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Carbonation of cementitious materials is one of main causes of reinforcement corrosion and CO₂ diffusivity influenced by microstructural characteristics of the cementitious materials is a decisive parameter for the carbonation rate. This study focused on establishing a multifactor functional model to calculate the CO₂ diffusivity of carbonated cementitious materials. Because CO₂ gas flows through carbonated zone, it is necessary to estimate CO₂ diffusivity of carbonated concrete. Many factors on the CO₂ diffusivity, such as the diffusivity in vapor, tortuosity, microstructural characteristics of cement paste, contribution of aggregate, and reduction of porosity due to carbonation, were considered. Apparent and effective CO₂ diffusivity were calculated according to the absence or presence of moisture in the pore system of concrete, and the results were compared with previous research.