Corrosion of Concrete by Water-Induced Metal–Proton Exchange

We have investigated basic mechanisms of concrete corrosion by studying wollastonite in aqueous environments. This well-defined crystalline mineral is well-suited as a model system of calcium–silicate phases, the main constituent of this important building material. A detailed peak-shape analysis of X-ray diffraction (XRD) signals recorded for wollastonite powders exposed to water allowed monitoring of dramatic changes in particle shape as a result of the so-called metal–proton exchange reaction (MPER). Since these experiments were carried out for well-defined particles with known orientation, state-of-the art calculations using density functional theory (DFT) could be employed to more precisely study this behavior, which previously has not attracted much attention. The free energies of the different crystalline surfaces of wollastonite are strongly affected when brought into contact with water, thus providing a strong driving force for changes in particle shape. A more detailed analysis of the corresponding Wulff constructions reveals, however, that a quantitative description of these phenomena also requires a detailed analysis of the kinetics. Implications for concrete corrosion will be discussed, and strategies for its prevention will be outlined.