The present study investigates the durability and leachate behavior of an alkali-activated fly ash (FA) as a binder to stabilize expansive soils in deep mixing applications. To activate FA, a liquid alkali activator (LAA) to binder ratio (LAA/FA) was introduced and varied from 1.0 to 1.5. Swelling-shrinkage, consolidation and unconfined compressive strength (UCS) characteristics under wetting-drying cycles (durability) were studies on deep mixed expansive soils. Leachate studies were performed to determine the environmental impact of alkali-activated fly ash. Heavy metal and sodium ions concentrations were measured in each leachate cycle. To examine the permanent microstructural and phase changes in the mixes, scanning electron microscopy (SEM) and X-ray diffraction (XRD) studies were conducted. The swelling-shrinkage behavior of soil could be marginally controlled for LAA/FA = 1.0 and 1.25 due to heavy weight loss during the wetting-drying cycles. However, even after twelve durability cycles, the unconfined compressive strength (UCS) of soil with LAA/FA =1.5 retained 1.0 MPa and showed non-critical swelling-shrinkage behavior. The heavy metals and sodium ion concentrations were in acceptable range for LAA/FA = 1.25 and 1.5. XRD and SEM studies revealed that soil treated with a binder ratio of LAA/FA = 1.5, showed high crystalline peaks and aggregated structures. © 2022 ICE Publishing: All rights reserved.