Effect of the Ca/Si Molar Ratio on the Micro/nanomechanical Properties of Synthetic C-S-H Measured by Nanoindentation

Calcium silicate hydrate (C-S-H), the main product in Portland cement hydration, influences the physical and mechanical properties of most cementitious materials. However, there are no structural models that currently relate chemical composition, nanostructure, and microstructure with the physicochemical and mechanical properties. In this work, the indentation technique was used to evaluate the micro/nanomechanical properties of synthetic C-S-H with different Ca/Si (CaO/SiO2) molar ratios. C-S-H was also characterized by X-ray diffractometry (XRD), Fourier transform infrared spectroscopy (FT-IR), and X-ray fluorescence (XRF). Analysis of the results verified that the elastic modulus and hardness increased when the Ca/Si molar ratio of C-S-H decreased, achieving elastic modulus values of 27 and 20 GPa for Ca/Si ratios of 0.7 and 2.1, respectively, corroborating calculations based on the force field method of Manzano et al.(1) Our results also determined that micro- and nanoporosity significantly influence the outcome. The research results are limited to synthesized C-S-H, but clarify the potential of the Ca/Si ratio to modify the mechanical properties, while permitting investigation of C-S-H without the presence of other phases of hydrated Portland cement.