Calcium carbonate – CaCO3 (Chalk).
Because of its limited solubility it is only effective when added directly to the mash. Use for making dark beers in areas of soft water. Monitor the mash pH with pH test papers to determine how much to add.
Brewing water pH and flavour can be adjusted (to a degree) by the addition of certain brewing salts. Unfortunately, the addition of salts to your tap water is not a matter of 2 + 2 = 4, it usually ends up being 3.9 or 4.1, depending on many factors. Water chemistry can be complicated; the rules contain exceptions and thresholds where other rules and exceptions take over.
There are a number of brewing water calculators available online and we suggest that you use one of these to calculate your salt additions to ensure correct water chemistry, and also to facilitate repeatability in your brews.