Limescale

Limescale is a hard, chalky deposit consisting mainly of calcium carbonate that often builds up inside kettles, boilers, and pipework, especially where hot water flows, since pipes carrying hot water are usually the ones leading out of a boiler. It is also commonly found as a deposit on the inner surfaces of old pipes and other surfaces where hard water has flowed, and in hard-water springs it forms naturally as travertine or tufa. This description is adapted from Wikipedia contributors under CC BY-SA 4.0; changes were made. https://creativecommons.org/licenses/by-sa/4.0/