Roman Concrete Mystery

Roman concrete has survived 2,000 years of saltwater exposure while modern concrete crumbles within decades. The secret: Romans mixed their concrete with volcanic ash (pozzolana).

When seawater seeps into a crack, it reacts with the ash to grow aluminous tobermorite crystals that seal the gap and strengthen the material over time. Modern concrete weakens in seawater; Roman concrete gets tougher. Engineers are now borrowing this chemistry, adding volcanic ash or fly ash to cut the carbon footprint and build concrete that could last centuries.