• Increasing HPMC dosage from 0 to 0.35wt% linearly reduces the uniaxial compressive strength of cementitious tungsten tailing backfill (CTB) specimens.
• The 75 mm × 150 mm CTB specimen demonstrates superior plasticity, toughness, and energy absorption, highlighting a significant size effect.
• HPMC improves anti-segregation and anti-bleeding properties, leading to more uniform hydration and reduced Ca/Si atomic ratio dispersion.
• Microstructural analysis reveals interwoven ettringite and C-S-H gels tightly wrapping tungsten tailings, explaining the combined tensile-shear failure modes.