Abstract: A curable gypsum-based composition for the production cured gypsum matrix. The composition comprises two-components, the first component includes calcined gypsum suspended in water, and a set retarding substance comprising (i) an organic acid containing at least two acid groups selected from the group consisting of carboxyl, sulphate, sulfonate, phosphate and phosphonate, the acid optionally also containing at least one hydroxy group per molecule; and/or (ii) inorganic anions selected from the group consisting of polyphosphate, polyborate and mixtures thereof. The second component comprises a set accelerating substance including water-soluble salts of multivalent metal ions. The water soluble salts in the second component form stable precipitates or complexes with the organic acid in the first component. The second component may also comprise easily soluble salts of ammonium and/or cations from the first group of the periodic table of the elements, which will accelerate the hardening process.