Calcium sulphate dihydrate aka gypsum is added by brewers who use distilled or RO water to increase the calcium and sulphate ions in their brewing water. According to the literature 1 gram per 3.785 liters adds 62 ppm of calcium and 147 ppm of sulphates. It is also used to decrease the pH of the brewing water. Calcium chloride will also decrease the pH of the brewing water. If you want to increase the pH of the brewing water or the mash the the addition of calcium carbonate works well. To adjust the pH of the mash either way just add 1/2 teaspoon at a time, mix well, and check the pH after each addition. As a general rule of thumb good brewing water should be moderately hard. Soft water has low calcium and magnesium, which has been replaced with sodium or potassium.
