Effects of red mud, desert sand, and ground granulated blast furnace slag on the mechanical properties and microstructure of fly ash-based geopolymer

The development of sustainable alternatives to traditional Portland cement is critical in reducing the global carbon emissions. This study explores the influence of red mud (RM), desert sand (DS), and ground granulated blast furnace slag (GGBFS) on the mechanical and microstructural properties of fly ash-based (FA) geopolymer. An orthogonal test is employed to optimize the mix ratio by varying the NaOH concentration, the Na₂SiO₃/NaOH mass ratio, and the alkali-activating solution to fly ash mass ratio (AAS/FA). The results indicate that the optimal mix—consisting of 16 mol/L NaOH, a Na₂SiO₃/NaOH mass ratio of 3.0, and an AAS/FA ratio of 0.40—achieves the highest compressive and flexural strength. The inclusion of GGBFS markedly enhances the geopolymers' mechanical properties, with 20 % GGBFS content resulting in an 83.03 % increase in compressive strength and a 93.75 % increase in flexural strength after 28 days. Microstructural analyses reveal that the addition of RM, DS, and GGBFS improved the density and reduced the porosity of the fly ash-based geopolymers, thereby enhancing their physical properties. The study demonstrates that the integration of these industrial by-products not only enhances the properties of fly ash-based geopolymer but also presents an efficient strategy for waste utilization. This approach advances environmentally sustainable construction materials, supporting global efforts to reduce carbon emissions.