While quarrying limestone on the slopes of Mt. Vesuvius, ancient Roman engineers stumbled upon a naturally occurring cement created by the extreme temperatures and pressures of the volcano. They discovered that the silica-alumina substance, when mixed with limestone and burned, would harden underwater as well as in the air, unlike mortar. They also found that the material was much harder and stronger than ordinary lime mortar. This material, which contained the essential elements of today's man-made portland cement, changed the character of Roman construction. Initially, columns, arches and vaults were built with masonry facings, then filled internally with the Vesuvian cement. Later, the plasticity and moldability of wet cement was exploited for the construction of new architectural forms: Roman
