Dry grinding of gypsum (CaSO4 . 2H(2)O) with hydrated minerals, such as talc, kaolinite or montmorillonite as an additive was conducted in a planetary ball mill in order to investigate the transformation of gypsum into its hemihydrate form (CaSO4 . 1/2H(2)O). This transformation of gypsum is accelerated by grinding and by the presence of hydrated minerals. The experimental results show that talc is the most effective additive, and the addition of 35 wt. % talc causes almost complete transformation after grinding for 2 h. The mechanism of the mechanochemical change of gypsum with the additives by mixed grinding is attributed to the transference of the water in the gypsum crystal structure into the additives.