In this study, the prediction model of carbonation progress rate of concrete using fly ash as an addition is developed, in which the reduction of not only the diffusion rate of CO_2 but also the amount of Ca(OH)_2 by pozzolanic reaction of fly ash. The model is based on the diffusions of CO_2 and its reaction with Ca(OH)_2, and considers the coexistence of CaCO_3 with Ca(OH)_2 in carbonation progressing region. The model can also explain the characteristics of carbonation resistance of concrete that the matrix is densified due to carbonation. The prediction by the model agreed well the experimental data in which the concrete using fly ash showed the higher rate at carbonation in early age and the lower rate in long-term age than the concrete without fly ash. Consequently the model can predict the rate of carbonation of concrete exposed in the atmosphere regardless of the water-to-cement ratio, the amount of the content of fly ash, etc.
