Unlocking Waste Potential: Hydration Products and Multiscale Mechanics of Engineering Geopolymer Composites Based on Slag and Coal Gangue under Alkali Control

Abstract This study focuses on the preparation of high-strength and high-ductility engineered geopolymer composites (EGCs) using ground granulated blast-furnace slag (GGBS) and coal gangue (CG). The effect of alkali equivalent (AE; 6% to 12%) on the multiscale mechanical properties and reaction products was systematically investigated through mechanical testing and multiscale characterization. The results indicate that an AE of 10% yields optimal performance, with a compressive strength of 84.56 MPa and an ultimate tensile strain of 11.48%, achieving a synergistic enhancement of strength and ductility in high-calcium EGCs. Microstructural analysis reveals that the primary hydration products are calcium sodium aluminosilicate hydrate [C-(N)-A-S-H] gel and gismondine crystals. Gismondine originated from Al 3 + supersaturation ( Al / Si ≥ 0.5 )–induced phase separation. Increasing AE enhanced gel polymerization and introduced a crystal nanofiller effect, raising the micro-elastic modulus of the matrix by up to 34.4%. Matrix densification also improved fiber/matrix bonding, increasing fiber bridging stress by 2.89 times. This led to multiple cracking and efficient stress transmission, resulting in a strain energy density of 329.01 kJ / m 3 . The findings advance the understanding of alkali activation in composite precursors and demonstrate a promising route for high-value CG usage.