Silicate Bond Characteristics in Calcium-Silicate-Hydrates Determined by High Pressure Raman Spectroscopy

The mechanical and thermal properties of the gigatonnes of concrete produced annually are strongly affected by the anharmonicity of the chemical bonds in its main binding phase, nanocrystalline calcium-(alumino-)silicate-hydrate (C-(A-)S-H). Improvements in C-(A-)S-H design increasingly depend on simulations utilizing a set of effective interatomic forces known as CSH-FF, yet these assumptions have never been directly examined at the chemical bond level, and there is no guidance for their improvement. In this work, we use high-pressure Raman spectroscopy to directly measure bond anharmonicity in a representative series of C-(A-)S-H samples with varying composition and two natural model minerals, 14 angstrom tobermorite and xonotlite. We find that structural water molecules effectively scatter thermal energy, providing a heuristic for improving the thermal resistance of concrete. A comparison of experimental and calculated bond anharmonicities shows that a stiffer Si-O interaction would improve the transferability of CSH-FF to the thermal properties of C-(A-)S-H. High-pressure Raman spectroscopy is suggested to improve the calculations of C-S-H and to characterize other complex, nanocrystalline materials.