Abstract: Peaty soil, a distinct category of soft foundation soil, exhibits unique physical and mechanical properties that are strongly influenced by its microstructure. Its high water content, organic matter content, low strength and permeability often result in significant engineering challenges. Enhancing the mechanical strength of peaty soil has thus become a central focus in geotechnical engineering. Using slag-based geopolymer to synergize with cement for solidification, the mechanical properties of peaty soil before and after stabilization were examined through unconfined compressive strength and direct shear tests. The mechanisms of improvement were further analyzed through microscopic techniques, including scanning electron microscope (SEM), X-ray diffraction (XRD), mercury intrusion porosimetry (MIP), and Fourier transform infrared spectroscopy (FTIR). The results demonstrate that all three alkali activators contribute to the enhancement of the mechanical strength of the peaty soil, with NaOH showing the highest activation efficiency. Cement stabilization of peaty soil improves shear strength by reducing pore space and strengthening interparticle bonding via ion exchange, hydration product crystallization, and the formation of CaCO3 and calcium silicate hydrate (C-S-H). Four stages i.e., dissolution activation, ion exchange, gel formation, and structural reorganization are identified in the reaction process of activated slag improving peat soil. The alkali activator facilitates the dissolution of the slag’s vitreous phase, promoting ionic polymerization that leads to the formation of calcium-alumino-silicate-hydrate (C-A-S-H) gel. Simultaneously, organic functional groups in the peaty soil engage in ion exchange, forming CaSiO3 precipitates and establishing a “calcium bridge” structure. These reactions collectively contribute to the formation of a dense composite matrix, thus enhancing compressive strength. Grey relational analysis reveals that compressive strength is most strongly correlated with pore area, while shear strength shows the highest correlation with the shape factor. Modified soil specimens undergo five dry-wet cycles, with a minimum strength loss rate of 27%. These findings provide a theoretical foundation for the partial replacement of cement with alkali-activated slag in peaty soil stabilization, contributing both to soft soil improvement and the valorization of industrial byproducts. Furthermore, these results offer valuable insights for ground improvement in peat-rich regions, such as Yunnan, China.

Key words: peaty soil, slag-based geopolymer, cement, mechanical strength, gray correlation method, microscopic mechanism