There is a growing demand for alternative low carbon binders. For these relatively new binder systems to be well received in industry it is important that detailed understanding of their durability performance is available. This paper investigates the hydrochloric acid resistance of geopolymer binders containing 100% fly ash and various blends of fly ash and blast furnace slag. Both physical and microstructural properties changes were examined to assess acid resistance and the leaching behaviour was also monitored. As the slag content of geopolymer binders increased, the resistance to hydrochloric acid also increased, evidenced by the reduced mass and strength loss observed. Findings also illustrate that geopolymer binders have hydrochloric acid resistance which exceeds that of traditional Portland cement binders in terms of the mass losses observed. The process of hydrochloric acid attack for each binder is also studied. • Fly ash/slag geopolymers had improved resistance to HCl than Portland cement. • As the slag content increased the resistance of geopolymers also increased. • Neat fly ash mixes had high apparent acid penetration but low surface deterioration. • Fly ash/slag geopolymers underwent decalcification of C-A-S-H type gel. • Neat fly ash mixes experienced dealumination of N-A-S-H gel.