The fluoroacid etching process to produce perforated cenospheres generates a considerable volume of waste etching water, yet no effective strategy has been proposed for the treatment or reutilization of this environmentally harmful wastewater. In light of this, the present study explores the feasibility of utilizing lime-treated etching wastewater (LTEW) as a potential source of mixing water for alkali-activated slag (AAS). LTEW exhibits a high concentration of Ca2+ and possesses strong alkalinity, making it potentially suitable for activating slag. The experimental findings revealed that the addition of LTEW enables a reduction in the concentration of the activator, NaOH solution, due to the presence of Ca2+ and NH4+ within LTEW. Consequently, the setting and hydration processes of AAS were retarded in correlation with the incorporation of LTEW. However, the reaction between Ca2+ in LTEW and NaOH resulted in the formation of Ca(OH)2, which could act as an auxiliary activator for AAS. Furthermore, the presence of expansive Ca(OH)2 crystals, possessing a higher elastic modulus compared to C-S-H gel, refined the pore structure of the mortars. The addition of increasing amounts of LTEW exhibited a gradual improvement in the compressive strengths of the mortars at all ages, ranging from 2.2% to 23.5% at 3 days, 2.4% to 21.6% at 7 days, and 6.0% to 24.6% at 28 days. The utilization of LTEW in AAS preparation offers significant economic and environmental benefits.