Now, engineers have found an inclusion that helps ancient concrete self-heal cracks, and shown how we can recreate the recipe to make new buildings last longer.

Concrete is the most commonly used building material in the world, but it's not impervious to damage. Weather and stress can lead to tiny cracks, which can grow into much larger cracks that eventually threaten the integrity of the entire structure. That can require expensive maintenance or replacement to prevent catastrophic failure.

In contrast, ancient Roman structures have stood the test of time for more than two millennia. To find out how, scientists have long examined samples of the material under microscopes to study the composition and uncover the ingredients that bestow such strength.

Pozzolanic material, made from volcanic ash from a specific region of Italy, features prominently. So does lime, and in previous studies it was found that this helps concrete actually grow stronger over time in marine environments like piers. One common inclusion – millimeter-sized chunks of white minerals called lime clasts – are usually disregarded as a by-product, but in the new study, researchers found that they might be there for a reason.

"The idea that the presence of these lime clasts was simply attributed to low quality control always bothered me," said Admir Masic, lead author of the study. "If the Romans put so much effort into making an outstanding construction material, following all of the detailed recipes that had been optimized over the course of many centuries, why would they put so little effort into ensuring the production of a well-mixed final product? There has to be more to this story."