As underground development extends to greater depths and broader scope, growing scenarios involving high water pressure and strong seepage are imposing stricter requirements on the anti-seepage performance of cement-sodium silicate (CS) grouts. In view of this, this study systematically investigated the modification effects of nano-silica (NS) on the engineering properties of CS grouts, particularly their impermeability performance, through comprehensive proportioning experiments and advanced multi-scale characterization techniques. The experimental results revealed that the incorporation of 0.5% NS yielded optimal performance improvements: the 28-day compressive strength increased from 10.4 MPa to 18.9 MPa (an enhancement of 81.7%), while the ultimate impermeability pressure rose from 0.4 MPa to 1.1 MPa (a remarkable 175% improvement). However, excessive NS addition led to agglomeration effects that compromised these benefits. The novelty of this study lies in the systematic elucidation of NS modification mechanisms in high-alkali, sodium silicate-rich CS grout systems through advanced microscale characterization. Specifically, the addition of NS can enhance the pozzolanic reaction progress, thereby continuously consuming calcium hydroxide (CH) while promoting the formation of more calcium silicate hydrate (C-S-H) gel and reducing its Ca/Si ratio. Moreover, 29 Si nuclear magnetic resonance (NMR) revealed that NS incorporation fundamentally restructured the C-S-H gel nanostructure by increasing the mean chain length (MCL) from 4.21 to 6.53 (a 55.1% increase) and elevating the Q²(1Al) content from 12.09% to 27.72%, resulting in a more highly polymerized and cross-linked silicate network. These microstructural modifications, combined with the reduction in total porosity by 11.7% and refinement of pore structure, collectively enhanced the density and uniformity of the hardened matrix, directly explaining the macroscopic improvements in mechanical strength and impermeability. This study provided a comprehensive understanding of NS modification mechanisms in CS grout systems at multiple scales and demonstrated a clear relationship between microstructure and macroscopic properties, offering significant theoretical guidance and practical implications for designing high-performance CS grouts for complex underground engineering applications under high water pressure conditions.
Zhao et al. (Sun,) studied this question.