Chloride binding in sound and carbonated cementitious materials with various types of binder
Résumé
Carbonation and chloride ingress are the main causes of reinforced concrete structures degradation. Usually, these two phenomena are studied individually. Here, chloride ingress has been investigated on sound and carbonated cementitious materials (cement pastes and concrete). The supplementary cementitious materials (SCM) are fly ash, slag and métakaolin. The materials were carbonated by exposure to accelerated conditions (1.5% CO2 and 65% RH) in laboratory. Chloride binding isotherms of sound and carbonated samples were assessed by the equilibrium method (exposure to various alkaline NaCl solutions). The microstructure was characterized not only by usual techniques such as XRD and TGA-DTA, but also by 29Si and 27Al NMR spectroscopy. The results show that carbonated samples bind less chlorides than sound ones. The use of a combination of various techniques of microstructural characterization reduces the number of the possible explanations of these findings. In carbonated materials, both the quantity of Friedel's salts and of chlorides bound by C-S-H decrease. The first decrease can be explained by a lack of portlandite and modification of aluminate phases equilibrium. The second decrease seems to be due to a modification of the C-S-H (change of surface charge) during the carbonation process. Moreover, differences according to the nature and the amount of supplementary cementitious materials (SCM) were observed.