The Romans were the first to achieve mastery in getting permanent cement and building strong arches. Romans produced cement from a mixture of lime and volcanic rock.
The species used for the underwater structures consisted of lime and volcanic ash, and the mortar thus obtained was mixed with the tuff and placed in wooden molds. After immersion in water, an immediate hot reaction occurred. The lime was hydrated and reacted with the ash. An extremely resistant cement was created.
Fortunately for us, the descriptions of used ash have survived. Vitruvius, engineer of the first emperor Augustus, and later Pliny the Elderly reported that the best cement for marine applications was made from volcanic ash from around the Bay of Naples. The one that is close to the modern city of Pozzuoli, called pucolana, was especially valued. We now know that this type of ash and volcanic rocks made of it occur in many places around the world. The latest research shows that thanks to the special way in which aluminum replaces silicon compounds, the Romans have managed to obtain an extremely durable cement.