Durability and Microstructural Evaluation of Geopolymer Mortars Exposed to Sulphuric Acid Using Industrial By-Product Fillers

Rapid urbanization and industrialization have increased atmospheric pollution, particularly via sulfur oxides (SOx) that form sulfuric acid and accelerate the degradation of cementitious materials. While Portland-cement systems have been widely studied, less is known about the acid resistance of geopolymer mortars. This study investigates the durability and microstructural evolution of metakaolin–red mud geopolymer mortars incorporating limestone, marble, and basalt powders as partial sand replacements (5, 10, and 15 wt %). Specimens were immersed in 3% H2SO4 for 30, 60, and 90 days, with performance evaluated via compressive and flexural strength, weight loss, and ultrasonic pulse velocity (UPV), alongside scanning electron microscopy (SEM), x-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR). After 90 days, the optimal basalt-filled mix (15 wt %) retained 84% of its initial compressive strength (46.8 MPa), compared with 61% for the control; mass loss decreased from 6.4% (control) to 3.2%, and UPV degradation was reduced by 35%. Microstructural analyses indicate denser gel phases and reduced microcracking in basalt- and marble-filled mixes. These results demonstrate that industrial by-product fillers can significantly improve sulfuric-acid resistance while supporting more sustainable binder technology.