Abstract: ABSTRACT Loess is a typical water-sensitive soft soil widely distributed in northwest China. It typically exhibits low cohesiveness and easy subsidence when immersed in water, thus severely restricting the development of infrastructure construction such as highways, railways, and bridges. To solve the issues of high carbon emissions, low economic benefits, and increased environmental alkalinity caused by conventional cement-based curing materials, alkali-activated slag/fly ash (S/F) geopolymers (referred to as “geopolymer”) were adopted to improve collapsible loess in this study. The influence laws of the precursor S/F mass ratios (i.e., 0∶10, 2∶8, 4∶6, 5∶5, 6∶4, 8∶2, and 10∶0), alkaline activator (i.e., sodium hydroxide/sodium silicate, N/G), mass ratios (i.e., 0∶10, 1∶4, 4∶1, and 10∶0), and curing age (i.e., 7 d and 28 d) on the physical and mechanical properties of solidified loess were systematically investigated through measurements of unconfined compressive strength (UCS) tests, moisture content, and dry density. The strength-formation mechanism of solidified soil was revealed through X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses. The results show that as the proportion of slag in the precursor increases from 0% to 100%, the UCS of solidified loess increases significantly. The UCS of solidified soil increases significantly with curing age, i.e., the UCS of the 28-d solidified sample is 2.1 ～ 3.8 times higher than that of the 7 d solidified sample. After 7 d of curing, the moisture content of the solidified samples decreases as the slag proportion increases. By contrast, the change in moisture content of the solidified samples after 28 d of curing shows the opposite law. The proportion of alkali activator imposes a threshold effect on the solidified soil: the UCS of the high-slag system (the slag content exceeding 80%) first increases and then decreases as the proportion of sodium hydroxide increases, whereas that of the high-fly-ash system (the fly-ash content exceeding 80%) increases continuously with the proportion of sodium hydroxide. Microstructure characterization based on XRD and SEM analyses shows that the alkali-activated S/F geopolymers in the solidified samples primarily generate hydrated calcium silicate (C–S–H) and hydrated calcium aluminosilicate (C–A–S–H) gels, which cement the soil particles. The cementation effect of these gels is identified as the dominant mechanism for strength enhancement (particularly the C–S–H/C–A–S–H in slag system), whereas the pore filling effect is a secondary contributing factor. The optimized formula (S∶F = 8∶2, N∶G = 8∶2) results in a 28 d UCS of 8.69 MPa, which significantly exceeds the performance of conventional cement-stabilized loess while offering favorable economic benefits. Alkali-activated solidification improvement reduces the collapsibility of loess as well as enhances its strength, bearing capacity, and road usability. This study provides a scientific basis for the resource utilization of industrial solid waste and the green reinforcement of loess subgrade. Additionally, it demonstrates the feasibility and superior performance of geopolymer technology.