This study investigates the carbonation behaviour of alkali-activated fly ash-based (AFA) concretes. AFA contains 20.67% of unburned carbon. GBFS and OPC were used as sources of calcium (20%). A mixture of NaOH and Na2SiO3 was used as activator. OPC concrete was used as reference material. AFA/GBFS, AFA/OPC and OPC concretes, with and without reinforcing steel, were exposed to accelerated carbonation in a chamber under controlled conditions (25 °C, 65% RH, 1% CO2). In the non-reinforced concrete, the carbonation depth and compressive strength were evaluated up to 360 days. FTIR and SEM were used as complementary tests. Given that carbonation is one of the causes of corrosion in reinforcing steel, the electrochemical behaviour of steels embedded in alkali-activated concrete was evaluated using half-cell potential test, polarization curves and linear polarization resistance techniques. According to the obtained results, AFA/GBFS and AFA/OPC concretes are more susceptible to carbonation than OPC with compressive strength losses of up to 40%. Regarding the corrosion behaviour of the reinforcing steel, the AFA/GBFS concrete showed very negative potentials and high corrosion densities; however, at the end of the exposure period (20 months), the reinforcing steel showed no signs of deterioration, unlike the steel embedded in the OPC concrete.