Ancient Roman concrete actually grows stronger and heals its own cracks when exposed to seawater

While modern concrete degrades over decades, Roman marine concrete has survived for 2,000 years due to a unique 'self-healing' mechanism. The recipe, which includes volcanic pozzolana and lime, facilitates the growth of rare alumino-tobermorite crystals. When seawater enters a crack, it reacts with the ash to grow these minerals, which expand by 20% to fill the void and reinforce the structure.