| Summary: | The early mechanical performances of low-calcium fly ash (FFA)-based geopolymer (FFA–GEO) mortar can be enhanced by soda residue (SR). However, the resistance of SR–FFA–GEO mortar to acid or sulfate environments is unclear, owing to the various inorganic calcium salts in SR. The aim of this study was to investigate the long-term mechanical strengths of up to 360 d and evaluate the resistance of SR–FFA–GEO mortar to 5% HCl and 5% Na<sub>2</sub>SO<sub>4</sub> environments through the losses in compressive strength and mass. Scanning Electron Microscopy (SEM), Energy-Dispersive Spectroscopy (EDS) and Fourier Transform Infrared Spectrometer (FTIR) experiments were conducted for the SR–FFA–GEO mortars, both before and after chemical attack, to clarify the attack mechanism. The results show that the resistances of the SR–FFA–GEO mortar with 20% SR (namely M10) to 5% HCl and 5% Na<sub>2</sub>SO<sub>4</sub> environments are superior to those of cement mortar. The environmental HCl reacts with the calcites in SR to produce CaCl<sub>2</sub>, CO<sub>2</sub> and H<sub>2</sub>O to form more pores under HCl attack, and the environmental Na<sup>+</sup> cations from Na<sub>2</sub>SO<sub>4</sub> go into Si-O-Al network structure, to further enhance the strength of mortar under Na<sub>2</sub>SO<sub>4</sub> attack. These results provide the experimental basis for the durability optimization of SR–FFA–GEO mortars.
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