Quantitative Evolution of Mineral Crystal Structure in Alkali-Activated Materials Derived from Multi-Source Coal-Based Solid Wastes via XRD Refinement

To address the rapid flash setting of NaOH-activated slag and promote high-volume utilization of coal-based solid wastes, this study investigated the synergy of fly ash (FA), coal gangue (CG), and gasification slag (GS) in partially replacing granulated blast furnace slag (GBFS) for NaOH activated binders, aiming to mitigate flash setting while preserving mechanical performance. Binary and ternary pastes with up to 90 wt% substitution were evaluated for setting time, fluidity, and compressive strength, supported by quantitative XRD and SEM. The G8F1G1 achieved the optimal balance, extending initial and final setting times from 28 and 32 min to 35 and 43 min, yielding a fluidity of 218 mm, and maintaining a 28-day strength of 48.5 MPa, equivalent to neat GBFS. The FA-GS synergy suppressed crystalline by damping the Ca2+ supersaturation peak through FA derived oligomers while providing nucleation sites via fine carbonaceous particles in GS, thereby retaining the amorphous C-A-S-H gel at approximately 77 wt%. This preserved the load-bearing gel network but eliminated early percolating crystalline frameworks, extending workability without strength loss. In contrast, CG acted as an inert diluent, introducing weak interfaces and porosity that severely degraded strength. A critical amorphous threshold near 64 wt% (G6F2G2, amorphous gel) governed the transition to a granular bed. The FA-GS system offers an effective route for high-volume valorization of coal-based solid wastes in sustainable construction materials.