Gypsum, also known as calcium sulfate, is used to add both calcium and sulfate ions to brewing water. Calcium comes into play in many chemical reactions during brewing, and proper levels are especially important for conducting a proper mash. Sulfate ions can add a crispness that accentuates hop bitterness. One gram per gallon of water will add about 60 ppm of calcium and about 150 ppm sulfate to brewing water. Typical dosage is 1 teaspoon in a 5 gallon batch.