Roman concrete has self-healing properties modern concrete lacks

Roman harbour structures built 2,000 years ago are still intact, while modern concrete crumbles in decades. Scientists discovered that seawater reacts with a rare mineral in Roman cement called tobermorite, growing interlocking crystals that seal cracks before they spread. Modern Portland cement doesn't contain this mineral and simply fails once cracked.

Source: Masic et al., 'Seawater as the Key to the Longevity of Roman Concrete', Science Advances (2023)