Toward a microscopic understanding of the calcium–silicate–hydrates/water interface

Abstract Calcium–Silicate–Hydrates (C–S–H) are the main binding phases in most concrete which is the primarily used composite construction material in the world. However, a big lack is cleaving between the actual knowledge about C–S–H, compared to what could be reached using state-of-the-art technologies of modern research. In this article, the formation of a C–S–H phase on a native oxide covered silicon wafer is investigated by means of in-situ attenuated total reflection infrared (ATR–IR) and ex-situ surface-enhanced Raman spectroscopy (SERS). The total thickness of the C–S–H phase is determined by X-ray photoelectron spectroscopy (XPS) to be 3 nm. The formation appears to be reversible depending on the environment pH value and can be performed at room temperature. Based on density functional theory (DFT) calculations, it is shown that the C–S–H phase in the presence of water will change its chemical composition in order to reach the thermodynamic ground state of the system. This change is achieved by a metal–proton exchange reaction. The stoichiometry of these metal–proton exchange reactions is nearly independent of the environment pH value. Electrokinetic measurements yield isoelectric points of 2.0 and 2.6 for the native oxide covered silicon wafer (SiO2) and the C–S–H phase. This is consistent with a predominance of Si–O sites at the C–S–H/water interface.