Mechanical properties and microscopic mechanisms of alkali-activated slag-fly ash modified collapsible loess

Loess, a typical water-sensitive weak soil widely distributed in Northwest China, exhibits low cohesion and collapsibility when saturated, severely infrastructure limiting development. To address the high-carbon emissions and poor economic efficiency of traditional cement-based binders, this study employs alkali-activated slag-fly ash geopolymers to improve collapsible loess. The effects of precursor slag/fly ash ratios, alkaline activator (NaOH/sodium silicate) ratios, and curing ages on the physical and mechanical properties of the stabilized loess were systematically investigated. The strength development mechanism of the improved loess was revealed through X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses. Results show that as the slag content in the precursor increases from 0 to 100%, the unconfined compressive strength (UCS) of the stabilized loess exhibits a significant upward trend. After 7 days of curing, the water content of the samples decreases with increasing slag content, but after 28 days, the water content changes show an opposite trend. The alkaline activator ratio exhibits a threshold effect on the stabilized soil: the strength initially increases and then decreases with increasing NaOH ratio in high-slag systems (slag ≥80%), while the strength continuously increases with rising NaOH ratio in high-fly ash systems (fly ash ≥80%). Microstructural characterization reveals that the geopolymers primarily form calcium silicate hydrate (C-S-H) and calcium aluminosilicate hydrate (C-A-S-H) gels, whose cementation effect is the dominant mechanism for strength enhancement, while pore-filling effects contribute secondarily. This study provides a scientific basis for the resource utilization of industrial solid waste and green reinforcement of loess subgrades.