This paper investigates the behavior of feldspar-based geopolymer mortars under nitric acid, sulfuric acid attack and seawater attacks and provides an understanding of their degradation mechanisms. Geopolymer mortars were cast using feldspar quarry waste (pegmatite) altered with 20–30 wt% of calcined clay and mixed with river sand, then, activated in an alkaline medium. The effect of alteration of calcined clay on the exposure of geopolymer mortars to physical and chemical attacks was investigated via visual observation, mass loss, mechanical strengths, Fourier Transform Infrared (FTIR), X-ray diffraction (XRD) and scanning electron microscope coupled with energy dispersive X-ray spectroscopy (SEM-EDS). From the results, the appearance of samples showed a degradation with the time of exposure and type of solution used from seawater to sulfuric acid. The compressive strength showed a loss after 90 days to seawater, nitric and sulfuric acids attack which decreased from 58.6 MPa to 30.8, 10.8 and 7.9 MPa, respectively. Similarly, significant deterioration of microstructure was observed after immersion in acid solutions, which is more pronounced in sulfuric acid compared to nitric acid. The results demonstrate an overall negative effect of feldspar-based geopolymer mortars due to the weak resistance of geopolymer gels formed under acid attacks. The outcomes of this study hold promise for tailoring geopolymer properties for various applications and provide a basis for further work in this area.