*2.4. Gypsum Waste (CaSO4*· *2H2O)*

Gypsum waste is a byproduct of the gypsum industry. It is widely used in dental applications, with its disposal representing a potential threat to the environment [129]. There is a possibility that when disposing of gypsum waste in landfill, there might be a reaction between the gypsum and biodegradable waste, which may produce poisonous and odorous hydrogen sulfide gas [130,131]. Gypsum waste can be processed for the recovery of CaS, which ultimately changes to calcium carbonate. A schematic diagram of the process of recovering calcium carbonate from gypsum waste is shown below in Figure 7, as reported by Yadav et al., 2021 [129].

**Figure 7.** Steps involved in the synthesis of calcium carbonate nanoparticles from gypsum waste, adapted from Yadav et al., open access journal Applied Sciences, 2021 [129].

Beer et al. [132] attempted to synthesize calcium carbonate nanoparticles from gypsum waste, producing elemental sulfur as a byproduct. In this process, the first step was the thermal reduction of the gypsum waste into calcium sulfide (CaS), followed by its direct aqueous carbonation, yielding low-grade carbonate products (i.e., 99 mass% as CaCO3) or precipitated calcium carbonate (PCC). The carbonate product was found to be predominantly composed of calcite (99.5%) with only 0.5% quartz. Calcite was the only CaCO3 polymorph obtained in the experimental process, while no vaterite or aragonite was found [133]. In addition, Mulopo and Radebe [129] investigated the batch recovery of calcium carbonate from gypsum waste slurry using sodium carbonate under ambient conditions. The results were applied to the pre-treatment of acid mine drainage (AMD) from coal mines. US patent no 2013/0288887 A1 reports a simple, cost-effective, and novel method for the recovery of nano-calcium carbonate from gypsum waste slurry [134]. Okumura et al. [135] extracted calcium oxide particles from the gypsum waste using reductive decomposition in a CO-CO2-N2 atmosphere. They also investigated the reductive decomposition of spent CaSO4 using a packed-bed reactor. The CaSO4 was used for the production of calcium oxide. Ramachandran and Maniam [136] reported a two-step method comprising chemical and thermal reactions for the regeneration of calcium oxide from gypsum waste. The chemical changes and confirmation of the formation of calcium oxide were determined using XRD. Mbhele et al. [137] attempted to recover sulphur from gypsum waste using the following sequence of steps: (1) reduction of gypsum to CaS2 (2) stripping of the sulphide with carbon dioxide gas, and finally (3) the production of S.
