Expansive soils undergo significant volumetric changes during wetting and drying, often leading to structural deterioration and engineering difficulties. Alkali-activated binders have been widely utilized to improve the mechanical performance and durability of such soils. This study examines the performance of γ-aminopropyltriethoxysilane (KH-550)-modified alkali-activated binder-stabilized expansive soils (AABS). As a result, the addition of KH-550 extended the setting times by up to 89% and enhanced fluidity by 6–27%, thereby improving the workability of AABS. The additive delayed early hydration while accelerating later-stage gel formation via hydrolysis and Si–O–Si bond generation, promoting the development of C-(A)-S-H. Microstructural observations indicated a refinement of pores and a reduction in capillary porosity, yielding a denser and more homogeneous matrix. Autogenous shrinkage was reduced by as much as 32.5%, and the unconfined compressive strength of 7 d AABS increased by 58.1% at an optimal KH-550 dosage of 1.0 wt.%, with mechanical performance remaining stable under wet–dry cycling. Overall, these results suggest that KH-550 serves as an effective organic–inorganic bridging agent, offering a viable strategy for the stabilization of expansive soils.