Mix design optimization and mechanism analysis of alkali-activated lithium slag–blast furnace slag composite binders

The large-scale production of lithium salts has led to the accumulation of lithium slag (LS), an industrial by-product rich in SiO₂ and Al₂O₃ with potential cementitious activity. This study used LS and ground granulated blast furnace slag (GGBFS) as aluminosilicate precursors to prepare alkali-activated materials (AAMs). An L₉ (3⁴) orthogonal design evaluated the effects of LS content, water-to-binder (W/B) ratio, and silicon-to-aluminum (Si/Al) ratio on workability, mechanical properties, and microstructure. Results indicate that the Si/Al ratio was the key factor controlling fluidity and strength. Increasing Si/Al from 2.4 to 3.2 raised fluidity by 40.67 mm and extended setting time by over 100 %. Optimal strength was achieved at a Si/Al ratio of 2.4, LS content of 30 %, and W/B of 0.35. Under this mix proportion, the AAMs achieved a 28-day compressive strength and flexural strength of 73.5 MPa and 5.1 MPa, respectively. SEM, XRD, and FTIR analyses confirmed that Si/Al = 2.4 favored a dense, highly cross-linked N(C)-A-S-H gel network. Solid-state NMR revealed the highest proportions of Q¹ and Q² species, the longest mean chain length (MCL = 14.95), and dominant Al(IV) tetrahedral coordination (83.15 %) under this condition, indicating excellent polymerization and connectivity. Higher Si/Al ratios caused excessive silicate polymerization, incomplete reactions, increased porosity, and aluminum coordination shifted from ordered Al(IV) to disordered Al(V)/Al(VI), weakening the gel and mechanical properties. This study provides new insights into the design of AAMs incorporating multiple industrial solid wastes and systematically elucidates the influence mechanisms of key parameters on their performance, supporting the sustainable utilization of LS.