Presented within is a critical overview of the effect of sulfates on cement hydration, properties, and optimum sulfate content in Portland cement. Calcium sulfate is used in Portland cement to control the C 3 A reaction to induce the optimum hydration of C 3 S to occur. The amount of calcium sulfate in the Portland cement influences the hydration, rheology, setting, phase assemblage, porosity distribution, and strength in cementitious materials. If added in excess, it can also lead to durability problems, thus a better understanding is needed about the mechanisms of sulfate on C 3 A and C 3 S hydration. The optimum sulfate content is well known as a key pathway to produce workable, good strength, and durable concretes. Despite many years of research, questions regarding sulfate optimization remain. Further investigation on the influence of clinker and calcium sulfate characteristics, the use of different supplementary cementitious materials and chemical admixtures, and the curing conditions on the sulfate demand of Portland cement are needed. The main methods used to determine the optimum SO 3 content are reported, and the advantages and disadvantages are examined. Finally, recommendation for future research is also discussed.