcium deposits found in the ancient concrete. They were previously thought of as a result of sloppy mixing practices.

Admir Masic is a professor of civil and environmental engineering.

If the Romans put so much effort into making an outstanding construction material, following all of the detailed recipes that had beenOptimised over the course of many centuries, why wouldn't they make a well-mixed final product? There needs to be more.

To prove that the lime clasts were responsible for the longevity, the team cracked the concrete and ran water through it.

The cracks healed in two weeks and the water stopped flowing. A chunk of concrete made without quicklime never healed, and the water flowed through the sample.

Roman concrete is being brought back as a commercially viable product.

Prof Masic said it was exciting to think about how these more durable concrete formulations could expand not only the service life of these materials, but also how it could improve the resilience of 3D-printed concrete.

The research was published in a peer-reviewed journal.