Mechanical properties and microscopic characterization of fiber-calcium carbonate whisker composite modified geopolymer sand materials

To mitigate the occupation of land resources and pollution caused by industrial solid waste, coal gangue powder (CGP) and slag (SL) were utilized as cementitious materials, carbide slag (CS) was used as an alkaline activator, and aeolian sand (AS) was employed as a solidification matrix to synthesize a fully solid-waste-based backfill material (CSCA). To overcome the limitations of single-form fibers in improving the mechanical performance of CSCA, the effects of individual and hybrid incorporation of calcium carbonate whiskers (CCW) and polypropylene fibers (PPF) on the mechanical properties and microstructure of CSCA were systematically investigated and compared. Unconfined compressive strength (UCS) and three-point flexural strength (FS) tests were conducted using digital image correlation (DIC) technology. Simultaneously, XRD, SEM-EDS, and AFM were employed to characterize the material's polymerization products, interfacial microstructure, and fiber failure modes. The results indicate that the hybrid fiber system significantly improves UCS and FS compared to CSCA reinforced with individual fiber types, demonstrating clear synergistic effects. Optimal mechanical performance is attained with 2% CCW and 0.6% PPF incorporation rates. DIC analysis reveals that CCW and PPF generate complementary reinforcement effects by leveraging their distinct dimensional characteristics and physico-mechanical properties. This multi-scale synergistic effect effectively inhibited crack initiation, propagation, and coalescence at both macroscopic and microscopic scales throughout various damage stages. Meanwhile, compared with the single-doped group, the samples with co-doping show increased accumulation of total material energy (U) and elastic strain energy (U^e), along with reduced energy loss. Microstructural analysis demonstrates that CCW incorporation substantially improves the pore structure characteristics of CSCA specimens. Moreover, the hybrid fiber reinforcement system induces a transition in matrix failure mode from isolated fractures governed by individual fiber types to an integrated 'fiber-whisker-matrix' multiphase interfacial mechanism exhibiting synergistic load transfer and strain compatibility.