Multiscale mechanistic insights into coal gangue for enhancing engineered geopolymer composites (EGCs): nano, micro and macro property correlations

• For the first time, coal gangue(CG) was used to prepare Engineered Geopolymer Composite (EGC). • The developed EGC had both high strength(84.60 MPa) and high ductility(11.48 %). • The mechanism of the Al/Si ratio on gel polymerization and crystalline phase formation was revealed. • We clarified the mechanism of CG-regulated Ca/Si and Al/Si effects on C-(N)-A-S-H gel. In this paper, coal gangue (CG) was used to prepare Engineering Polymer Composites (EGCs). The nano, micro, and macro properties were elucidated through macro-mechanical testing and microstructural characterization. EGC with 30 %CG resulted in an impressive tensile strain capacity of 11.48 % and a compressive strength of 84.60 MPa. The results show that the addition of CG evolved the hydration product from a single-phase C-(A)-S-H gel (S100G0) to a biphasic combination of C-(N)-A-S-H gel and gismondine crystals. This transition is attributable to the CG-induced optimization of the Al/Si ratio, which not only promotes gel polymerization but also facilitates the formation of gismondine through phase separation (Al/Si ≥ 0.5). Furthermore, the effects of Ca/Si and Al/Si regulated by CG on the properties of C-(N)-A-S-H gel were clarified by altering its molecular structure. Notably, matrix densification enhanced fiber/matrix interfacial bonding, resulting in a 48.78 % increase in fiber bridging stress (S100G0 → S70G30), which achieved effective stress transfer and multiple cracking. The research results not only give a theoretical foundation for artificially regulating the multiphase microstructure of geopolymers, but they also provide valuable technological support for the high-value application of CG.