Limescale is a hard, chalky deposit, consisting mainly of calcium carbonate (CaCO3). It often builds up inside kettles, boilers, and pipework, especially those used for hot water since pipes containing hot water are usually the ones that come out of the boiler. It is also often found as a similar deposit on the inner surfaces of old pipes and other surfaces where hard water has flowed. Limescale also forms as travertine or tufa in hard water springs.