The carbonation resistance of a blended slag and low-calcium fly ash (FA) geopolymer concrete was investigated. The geopolymer binder studied was composed of 90% low-calcium FA and 10% ground granulated blast-furnace slag. The alkalinity of the pore solution plays a pivotal role in carbonation progression and subsequent corrosion initiation. pH profiles were measured to assess the pore solution alkalinity. Phenolphthalein indicator was used to measure the carbonation depth. X-ray diffraction (XRD) and quantification were carried out to identify and quantify the carbonation products. The obtained pH profiles illustrated a wider semi-carbonation zone in the geopolymer specimens, although the pH drop was insignificant in most cases. XRD analysis revealed that nahcolite mainly formed at 3% carbon dioxide concentration and led to a significant drop in pH values. The results further demonstrated that 1% accelerated carbonation replicated the natural carbonation process well, with only natron identified as a carbonation product. This work contributes to the assessment of the risk of carbonation-induced reinforcement corrosion in low-calcium FA geopolymer concrete.