William Resinger
The Romans didn’t just conquer the Mediterranean with
legions. They conquered it with cement.
From the 2nd century BC onward, engineers along the Italian coast started using
a bizarre building material in their harbors at places like Cosa, Portus,
Baiae, and Caesarea Maritima. They mixed quicklime with volcanic
ash—pozzolana—and then poured the slurry straight into the sea inside giant
wooden forms.
You’d expect seawater to destroy it. Instead, it turned the mix into stone.
What’s strange is not that Roman concrete survived—it’s that it actually
improved over time. Cores drilled from submerged Roman breakwaters show that,
centuries after setting, new crystals had grown inside the concrete. Minerals
like aluminous tobermorite and phillipsite slowly formed as seawater seeped
through the mix, knitting cracks together and locking the structure even tighter.
In other words, Roman marine concrete was self-healing. Waves, salt, and time
didn’t weaken it; they helped finish the job.
Compare that with much of our modern concrete. We rely on Portland cement,
steel rebar, and fast construction. It’s strong in the short term, but
saltwater corrodes the steel, cracks spread, and entire structures can become
unsafe within a human lifetime. Roman harbors, built with no steel at all, have
been punched by storms for 2,000 years and are still sitting there like man-made
cliffs.
For centuries, the recipe was a mystery buried in ruined quarries and
half-translated texts. Only in the last couple of decades have geologists and
engineers seriously analyzed Roman harbor cores under microscopes and in labs.
Now, some researchers are experimenting with modern versions of Roman-style
concrete—low-carbon, volcanic-ash mixes that might last centuries and emit far
less CO₂ to produce.
The future of “green” construction might just depend on a technology Romans
invented to park their ships.
