This study evaluated the performance of geopolymer concretes based on a binary mixture of fly ash (FA) with blast furnace slag (GBFS) in an 80/20 ratio and activated with a mixture of sodium silicate and sodium hydroxide. FA/GBFS and portland cement (OPC) concretes were immersed in 5% by weight sodium sulfate and magnesium sulfate solutions. Volumetric expansion and mechanical resistance loss were measured, and the reaction products were characterized by X-ray diffraction and scanning electron microscopy. The highest levels of deterioration were found in the probes exposed to MgSO4, indicating the higher aggressiveness of this solution. After aging for 360 days in MgSO4, the FA/GBFS concrete expanded by 0.04% and decreased its mechanical resistance by 33%, compared to 0.08 and 48%, respectively, for similarly treated OPC concretes. However, the same exposure to Na2SO4 increased the observed FA/GBFS resistance while reducing the mechanical resistance of OPC by 30%. This relates to their respective expansion percentages of 0.0068 and 0.412% and the presence of expansive ettringite in the OPC concretes.