The rehydration of anhydrite (CaSO₄) leads to the crystallization of gypsum (CaSO₄⋅2H₂O) has been widely studied. Different process parameter had been conducted on anhydrite from the flue gas desulphurization (fgd) sludge to identify its ability to be reused as synthetic gypsum. A mechanochemical process using a high-energy planetary mill would break down the particle size and enhance the reaction of Ca²+ and SO₄ ²− with water while additives modify the number of ions present in the sample during the hydration. Difference hydrothermal temperatures (80 and 130 °C) for 30 min were conducted to dehydrate the synthetic gypsum from fgd sludge to hemihydrate (CaSO₄⋅0.5H₂O) to identify its potential to replace natural gypsum during the plaster of Paris production. The sample without a mechanochemical process wasn’t able to crystallize a high amount of gypsum while sample with 5 × 10–4 mol−1 of H₂SO₄ additive was able to crystallize a high amount of gypsum from the fgd sludge. After undergoing the hydrothermal process, all of the samples were able to crystallize hemihydrate at low-intensity peak due to the favourable of hemihydrate crystal’s growth at plane [31-3] only, which leads to the preferred orientation of hemihydrate crystal.
