Degradation mechanism of cement–sodium silicate hardened grout bodies under ion erosion

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Cement-sodium silicate (C-S) grout is extensively employed as a grouting material in shield tunnel synchronous grouting projects. However, in water-rich areas, its hardened grout bodies are vulnerable to erosion damage caused by various ions. In order to elucidate the degradation mechanism of C-S hardened grout bodies' strength under salt ion erosion. This study analyzed the influence of SO42- ions, Cl- ions, and SO42- and Cl- mixed ions at different concentrations on the compressive strength of C-S hardened grout bodies through laboratory experiments. Moreover, microscopic techniques (XRD, SEM-EDS) were employed to illustrate the phase characteristics and microstructure of ion-eroded C-S hardened grout bodies, and the distribution of surface calcium elements. Molecular dynamics (MD) simulation investigated the adsorption, diffusion degree, and energy change of erosive ions in C-S-H gel. The results show that the higher the concentration of erosion ions, the faster the erosion rate. A nonlinear spatiotemporal strength degradation prediction model was developed, assessing the deterioration degree of hardened grout bodies due to ions: SO42- > SO42-and Cl- mixture > Cl- > water. Additionally, microscopic observations reveal a tight correlation between the degradation level of C-S hardened grout bodies and the loss of Ca2+ ions inside. The more significant the loss of Ca2+ ions from hydration products, the more evident the strength degradation of C-S hardened grout bodies. MD simulations demonstrate that SO42- ions and Cl- ions occupy adsorption sites like SiOCa+ and SiOH, altering interatomic Coulomb forces and intermolecular van der Waals forces, changing system energy, thereby influencing the structure of C-S-H gel, leading to strength degradation in C-S hardened grout bodies.