To advance the state of knowledge and the conventional method of processing soda residues/sludges, this work presents a novel design for the preparation of high-strength CaSO4 center dot 0.5H(2)O (a-gypsum). The two steps included are to (1) produce CaSO4 center dot 2H(2)O (dihydrate, DH) using the discharged soda residues and H2SO4 and (2) transform DH into a-gypsum in a mixed solution with additives at 368 K. A critical step was included to reduce the iron content of the discharged liquor. Thermodynamic properties and phase transition mechanisms were investigated for the solid-liquid systems that involve CaSO4. The composition of the obtained raw soda residues, containing mostly CaCO3 (similar to 60%), was quantified. The influence of the following parameters was studied on the obtained CaSO4 center dot 2H(2)O crystals: the duration of the reaction (1.5-3 h), temperature (333-353 K), solid/liquid ratio of sludge (1:2-1:4), concentration of acids (0.95-2.54 mol/L), and amount of seeded crystals added (5-10%). Marked effects on the evolution of particle sizes and morphological structures were examined. The produced a-gypsum crystals possess an elongated-prismatic or spherical structure, much favored for their excellent injectability and high compressive strength. The most optimal conditions deduced from these experiments can be applied to aid in real industrial design and for the scale-up of the mass production of a-gypsum.