Prepn. of anhydrite-gypsum mixts. - using activator and water including mixing compacting calcining wetting crushing

Method of preparing a homogeneous mixt. of anhydrite and gypsum in the form of compacted elements, from finely divided synthetic gypsum from desulphurisation of waste gases or from phosphoric acid mfr. is described in which (a) an activator is added to a CaSO4 based material brought to neutrality; (b) the mixt. is compacted without binders addition; (c) the material is stored for at least 20 h.; (d) the mixt. is crushed to 0-16mm particles, pref. obtaining a granulometery comprising 10-40% below 0.2mm, 20-45% 0.2-1mm; 15-40% 1-3.15 mm, 5-30% 3.15-16mm; the mixt. comprising 70-92 wt.% anhydrite, 8-25% gypsum and having an initial setting time below 12 mins. Specifically, before storing, the mixt.is calcined at 400-700 deg.C to form anhydrite II, then 1.7-5.25 wt.% water is added to this after coating to ambient temp. The mixt. is used in mixing for construction of barriers accompanying the working face, and supporting pillars for back filling and consolidation. The product yield is greater than in prior art (DS2904071).