Is ancient Roman concrete better than today’s? Most modern concrete starts with Portland cement, a powder made by heating limestone and clay to super-high temperatures and grinding them up. The ancient builders mixed materials like burnt limestone and volcanic sand with water and gravel, creating chemical reactions to bind everything together. Now, scientists think they’ve found a key reason why some Roman concrete has held up structures for thousands of years: The ancient material has an unusual power to repair itself. Even though Roman concrete lasted a long time, it couldn’t hold up heavy loads: “You couldn’t build a modern skyscraper with Roman concrete,” Oleson said.