This study systematically investigates the mechanical behavior and failure mechanisms of cemented tailings backfill (CTB) prepared from classified and unclassified tailings across cement-to-tailings (C/T) ratios of 1:8, 1:6, and 1:4 and slurry concentrations of 60%, 65%, and 70%. Specimens were evaluated by uniaxial compression (UCS) tests, failure mode observation, and SEM. The results show that increasing C/T and concentration markedly enhances compressive strength: the maximum 28-day UCS reached 5.38 MPa under unclassified tailings, C/T = 1:4, 70%. Moreover, unclassified tailings exhibited a later-age strength gain of 244.9%, far exceeding the 58.5% observed for classified tailings. Failure modes evolve from brittle splitting to shear-dominated behavior as mixes densify, reflecting a transition from near-symmetric early-age stress/microstructural fields to asymmetric localized failure (symmetry breaking). SEM reveals that higher binder ratios and concentrations promote C-(A)-S-H-dominated gel formation, improved ITZ continuity, and reduced apparent porosity, thereby restraining microcrack initiation and coalescence. These findings elucidate the micro-to-macro mechanisms governing CTB strength and failure and provide field-relevant guidance for mix optimization and safe, efficient underground backfilling.