Roman concrete — opus caementicium — has attracted enormous attention in recent years, largely because modern Portland cement structures routinely deteriorate within decades while Roman harbors and domes remain structurally sound after two thousand years. The Pantheon's unreinforced concrete dome is the most famous example, but the harbor at Caesarea Maritima is equally remarkable.

The key ingredient appears to be volcanic ash (pozzolana) mixed with seawater and lime. UC Berkeley research published around 2017 identified the mechanism: seawater infiltrating cracks deposits aluminous tobermorite crystals in the voids, actually strengthening the concrete over time rather than degrading it. Modern concrete, by contrast, cracks and allows corrosive saltwater to reach steel reinforcements.

The Romans didn't understand the chemistry, of course — they knew empirically that ash from the Pozzuoli region produced better results, and Vitruvius documented the recipes. The irony is that the ingredient that makes Roman concrete so durable is precisely what's unavailable at scale for modern use: high-quality volcanic ash from the right geological formations isn't universally accessible.

There's ongoing research into whether the volcanic ash component can be approximated with modern materials, but the short answer is: Roman concrete is better because it's a fundamentally different chemistry, not just a better version of what we use now.