Effects and Mechanisms of Hooked-End Steel Fibers and Carbon Nanotubes on the Shrinkage of Lithium Slag-Based Geopolymers: Role of Fiber-Confined Zones.

Lithium slag-based geopolymer (LSG) provides a promising route for the high-value utilization of lithium slag; however, its pronounced shrinkage deformation continues to restrict engineering applications. To clarify the multi-scale mechanism by which fibers regulate LSG shrinkage, this study investigated the effects of single and hybrid additions of carbon nanotubes (CNTs) and hooked-end steel fibers (HSFs). The 90 d drying shrinkage test quantified shrinkage-reduction efficiency for mono- and hybrid-fiber systems. MIP was used for porosity and pore size distribution analysis; SEM examined fiber dispersion, interfacial bonding, and CNT-HSF synergy. At 90 d, 0.15% CNTs and 1.5% HSFs reduced shrinkage by 5.7% and 26.8%, respectively, whereas the C0.15-H1.5 hybrid mixture achieved a maximum reduction of 31.33% relative to the control. HSF addition increased porosity from 15.5% to 23.1% and the average pore diameter from 23.36 to 50.82 nm. Macroscopic shrinkage was reduced because interfacial friction and hooked-end anchorage provided mechanical restraint. In addition, based on fiber pull-out behavior and a simplified interfacial bond-slip model, the effective confinement radius at the fiber-matrix interface was analyzed, and the concept of fiber-confined zones was proposed. Results show that CNTs refine pore structure and bridge microcracks, whereas HSFs provide mechanical restraint through interfacial friction and anchorage. The hybrid CNT-HSF system achieved a maximum drying shrinkage reduction of 31.33%, higher than the corresponding theoretical additive value. The spatial overlap of fiber-confined zones is identified as the key mechanism for forming a weakly rigid framework that suppresses macroscopic shrinkage.