The competency of CaS in reducing SO2 into pure sulphur was well-validated in past research. However, all pertinent past works monotonically derived CaS from CaSO4 or CaSO4-based materials while overlooking the potential of other Ca-materials in reducing SO2. Current study closes such research gap by assessing the theoretical feasibilities of converting CaCO3, Ca(OH)(2) and CaO into CaS via minimization of Gibbs energy. Significantly, two different synthesis paths, namely solid-solid and gas-solid reaction, were conceptualized and assessed herein. In solid-solid synthesis, the formation of CaS is accomplishable in S-8-sulphidization of Ca(OH)(2) and CaO, with the former one attained lower CaS yield due to low equilibrium conversion. Whereas, in gas-solid operation, H2S appeared to be the strongest sulphidizing agent, with the potential of attaining 100% CaS yield from all three Ca-based precursors, followed by S-2 (75 % CaS yield) and SO2 (25 %) under appropriate condition. Auspicious CaS formations are commonly attained at low temperature operation, typically < 1218.15 K for all Ca-precursors. As temperature surpasses 1218.15 K, CaS formation was pronouncedly suppressed, bestowed to the unfavourable thermodynamic restriction. Significantly, the findings in current study revealed the thermodynamic hurdles in synthesizing CaS from different Ca-species, concurrently propagating CaS-mediated SO2 reduction in future studies. (C) 2021 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.