Precipitated Chalk (Calcium Carbonate) is used to reduce the acidity in wine. Added at the rate of around 1½ teaspoons per gallon, it will reduce the acidity by 1½ parts per thousand. It is also useful for adjusting the sharpness of homemade cider.
When it is added to the must, it will start to foam because of the gas given off by the chemical reaction, so it should only be used in a vessel that has sufficient free headroom to "froth up" without overflowing.
Some home brew recipes used by All Grain brewers, also recommend the addition of 1 teaspoon per 5 gallons to the mash if making Irish Stouts in a "soft water" area.