<p>This study investigated the compressive strength, pH value, and total chloride ion concentration distribution of one-part fly ash geopolymer cement mortars under three different conditions: immersion in chloride salt solution followed by carbonation, carbonation with standard curing followed by chloride salt immersion, and chloride salt immersion-drying-carbonation cycles. Combined with XRD and SEM, the interaction mechanism of carbonation and chloride ion penetration on the durability of one-part fly ash geopolymer was clarified. The experimental results indicate that immersion in chloride salt solution leads to the formation of Friedel’s salt, which refines the surface pore structure and thus slows down carbonation. Carbonization promotes the compactness of the structure, thereby slowing down the erosion of chloride ions. In the chloride salt immersion-drying-carbonation group, the fluctuation range of compressive strength decreases. With an increasing number of cycles, due to the dissolution of Na₂CO₃ and the saturation of the carbonation reaction, the mortar structure becomes loose, resulting in an increase in the amplitude of strength fluctuations. The chloride salt immersion-drying-carbonation group shows a higher overall pH distribution than the water immersion-drying-carbonation group, because the chloride immersion improves the microstructure. Furthermore, the total chloride ion concentration distribution in the chloride salt immersion-drying-carbonization group increased due to the compactness of the structure and the increase in the number of dry–wet cycles, resulting in an increased fluctuation range of chloride ion concentration.</p>