The present invention relates to hydrophobic gypsum and to a process for making gypsum hydrophobic, in particular it relates to a powdered hydrophobing additive, a hydrophobic gypsum composition and method for preparing same using granules containing organosilicon compounds as hydrophobing additives. The invention provides a granulated additive for rendering gypsum material hydrophobic, comprising a particulate carrier on which is deposited an organosilicon component, a binder polymer and an emulsifier for the organosilicon component. Such a granulated hydrophobic additive is able to provide a high initial hydrophobicity to gypsum materials to which it is applied and that the obtained hydrophobicity can last during a long period of time.