Abstract: Toward the successful implementation of the “dual carbon” strategy, large-scale backfilling technology utilizing coal gangue has emerged as a pivotal approach for ecological restoration and solid waste disposal in coal mine goaf areas. However, owing to its high clay mineral content (43.5%) and porous structure (specific surface area: 11.14 m²·g⁻¹; pore volume: 0.037 cm³·g⁻¹), coal gangue powder tends to compete with Portland cement for the adsorption of the polycarboxylate superplasticizer (PCE). This leads to deteriorated rheological properties of the slurry, manifested as increased viscosity and greater pumping resistance, thereby posing engineering challenges. The purpose of this study is to elucidate the competitive adsorption mechanism between coal gangue powder and cement for the PCE and to enhance the pumping performance of the backfill slurry with high gangue powder content (>30%) through molecular structure optimization. Based on scanning electron microscopy (SEM), Zeta potential analysis, and adsorption capacity determination, the adsorption behavior and dispersion mechanisms of two types of superplasticizers, namely, conventional comb-type (PCE-A) and short main-chain, long side-chain type (PCE-B), were systematically investigated in the coal gangue–cement system. First, the morphological characteristics of coal gangue powder were observed through SEM, revealing its lamellar structure with numerous micrometer-scale fissures and nanometer-scale pores. Quantitative X-ray diffraction (XRD) indicated that coal gangue powder contains high levels of kaolinite (13.1%), illite (15.4%), and montmorillonite (15.0%), among other layered clay minerals. Furthermore, the Zeta potential test results suggested that the absolute value of the negative potential on the surface of coal gangue powder (−7.21 mV) is significantly lower than that of cement particles (−14.41 mV), leading to system stability reduction. Adsorption kinetics experiments demonstrated that the adsorption amount of the PCE by coal gangue powder increases in two stages with increasing dosage: rapid physical adsorption stage (1–15 min; rate: 0.0057–0.0136 mg·g⁻¹·min⁻¹ and slow chemical adsorption stage (15–90 min; rate: 0.0005–0.0007 mg·g⁻¹·min⁻¹). The Langmuir isotherm adsorption model indicated that the saturation adsorption amount of PCE-A on coal gangue powder (1.79 mg·g⁻¹) is 2.4 times that on cement (0.75 mg·g⁻¹), while the saturation adsorption amount of PCE-B on coal gangue powder (1.04 mg·g⁻¹) is 42% lower than that of PCE-A, and its competitive adsorption coefficient (α=1.55) is significantly lower than that of PCE-A (α=2.39). Molecular structure analysis indicated that the short main chain of PCE-B (degree of polymerization (DP) is 15) inhibits interlayer intercalation (montmorillonite layer spacing: 1.2 nm), while its long side chain (the molecular weight of polyethylene glycol (PEG) is 2400) enhances steric hindrance, increasing the interparticle interaction distance beyond the range of van der Waals forces. Rheological tests confirmed that when the coal gangue powder content reaches 50%, the slurry viscosity in the PCE-A system increases sharply from 920 to 2300 mPa·s (150% increase), whereas the viscosity in the PCE-B system only rises from 503 to 880 mPa·s (75% increase), significantly optimizing dispersion performance. Overall, the results indicated that (1) the high porosity and clay mineral composition of coal gangue powder result in ineffective adsorption of PCE, which is the main cause of slurry rheological deterioration. (2) The molecular design of PCE-B balances the adsorption competition between heterogeneous powders by inhibiting intercalation and enhancing spatial repulsion. When the content of coal gangue powder in the slurry increases from 0 to 50%, the viscosity increase of PCE-B system decreases from 150% to 75% and the viscosity increase decreases by 50% compared with PCE-A system under the condition of 0.1% PCE content. (3) For backfilling slurries with high coal gangue powder content, a short main-chain, long side-chain PCE can effectively enhance pumping performance, providing a theoretical basis for selecting superplasticizers suitable for solid-waste-based backfilling materials.