In this paper we bring up new aspects of the metal-proton exchange reaction (MPER, also called early stage hydration): (1) dependence of the number of protons consumed by the preferential exchanged cations on the pH value applied at the water/wollastonite interface and (2) strong anisotropic characteristics detected in atomic force microscopy (AFM) and low energy ion scattering spectroscopy measurements (LEIS). First we apply density functional theory (DFT) calculations to compare the kinetics of the reaction on different wollastonite surfaces and combine it with ab initio thermodynamics to set up a model describing (1) the release of Ca in exchange with H coming from the water/wollastonite interface, (2) the dependence of the MPER on the chemical potential of protons. In the second part of the paper we carried out in situ AFM and inductive coupled plasma optical emission spectroscopy (ICP-OES) measurements in order to evaluate the predicted values. While a good agreement is found in the basic and neutral regime (pH values from 14 to 4), an increasing mismatch appears in the acidic regime (pH value lower 4). This is finally explained by nonequilibrium etching, dominating over the MPER in the very acidic regime.

In this study, ultrathin calcium-silicate-hydrate (C-S-H) phases on silicon wafers were prepared, which are partially terminated by calcium carbonates. First, a density functional theory (DFT) analysis was performed, to define the nature of the carbonates that are stable in the structure, concluding that two different kinds of them will be present on the surface. Then, by means of four different experimental handling techniques, the C-S-H phases were activated by disposing the carbonate termination: (1) UV-light (365 nm) radiation as a function of time, (2) direct heating between room temperature (RT) and 840 °C, (3) wet chemical treatment by an aqueous solution with a defined pH value as a function of time and (4) Ar/O2-plasma treatment. Fourier transform infrared (FTIR) spectroscopy was implemented to confirm that every method successfully reduced the carbonate termination of the ultrathin C-S-H phases. Interestingly, the effects of the diverse treatments on the C-S-H phases are very different. UV-light radiation eliminates partially carbonates from the C-S-H phases; but in contrast to the other treatments, the rate of this activation is very low. Temperatures up to 700 °C are necessary to remove the carbonates by direct heating. Remarkably, at these high temperatures, the remaining calcium-silicate (C-S) phases start to change their crystal structure, which was proved by means of X-ray diffraction (XRD). During wet chemical treatment, in addition to the carbonates removal, C-S-H phases were also affected, due to the low pH value (≤4) of the implemented solution. Finally, the most rapid activation at RT was provided by Ar/O2-plasma treatment, without drastic impacts on the C-S-H phases.

In this work, high-temperature-resistant phosphate molecules are applied to characterize ultrathin (100 nm) calcium silicate (C-S) phases. These C-S phases are synthesized on silicon wafers, and the interaction of phosphates with the C-S phases is studied by means of in situ transmission Fourier transform infrared (FTIR) spectroscopy. At room temperature, the chemistry of the system is dominated by the formation of calcium phosphates (C-P). In the case of temperature rising to 1000 °C, the C-S phases are regenerated. FTIR results are analyzed on the basis of first-principles calculations and further supported by complementary time-of-flight secondary ion mass spectrometry (ToF-SIMS) experiments. This study provides a detailed and self-consistent picture of the chemical and structural properties of interfaces such as the one between the atmosphere and ultrathin C-S phases (gas/C-S) and the one between them and silicon wafers (C-S/Si bulk). The material combination of ultrathin C-S phases grown on silicon wafers might in the future have great potential in selective chemistry, catalysis, and sensing technology as well as in semiconductor manufacturing.

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.

No abstract