Correlation between Composition and Mechanical Properties of Calcium Silicate Hydrates Identified by Infrared Spectroscopy and Density Functional Theory

Building and construction industries are at the same time the backbone and the driving force of our modern society. Nearly all our technical infrastructure today is based on cement-based materials. Detailed spectroscopic investigations of model reactions on well-defined mineral substrates under UHV conditions are largely lacking, thus prohibiting a validation of theoretical methods. Even simple chemical processes are poorly understood. As a result, the rational design of anticorrosion strategies is virtually impossible. In this manuscript, we have worked on eight different calcium-silicate (CS) or calcium-silicate-hydrate (CSH) phases, namely, tobermorite 14 Å, tobermorite 11 Å, tobermorite 9 Å, wollastonite, jaffeite, jennite, γ-C 2 S, and α-C 2 SH. These representative model phases are calculated with the help of the density functional theory modeling method. Initially, we take care of the mechanical properties of the material. Our results revealed that jaffeite, γ-C 2 S, and α-C 2 SH have a linear bulk modulus because of the monomer structure of silicate tetrahedra. Tobermorite 14 Å and γ-C 2 S exhibit the lowest and highest bulk modulus, respectively. In the second part, the optimized geometries allow for the precise calculation of vibrational eigenmodes and frequencies by the force constant approach. The proportions of C/S and H/C are major criteria for the classification of the calculated wavenumber of ν(Si–O) for all phases in our model system. However, γ-C 2 S and α-C 2 SH have an equal C/S ratio. As a result of the different H/C ratios, wavenumbers were found for four characteristic vibration modes. The contribution of the C/S ratio for α-C 2 SH in comparison to jaffeite is another aspect to be considered for phases with equal H/C ratios. Because of the lower C/S ratio, α-C 2 SH exhibits a higher wavenumber than jaffeite.