Originally, a tuff found near Pozzuoli, in southern Italy, that was used in mortars in Roman times. Today the most widely used pozzolan is pulverized fly ash. A pozzolan is any substance containing silicic acid (H4SiO4) that has the properties of a cement when mixed with calcium hydroxide (Ca(OH)2). The cement is a calcium silicate hydroxide (CaH2SiO4.2H2O):