The brownmillerite, also known as tetracalcium aluminoferrite (C(4)AF), represents a solid solution series of Ca-2(AlyFe(2-y))O-5 (0 >= y <= 1.33), which is one of the most important components in ordinary Portland cement. The hydration of brownmillerite in the presence of gypsum attracts massive attentions, but the hydration mechanism remains controversial nowadays. In particular, the influence of Al/Fe ratio on brownmillerite hydration with gypsum is obscure and out of favor. This paper studies the hydration characteristics of a binary system containing brownmillerite and gypsum with variables including two Al/Fe ratios and gypsum contents in an attempt to interpret the retarding mechanism of sulfate ions. When the molar ratio of brownmillerite to gypsum is 1.0, the hydration process of Ca-2(Al1.3Fe0.7)O-5 shows five distinct stages in the conductivity test, while only two stages are observed in the case of Ca-2(Al1.0Fe1.0)O-5. A decrease of Al/Fe ratio in brownmillerite leads to severe stagnation of the hydration process. When the molar ratio of Ca-2(Al1.0Fe1.0)O-5 to gypsum is increased to 2.7, the hydration is able to proceed and also displays five stages similar to Ca-2(Al1.3Fe0.7)O-5. An adsorption layer of sulfate ions on the surface of brownmillerite contributes to the postponement of brownmillerite hydration. In addition, the concentration of aluminum in the solution is the key to break this adsorption layer and dominate the hydration proceeding of brownmillerite.