Diffusion de gaz, carbonatation et retrait de la pâte de ciment durcie

Though the carbonation of cementitious materials has been widely studied, numerous problems remain unsolved. The lack of quantitative data especially appears when one tries to model the carbonation process with the aim of predicting the lifetime of a reinforced concrete structure. This reason has justified this study which deals essentially with four subjects namely: the modification of the microstructure caused by carbonation, the measure of the diffusivity of CO2 and O2 through carbonated hydrated cement paste (hcp) and mortar, the carbonation shrinkage and the progress of the carbonation into mortars exposed outdoors. The accelerated carbonation of hcp and mortars leads to large modifications of the porous system. The lower the water/cement ratio or, in other words, the porosity, the higher the reduction of porosity. The pore-size distributions, measured by mercury intrusion porosimetry, show that the pores of all sizes are affected. Therefore, in hcp of W/C ≤ 0.5, the volume of the pores with a radius < 0.1 μm especially decreases. The water vapor sorption isotherms are strongly modified by carbonation. The equilibrium water amounts decrease considerably at all relative humidities after carbonation. The hcp of different W/C get similar sorption isotherms. The BET specific surface area is reduced of 50% by carbonation. The porosity of the studied mortars is little modified by carbonation. This is due to the high level of big pores in these mortars, which is much higher than that of hcp. The influence of the water content of hcp on the diffusivity of CO2 and O2 is clearly lower than that observed on non-carbonated materials. It is only above 90% rh and for the less porous hcp where micro- and mesopores dominates, that the influence of the water content becomes noticeable. These facts are a direct consequence of the strong decrease of the internal surface due to carbonation. On the studied mortars the influence of the water content is still weaker due to their coarser porosity. The diffusion coefficients of CO2 and O2 strongly depend on the porosity and are an exponential function of the latter. The same diffusion coefficients determined on the mortars seem to show a minimum as the sand concentration is increased. In our case this minimum is located at a sand concentration of about 50% by volume. At first sight, this behavior appears peculiar because the amount of nonporous sand has been increased. This can be explained by the fact that as the concentration in cement decreases, a lot of cement particles are too big to fill the porosity between sand particles which so acts as a sieve. The lack of cement paste to fill the voids and the augmentation of the "auréoles de transition" can also play a role. The diffusion coefficient Of CO2 is between 60 and 65% of that of O2. This is due to the bigger size of the molecule of CO2. It is shown that the Knudsen diffusion predominates in hcp of W/C ≤ 0.5, whilst in that of W/C = 0.8 and in the studied morta