The utilization of red mud (RM) as an aluminosilicate precursor (AP) for geopolymer production is attracting increasing research interest. This use of RM offers a desirable outlet for the increasing RM inventory and provides a sustainable solution for the immobilization of heavy metals and the valorization of waste as eco-friendly building materials. However, the effect of RM and its caustic residue on geopolymerization remains uncertain because of the extremely varied characteristics of RM. This study investigates the potential of using RM supplied from a local alumina refinery as an AP. First, RM was used to partially replace fly ash (FA) and the strength activity index (SAI) of the RM was evaluated. Second, to gain a deeper insight into the effect of caustic residue on geopolymerization, performances of traditional FA-based geopolymer and red-mud–fly-ash (RMFA)–based geopolymer under different NaOH concentrations were investigated and compared. Third, based on alkali leaching tests, the effective molar ratios considering the reactive content of RM and FA were developed, compared with conventional nominal molar ratio, and verified intensively using scanning electron microscopy–energy-dispersive spectroscopy (SEM-EDS).