Efflorescence Suppression and Performance Enhancement in CaO-Activated Lepidolite Lithium Slag-Based Geopolymers

Lepidolite lithium slag (LLS), a byproduct of the lithium extraction industry, exhibits high structural stability and low reactivity, posing significant challenges for its valorization. Although geopolymerization offers a potential pathway for LLS recycling, conventional alkali activation is often associated with high alkalinity, efflorescence issues, and environmental drawbacks. In this study, a combined approach of CaO activation and mechanical activation was developed to enhance LLS reactivity while suppressing efflorescence. Systematic tests demonstrated that 30 min mechanical activation maximized reactivity, yielding a 92.9% increase in 28-day compressive strength compared to untreated LLS. The optimal mixture proportion (LLS: nickel slag = 8:2, 20 wt % CaO, liquid-to-solid ratio = 0.40) achieved a compressive strength of 22.87 MPa and showed marked resistance to efflorescence. Microstructural and phase analyses revealed that CaO activation accelerates reaction kinetics through exothermic hydration, promoting C–(A)–S–H gel formation and matrix densification. The continuous release of Ca2+ further enables ettringite crystallization via reaction with SO42–, contrasting with Na-based activations where Na2SO4 formation causes surface efflorescence. This calcium-dominated reaction pathway effectively immobilizes sulfate ions and inhibits soluble salt accumulation. Leaching assessments confirmed compliance with relevant environmental regulations, highlighting the environmental safety of the developed system. The proposed activation strategy offers a viable approach for upcycling LLS into durable, low-alkalinity construction materials, addressing both industrial waste management and sustainable building materials development.