The properties of sodium sulfate-activated ground-granulated blast-furnace slag (shortened as slag) pastes under carbonation attack were analyzed and compared with the uncarbonated specimens in this paper. A slag with two different finenesses--namely, 250 and 500 m2/kg—was activated by sodium sulfate at two concentrations (1 and 3% Na2O equivalent). After the initial 28 days of curing, the hardened pastes were carbonated in 5% CO2 and relative humidity (RH) of 65% at 20 ± 1°C for 2, 4, and 12 weeks. The carbonation depth, compressive strength, and pH value of the carbonated specimens were measured and compared with the uncarbonated counterparts exposed to a natural concentration of CO2. X-ray diffraction, thermogravimetric analysis, and scanning electron microscopy were employed to characterize the reaction products and microstructure of both carbonated and uncarbonated alkali-activated slag (AAS) samples. The results indicated that the specimens prepared with the coarse slag and low Na2SO4 concentration (1% Na2O equivalent) showed the worst carbonation resistance. Increasing slag fineness has a leverage effect on increasing the carbonation resistance. However, increasing Na2SO4 concentration (3% Na2O equivalent) led to more notable carbonation resistance than increasing slag fineness. By combining the fine slag with high Na2SO4 concentration, almost no changes in the pH, carbonation depth, and compressive strength were noticed even after 12 weeks of carbonation, showing a superb resistance to carbonation attack.