Roman Concrete

A study in the journal Science Advances found that calcium-rich mineral deposits called “lime clasts,” commonly found in Roman-era concrete, gave buildings and structures “a previously unrecognized self-healing capability.” The deposits are not found in modern concrete. Such deposits are viewed as impurities by today’s concrete manufacturing standards. Aided by spectroscopic examinations and high-resolution, multiscale imaging and chemical mapping, researchers showed how lime clasts were used by Roman concrete-makers. The team produced samples of “hot-mixed concrete” using Roman and modern methods. After the materials hardened, scientists cracked the samples and ran water through the cracks.

The sample using ancient mixing techniques completely healed within two weeks, and water no longer flowed through the material. Meanwhile the modern concrete without the lime-clast structure never healed, and the water kept flowing through the sample. The Roman process involved a highly chemically reactive form of lime called quicklime.

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

Roman concrete, '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 that outlasted the empire. Tests revealed a rare chemical reaction, with aluminous tobermorite crystals growing out of another mineral called phillipsite.

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

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.