Thermal pretreatment has proven to be an effective method to enhance the reactivity of red mud for geopolymer production. It enables the transformation of red mud into a cleaner and more value-added construction material. However, the inconsistent properties of red mud, such as variations in chemical composition, particle size, and amorphous content, present challenges and affect the efficacy of thermal pretreatment, limiting the scalability of red mud valorization. In this study, thermal pretreatment was applied to five types of red mud sourced from three refineries, and its efficacy on each type of red mud was examined. In the raw material phase, the particle size, specific gravity, chemical composition, and crystalline phase of each red mud before and after thermal pretreatment were characterized to examine the effect of thermal pretreatment. Further, the final product with ambient-cured geopolymer was synthesized by mixing each thermally pretreated red mud with ground granulated blast furnace slag. The mechanical properties of each resulting geopolymer were tested and compared to assess the differences between each thermally pretreated red mud. Lastly, alkali leaching tests were conducted on each thermally pretreated red mud to quantify the dissolution of Si and Al, and this was correlated with the performance of the geopolymer.