Synthesis of geopolymer composites utilizing PVA-modified basalt fibers and foundry waste: Fundamental properties, cracking resistance and fracture toughness

Coal gangue/slag-based geopolymer composites (CSGC) have garnered significant attention in sustainable construction due to their low-carbon footprint, but their high brittleness remains a critical limitation. This study addresses this challenge by modifying basalt fiber (BF) with polyvinyl alcohol (PVA) to enhance CSGC. A novel porosity detection program, with an average accuracy exceeding 98 % for pores larger than 10 pixels (0.4 mm), was developed to analyze pore evolution. Combined with a multi-scale testing approach, the study evaluates shrinkage, fracture toughness, compressive strength, and flexural strength of fiber-reinforced CSGC. Results revealed that PVA coating enhanced fiber-CSGC matrix bonding via chemical crosslinking, such as O-Al-O bonds, and mechanical interlocking, achieving 15.9 % and 19.1 % improvements in the 28-d flexural and compressive strength of modified basalt fibers (MBF)-reinforced CSGC, respectively. Acoustic emission (AE) analysis indicated that fibers shifted the dominant cracking mode from shear to a mixed tensile-shear mode (20.99 % tensile cracks), thereby enhancing CSGC toughness. Moreover, double-K fracture model analysis, based on digital image correlation (DIC) test results, confirmed that MBF outperforms unmodified fibers in toughening effects, with increases of 50.0 % and 32.4 % in initiation and unstable fracture toughness, respectively. This study presents an effective strategy for reducing brittleness in geopolymers through interface-engineered fiber reinforcement, which is expected to promote the development of high-performance, sustainable construction materials.