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To address the inherent conflict between fluidity and stability in high-concentration unclassified tailings slurries for blasthole stemming slurry (BSS), this study establishes an evaluation system based on “equal fluidity” to screen and optimize chemical admixtures suitable for high-concentration BSS. Three typical superplasticizers—polycarboxylate (PCE), naphthalene-based (NF), and melamine-based (MF)—were selected to systematically compare their effects on rheological parameters and bleeding performance under a controlled, consistent fluidity condition (16.0 ± 0.5 cm). The results indicate that the effectiveness of superplasticizers exhibits noticeably concentration dependence. While NF demonstrates the highest dispersion efficiency at low concentrations, PCE emerges as the sole effective admixture capable of maintaining the fluidity of high-concentration BSS (71% solid mass fraction), attributed to its robust steric hindrance effect. Rheological analysis reveals that the PCE-modified BSS exhibits a unique state characterized by “low yield stress and high differential viscosity,” which effectively decouples the contradiction between macroscopic flow and microscopic stability. Furthermore, the synergistic effect of high concentration and PCE constructs a kinetically stable suspension system, achieving “zero bleeding.” This study confirms that PCE is the optimal choice for preparing high-concentration pumpable BSS, providing a theoretical foundation for the design of deep-hole stemming materials in mining engineering. Full article