Identification of Calcium Sulphoaluminate Formation between Alunite and Limestone

This study was carried out to identify the conditions of formation of calcium sulphoaluminate (3CaO·3Al(2)O(3)·CaSO(4)) by the sintering of a limestone (CaCO(3)) and alunite [K(2)SO(4)·Al(2)(SO(4))(3)·4Al(OH)(3)] mixture with the following reagents: K(2)SO(4), CaCO(3), Al(OH)(3), CaSO(4)·2H(2)O, and SiO(2). When K(2)SO(4), CaCO(3), Al(OH)(3), CaSO(4)·2H(2)O were mixed in molar ratios of 1:3:6:3 and sintered at 1,200∼1,300 °C, only 3CaO·3Al(2)O(3)·CaSO(4) and calcium langbeinite (2CaSO(4)·K(2)SO(4)) were generated. With an amount of CaO that is less than the stoichiometric molar ratio, 3CaO·3Al(2)O(3)·CaSO(4) was formed and anhydrite (CaSO(4)) did not react and remained behind. With the amount of CaSO(4) that is less than the stoichiometric molar ratio, the amounts of 3CaO·3Al(2)O(3)·CaSO(4) and 2CaSO(4)·K(2)SO(4) decreased, and that of CaO·Al(2)O(3) increased. In the K(2)SO(4)-CaO-Al(2)O(3)-CaSO(4)-SiO(2) system, to stabilize the formation of 3CaO·3Al(2)O(3)·CaSO(4), 2CaSO(4)·K(2)SO(4), and β-2CaO·SiO(2), the molar ratios of CaO: Al(2)O(3): CaSO(4) must be kept at 3:3:1 and that of CaO/SiO(2), over 2.0; otherwise, the generated amount of 3CaO·3Al(2)O(3)·CaSO(4) decreased and that of gehlenite (2CaO·Al(2)O(3)·SiO(2)) with no hydration increased quantitatively. Therefore, if all SO(3)(g) generated by the thermal decomposition of alunite reacts with CaCO(3) (or CaO, the thermal decomposition product of limestone) to form CaSO(4) in an alunite- limestone system, 1 mol of pure alunite reacts with 6 mol of limestone to form 1 mol of 3CaO·3Al(2)O(3)·CaSO(4) and 1 mol of 2CaSO(4)·K(2)SO(4).