Calcium Sulphate (Gypsum) 100g is a brewing water treatment additive used to compensate for water chemistry deficiencies. It raises sulphate levels, which accentuates hop bitterness and dryness — the key mineral for pale ales and IPAs, and the basis of "Burtonisation". It's also used in sherry-style fermentation to improve clarity and flavour stability.
Add in small amounts (around 1–2g per 23 litres) to your brewing water. Test your water hardness first and use alongside other water treatment salts as needed. Read our full guide: Brewing Water Guide.