Micromechanical insights into high-ductility geopolymer composites incorporating red mud (RM): Role of in situ spherical RM-balls at the fiber-matrix interface

Red mud (RM), an aluminosilicate-rich solid waste, finds its primary resource utilization in replacing cementitious binders in concrete systems. This study successfully fabricated red mud-based high-ductility geopolymer composites (RM-HDGC) through alkali activation, utilizing 70 wt% red mud as the primary raw material, supplemented with ground granulated blast-furnace slag and fly ash. Micromechanical analysis revealed that an increase in the Ca/Si ratio initially enhanced fiber dispersion, reaching an optimum before declining. In contrast, the tensile ductility consistently increased, achieving a maximum elongation of 4.71 %. This phenomenon deviates from the predicted optimal ductility range based on rheological parameters and contradicts classical micromechanical interfacial theories. Microstructural analysis revealed the presence of characteristic micrometer-sized spherical products (RM-balls) at the fiber-matrix interface. Their chemical composition, which is similar to that of N-A-S-H gel, was further confirmed by in situ Energy Dispersive X-ray Spectroscopy (EDS). Considering the 60 °C steam-curing high-alkali environment these structures are determined to be gel-phase transformation products formed through thermo-alkali coupled activation of reactive RM components. RM-balls synergistically optimize interfacial performance via dual mechanisms: firstly reducing chemical bonding energy G_d; secondly their spherical geometry induces localized rolling effects during fiber slip significantly decreasing the slip-hardening coefficient β thereby promoting thorough debonding-slip development. The core contributions of this work are threefold: demonstrating engineer-grade ductility (4.71 %) achievable in HDGC with 70 % RM incorporation; first discovery of regularly-shaped spherical gel RM-balls growing in-situ on PVA fiber surfaces which structurally overcomes traditional ductility constraints imposed by rheology and fiber dispersion; revelation of RM-ball-induced in-situ hydrophobization effects on PVA fibers. These findings provide mechanistic explanations for anomalous mechanical behavior and establish a theoretical foundation for constructing precise micromechanical constitutive models for RM-HDGC.