This study characterizes alkali-activated fly ash/slag (AFS) blends with reactive MgO under an accelerated carbonation condition. XRD, DTG, FT-IR, solid-state 29Si, and 27Al MAS NMR analyses were conducted to investigate the microstructural and gel phase changes caused by the addition of MgO under a high CO2 condition. The addition of reactive MgO to the AFS blends confirmed the formation of a hydrotalcite-like phase with a higher Al uptake level rather than Al substitution for Si in C-S-H gel. The incorporated MgO reacted with CO2 to precipitate as magnesium carbonate and an amorphous hydrotalcite-like phase, which mitigated the carbonation of the C-S-H gel by retarding the penetration of CO2. The carbonated samples showed the presence of aluminosilicate gel with Q3 and Q4 sites stemming from the decalcification of C-(A-)S-H gel and highly cross-linked C-(N-)A-S-H gel.