Abstract: Coal gangue-based geopolymer aggregate slurry is an emerging green material with excellent stability and suitability for use as backfill in coal mine goafs. This approach not only addresses the remediation of coal mining subsidence areas but also resolves large-scale coal-based solid waste disposal. To investigate the transport properties of the slurry and the mechanical performance of its cured form, this study prepared coal gangue-based geopolymer aggregate slurry using alkali-activated calcined coal gangue as the cementitious material and crushed coal gangue as the aggregate. By controlling aggregate particle size and content, uniaxial compressive strength tests, low-field nuclear magnetic resonance (LF-NMR), and scanning electron microscopy (SEM) were employed to analyze the mechanical properties, pore distribution, and microstructure of the cured slurry. Additionally, rheological tests and slump flow tests were conducted to evaluate the transport characteristics of the slurry. The experimental results demonstrate that at a constant binder-to-aggregate ratio, the compressive strength of the cured material increases with larger aggregate particle sizes. Aggregates sized 20 mm, 10 mm, and 5 mm formed robust load-bearing skeletons while maintaining sufficient hydration reactions, leading to significantly higher strength compared to finer aggregates (0.15 mm and 0.075 mm). For materials with identical aggregate sizes, the strength development initially decreases before increasing with higher aggregate content, due to the combined effects of internal pore structure evolution and improved mechanical interlocking between the cementitious matrix and aggregates. The slurry's flowability exhibits an inverse correlation with rheological parameters. Increasing the aggregate content reduces the slump flow while elevating the yield stress, plastic viscosity, and thixotropy, consequently raising pumping energy consumption, decreasing transport efficiency, and increasing pipeline clogging risks, thereby adversely affecting pipeline conveyance performance. The findings of this study provide theoretical support for engineering applications of gangue-based geopolymer aggregate slurry.