Raises Residual Alkalinity and mash pH

Also known as chalk, Calcium Carbonate is suitable for brewing dark beers, which require a CO32- concentration of at least 150 ppm. It is used for increasing Residual Alkalinity in the brewing water as well as raising the mash pH. Additions can be put in the brewing water or directly into the mash.
 
1 gram of CaCO3 per litre of water contributes:
400 ppm Ca2+
600 ppm Total Alkalinity as CaCO3
20 mEq/L Alkalinity as CO3-2


Calcium (Ca)
Calcium is the primary ion determining the “permanent hardness” of the water. Calcium plays multiple roles in the brewing process including lowering the Ph during mashing, aiding in precipitation of proteins during the boil, enhancing beer stability and also acting as an important yeast nutrient. Calcium levels in the 100 mg/l range are highly desirable, and additives should be considered if your water profile has calcium levels below 50 mg/l. The range 50mg/l to 150 mg/l is preferred for brewing.
 
Carbonate and Bicarbonate (CO3 and HCO3)
Carbonate is considered the most important ion for all grain brewing. Carbonate (or bicarbonate), expressed as “total alkalinity” on many water reports, is the ion that determines the acidity of the mash. It also is the primary determinant in the level of “temporary hardness” of the water. If carbonate levels are too low, the mash will be too acidic, especially when using darker malts (which have higher acidity). If carbonate is too high, mash efficiency will suffer. Recommended levels are 25-50 mg/l for pale beers and 100-300 mg/l for darker beers. Note that bicarbonates and temporary hardness can be reduced by pre-boiling the water – the precipitate that falls out after boiling is primarily bicarbonate.

 
Adjusting your Water
Different styles of beer require different water profiles. Often a particular beer is associated with the water profile of the city in which the beer originated. For a listing of water profiles for popular brewing cities of the world, you can visit BeerSmiths water profile listing. If you have a target profile in mind, you can adjust your water to match that profile.
You can dilute your local tap water with distilled water if some ion counts are too high for your target water profile. Similarly you can use additives to increase the level of key ions. Popular additives include table salt (NaCl), Gypsum (CaSO4), Calcium Chloride (CaCl), Epsom Salts (MgSO4), Baking Soda (NaHCO3), and Chalk (CaCO3).
Unfortunately the additives do not add a straightforward amount of ions to the water profile, so its best to use some kind of water profile tool to adjust your local water supply to reach a target profile. Usually only a few grams of additives is required to achieve your target profile. BeerSmith has a water profile tool available to perform this very function. Other water profile tools are also available online.