Study on the strength mechanism of water glass-activated fly ash-cement solidified loess based on macro-micro scale analysis

In order to address the technical challenge of “excavation-backfilling in one step, support-free rapid demolding” during fast backfilling of narrow and elongated foundation trenches without temporary support, this study takes the in-situ excavated loess from the Xiong’an New Area Start-up Zone as the research object. Using sodium silicate as an alkali activator and fly ash as a partial cement replacement to form a composite curing agent, the effects of water content, sodium silicate content, curing agent dosage, and fly ash/cement mass ratio on the unconfined compressive strength and water dispersion loss were investigated through single-factor and response surface methodology experiments. Scanning electron microscopy and energy dispersive spectroscopy were further applied to reveal the microscopic evolution of strength development.The results show that the optimal mix proportion consists of a fly ash/cement mass ratio of 1, water content of 51.25%, sodium silicate content of 2.0%, and curing agent dosage of 35%. The curing agent dosage is the most significant factor affecting strength and water dispersion loss. Within the range of 25–35%, strength increases with its content, while water dispersion loss first decreases and then rises. An appropriate amount of sodium silicate (2.0%) promotes the formation of acicular/rod-like AFt and fine flaky C-A-S-H, effectively filling pores and improving compactness. In contrast, excessive sodium silicate leads to large laminated C-A-S-H, which reduces the filling effect, and this deterioration aggravates with curing age.It is concluded that the strength originates from the synergy between physical compaction & filling (based on initial high density) and the cementitious reaction activated by sodium silicate, where fluidity ensures initial compaction during construction and water dispersion behavior governs subsequent strength development. This work not only provides a reliable mix design and theoretical basis for the pumpable and rapid-setting backfilling of in-situ loess in confined spaces, but also proposes a universal methodological framework for performance optimization of alkali-activated solidified soils through coordinated control of fluidity and water dispersion behavior, which can serve as a reference for the treatment of similar silico-aluminous engineering waste soils.