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 (SO_x) 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.