This study investigated the mechanical strength and durability of alkali-activated binders composed of blends of fly ash (FA) and ground granulated blast furnace slag. Five samples with FA/slag ratios of 100/0, 75/25, 50/50, 25/75, and 0/100 by mass were employed to prepare alkali-activated FA/slag (AAFS) concrete. Sodium oxide (Na2O) concentrations of 6% and 8% of binder weight and activator modulus ratios (mass ratio of SiO2 to Na2O) of 0.8, 1.0, and 1.23 were used to prepare alkaline activators. Test results revealed that higher slag contents, Na2O concentrations, and activator modulus ratios increased the compressive strength and splitting tensile strength of AAFS concrete. The total charge passed through AAFS concrete was between 2500 and 4000 C, higher than that passed through reference ordinary Portland cement (OPC) concrete. However, AAFS concrete demonstrated higher performance than that of OPC concrete when exposed to sulfate. According to scanning electron microscopy observations, the main hydration products of AAFS concrete were amorphous alkaline aluminosilicate and low-crystalline calcium silicate hydrate gel. As the slag content increased, the amount of C-S-H gel increased and that of A-S-H gel decreased. According to the results, 100% slag-based AAFS concrete with a Na2O concentration of 8% and activator modulus ratio of 1.23 offers superior performance.

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