Concrete is one of the most important and widely used construction materials in the world, especially in the last few centuries since the creation of Smeaton’s Tower, but it was also a material that built much of the ancient world.

Known as opus caementicium, the concrete used in Ancient Rome was seen as particularly strong and durable, with the material capable of feats that to this day have not been replicated.

For example, the Pantheon is still to this day the largest concrete dome without any form of reinforcement, and many concrete buildings from that era remain in incredible condition.

Researchers have spent decades trying to figure out what made Roman concrete so durable, particularly in conditions that are considered to be less than ideal for modern concrete mixes without additional reinforcement or additives.

It took until 2023 for the answer to be found, an answer that was originally believed to be a mistake or a byproduct of low-quality materials.

Roman concrete used volcanic ash as part of the quicklime mortar mix. Specifically, Roman concrete used pozzolana, a type of volcanic ash found in the Bay of Naples and commonly exported to the rest of the Roman Empire.

This material contained distinct white mineral chunks typically known as “lime clasts”, which were for a very long time dismissed as the result of sloppy mixing, but in practice created a type of self-healing system.

The lime clasts react to water and create a calcium carbonate solution that fills cracks when they emerge or strengthens the composite material, healing any cracks before they could spread.

This discovery, one several thousand years in the making, not only is important to historians curious as to how many Roman buildings stand to this day but also has major potential consequences for modern concrete construction.

A self-repairing concrete means buildings that can survive longer and therefore rely on less concrete throughout the lifetime of the structure.