Coupled moisture-carbon dioxide-calcium transfer model for carbonation of concrete

The carbonation mechanisms of concrete are analyzed in this paper, accounting for evolutions of relative humidity in the porous material. The model is based on macroscopic mass balance equations for the water, the carbon dioxide contained in the gaseous phase and the calcium contained in the pore solution, which are supposed to completely define the problem of atmospheric carbonation in concrete. These equations govern the diffusion and permeation processes of the three variables: saturation degree, carbon dioxide partial pressure and calcium concentration in pore solution. By using an idealized description of the main hydrated products of the cement paste, the dissolution phenomenon can be regarded as depending only on the calcium concentration in the aqueous phase. The calcite formation and the hydrates dissolution are introduced in the mass balance as source terms and conduct to significant variations of the porosity. The mass balance equations are discretized in time and space in the one-dimensional case. The simulation of a concrete wall subjected to combined drying and accelerated carbon dioxide attack is then performed, and the results are analyzed and compared with experimental data in terms of carbonation depth. (C) 2004 Elsevier Ltd. All rights reserved.