As an industrial waste characterized by huge volume and high alkalinity, red mud has become a serious environmental problem. The reuse of red mud has been explored in previous studies, including as building materials and for soil and waste water treatment. In this study, an innovation was made for the reuse of red mud to create a synergistic effect. Red mud was first used in flue gas desulfurization (FGD), and then the desulfurized red mud was again reused to make a geopolymer material. By using one type of original red mud and three types of fly ash, this study revealed that with high alkalinity and desulfurization capacity, the red mud could serve as an excellent FGD sorbent. After FGD, the sodium sulfate in the desulfurized red mud acted as a chemical activator for geopolymer made with class C fly ash. A 25% increase in strength was observed between the geopolymers with the red mud after FGD and with the original one. There are no significant benefits of FGD on the class F fly ash-based geopolymers and further study is required.