Abstract

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.

Download