The authors presented a computational system that can deal with pH fluctuation and decomposition of cementitious materials exposed to various environmental actions. In order to evaluate a drop in pH due to carbon dioxide, the material models were developed for transport and equilibrium of carbon dioxide, ionization, ion equilibriums, and carbonation reaction based on thermo-dynamic theory. The material properties of concrete are evaluated considering the inter-relationship of hydration, moisture transport and pore-structure development processes based upon fundamental physical material models. The proposed system can reasonably predict the carbonation phenomena and pH profile in concrete for arbitrary conditions.