Impact of alkali hydroxide selection on strength and shrinkage of multi-component alkali-activated mortars

• Lithium hydroxide evaluated as an alkali activator in hybrid AAM systems • Workability improved with increasing alkali ion size from Li⁺ to K⁺ • LiOH delayed setting due to formation of poorly soluble lithium silicate • Strength varied with binder type and nature of alkali activator used • Ionic size and charge influenced gel structure and microstructural features The growing demand for sustainable alternatives to ordinary Portland cement has accelerated research on alkali-activated materials (AAMs), yet their performance remains highly sensitive to activator chemistry, and the role of lithium hydroxide (LiOH) is still poorly understood. This study systematically compares LiOH with conventional NaOH and KOH activators in multi-component AAM mortars. Ground granulated blast furnace slag (GGBS), Class C fly ash (FA), and silica fume (SF) were used as binders, and the effects of activator type and binder composition on flow, setting time, compressive strength, and drying shrinkage were evaluated. Complementary microstructural analyses—gravimetric Si dissolution, X-ray diffraction (XRD), ²⁹Si magic-angle spinning nuclear magnetic resonance (²⁹Si MAS NMR), scanning electron microscopy (SEM), and mercury intrusion porosimetry (MIP)—were performed to clarify reaction mechanisms and gel development. Workability increases with ionic radius (Li < Na < K), whereas setting time is strongly prolonged in LiOH-activated systems owing to Li₂SiO₃ precipitation and extensive hydration shells around Li⁺. Compressive strength depends on binder chemistry: NaOH provides the highest strength in GGBS-rich mixtures, KOH is most effective in GGBS-only systems, and LiOH yields the highest strength in FA-only binders. Drying shrinkage is lowest in Na-based mortars and highest in Li-based mortars because of residual micropores and a less refined pore structure. Overall, LiOH is a promising specialized activator for FA-based AAMs, but its use in structural applications will require optimization or blending with NaOH or KOH. These findings support the design of AAMs for building environments where dimensional stability and durability are critical.