Why does a Roman harbour wall submerged in salt water for 2,000 years keep growing stronger while modern concrete piers built in the same sea crumble within decades?

Roman harbour walls poured before the birth of Christ are still gaining strength in salt water while modern piers spall and rust within decades. The reason is a hot-mixed recipe of quicklime and volcanic ash that lets seawater trigger its own repair. The post Why does a Roman harbour wall submerged in salt water for 2,000 years keep growing stronger while modern concrete piers built in the same…