Why has Roman harbour concrete survived 2,000 years of seawater when ours lasts decades?

Roman harbour concrete lasts because seawater, which destroys most modern concrete, kept building it up. The Romans mixed lime with volcanic ash from around Naples. When seawater seeped into tiny cracks, it reacted with minerals in the ash and grew a rare, stiff crystal called aluminous tobermorite, which modern concrete almost never forms. Instead of corroding the structure, the sea helped cement it.

The recipe was written down. Around 25 BC, the engineer Vitruvius recommended pozzolana, the volcanic sand of Pozzuoli, and specified one part lime to two parts ash for work underwater. Builders packed this mortar with chunks of volcanic rock into wooden forms in the sea, where the water set off a hot chemical reaction. The piers of the Roman port at Cosa are still in generally excellent condition after more than 2,100 years, while modern concrete in saltwater can deteriorate within decades.

That doesn't make Roman concrete simply 'better'. It was laid in chunks rather than poured, walls had to be thick, and it took decades to reach full strength. Modern concrete is good enough that the world uses about 19 billion tons a year. But Roman chemistry is now inspiring greener recipes: the Romans used far less lime, baked at 900 °C or less, while making Portland cement needs 1,450 °C and produces about seven percent of industry's carbon dioxide.