Calcium carbonate (CaCO3) is a common substance found in rocks as the minerals calcite and aragonite (most notably as limestone, which is a type of sedimentary rock consisting mainly of calcite) and is the main component of pearls and egg shells.
In brewing calcium carbonate may be used during the mash to increase the mash pH (reduce acidity), however it may not increase mash pH as much as desired. Click hereto read a detailed analysis of calcium carbonate in brewing written by A.J. deLange.
In wine making calcium carbonate may be used to reduce acidity levels in must. It should only be used before fermentation and will reduce tartaric acid levels before malic or citric acids. It does not require cold stabilisation to precipitate out like potassium carbonate, but it will take longer and may affect flavour.
