Water containing calcium ions often leads to scaling problems in hot-water pipes, heaters, and boilers. The building up of this scale deposit is a common and costly problem in many industrial processes. Recently, glasses with a special composition have shown the ability to remove some hazardous ions from waste solutions. In the present study, a 45SiO(2)-4P(2)O(5)-16B(2)O(3)-20Na(2)O-15K(2)O glass system was used to remove calcium ions from an aqueous solution. When this phosphosilicate glass was reacted in a solution containing calcium ions, the calcium ions were eliminated by forming calcium phosphate crystals on the glass surface. For this calcium removal process to occur, phosphorus ions must be supplied from the glass through ion leaching. Reaction under a dynamic condition and an increased surface area of the glass enhanced the removal rate of calcium ions from the solution. The calcium ion removal efficiency was also increased by the crystallization of the glass.