This study investigates the durability of precast fly ash–based geopolymer concrete (GPC) exposed to an outdoor atmospheric environment for 8 years. Core specimens from GPC culverts are tested to determine the effect of carbonation, permeation properties, and poresize distribution, and the durability is compared with that of ordinary portland cement (OPC) concrete from the same exposure environment. It is found that the GPC has lower carbonation resistance than OPC concrete. According to mercury intrusion porosimetry (MIP) test results, the porosity of the GPC surface increased with carbonation under field-exposed conditions, whereas no significant changes occurred between laboratory-prepared carbonated and uncarbonated GPC specimens. The GPC produced sodium-based carbonation products that are soluble in water. The surface porosity of the GPC therefore increased, and this process accelerates the carbonation in field conditions. In addition, sorptivity test results correlate well with the MIP analysis and carbonation resistance. Therefore this study reveals that the fly ash–based geopolymer concrete is more susceptible to carbonation in an atmospheric environment.