Calc-sinter

Calc-sinter (Sinter is an important category in German chemistry)
Term for a solid calcium carbonate deposit in a solution during or after the chemical reaction of precipitation. The deposit is found in caves, mineral springs and also - as a coating or even thick layer - in old aquaeducts or waterpipes. If it were more compacted (e.g. recrystallized) and deposited uniformly it could be called Travertine.