Roman Concrete

Roman concrete, known to the Romans as opus caementicium, was a hydraulic setting building material made from lime based cement mixed with volcanic ash or other aggregate. The Romans used it widely from around 150 BC onward, often facing it with brick or decorating it with stucco and fresco, and it could set even underwater, which made it valuable for harbors and other marine construction. Many Roman concrete structures, including the Pantheon's dome, the world's largest and oldest unreinforced concrete dome, still stand today, and 2023 research found that small lime deposits within the material called lime clasts let cracks self heal over time.