Roman Concrete

Sunken structures off the Italian coast doesn't sound impressive but the marvel is in the material.

Roman concrete, also called opus caementicium, was a material used in construction until the fading of the Roman Empire. Roman concrete was based on a hydraulic-setting cement. Roman builders constructed seawalls and harbour piers. The concrete they used outlasted the empire.

The concrete, a mixture of volcanic ash and quicklime, has withstood the sea for two millennia.

Tests revealed a rare chemical reaction, with aluminous tobermorite crystals growing out of another mineral called phillipsite. The key ingredient proved to be seawater.

As seawater percolated in the cracks in the Roman concrete it reacted with phillipsite found in the volcanic rock and created tobermorite crystals.

Microscopic image shows the lumpy calcium-aluminum-silicate-hydrate (C-A-S-H) binder material that forms when volcanic ash, lime, and seawater mix. It is even stronger than when it was first mixed.

Caesarea Concrete Bath

The Romans mined a specific type of volcanic ash from a quarry in Italy. Modern seawalls require steel reinforcement. The Romans didn’t use steel. Their reactive concrete was strong enough.