This study was carried out to identify the conditions of formation of calcium sulphoaluminate (3CaO·3Al
2O
3·CaSO
4) by the sintering of a limestone (CaCO
3) and alunite [K
2SO
4·Al
2(SO
4)
3·4Al(OH)
3] mixture with the following reagents: K
2SO
4, CaCO
3, Al(OH)
3, CaSO
4·2H
2O, and SiO
2. When K
2SO
4, CaCO
3, Al(OH)
3, CaSO
4·2H
2O were mixed in molar ratios of 1:3:6:3 and sintered at 1,200~1,300 °C, only 3CaO·3Al
2O
3·CaSO
4 and calcium langbeinite (2CaSO
4·K
2SO
4) were generated. With an amount of CaO that is less than the stoichiometric molar ratio, 3CaO·3Al
2O
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
2O
3·CaSO
4 and 2CaSO
4·K
2SO
4 decreased, and that of CaO·Al
2O
3 increased. In the K
2SO
4-CaO-Al
2O
3-CaSO
4-SiO
2 system, to stabilize the formation of 3CaO·3Al
2O
3·CaSO
4, 2CaSO
4·K
2SO
4, and β-2CaO·SiO
2, the molar ratios of CaO: Al
2O
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
2O
3·CaSO
4 decreased and that of gehlenite (2CaO·Al
2O
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
2O
3·CaSO
4 and 1 mol of 2CaSO
4·K
2SO
4.
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