<p>An industrial by-product of the flue gas desulphurization process in coal-fired power plants—flue gas desulphurization gypsum (FGDG)—was used to produce a urea adduct. The urea adduct (calcium urea sulphate), a slow release fertilizer, was formed by reacting urea with partially dehydrated FGDG. The nutrient release from the adduct-bound urea is delayed, and ammonia losses to the atmosphere are reduced. Test methods and the thermal properties of calcium urea sulphate obtained by both granulation and mechanochemistry were investigated and discussed. A new, rapid, waste- and reagent-free method for measuring the degree of urea conversion to the adduct form was presented. Differential scanning calorimetry (DSC) and thermal properties of urea were used to calculate the degree of conversion of urea to the adduct. The presence of the urea adduct in the samples was confirmed by X-ray powder diffraction (XRPD). In addition, in situ changes in the samples during heating were studied using high-temperature X-ray powder diffraction (HT-XRPD) method.</p>