We Lost the Recipe for Concrete That Lasts 2,000 Years 😱 Modern concrete often cracks and fails within decades, but Roman structures like the Pantheon stand strong after millennia. Their secret was mixing volcanic ash with quicklime and seawater. This created a chemical reaction that allows the concrete to "heal" its own cracks over time. Roman builders in the 2nd century BCE discovered that mixing volcanic ash (pozzolana) with lime made a mortar that could set underwater. Vitruvius wrote down