Low-activity copper tailings (CTs) were mechanochemically activated and used to develop a copper tailings–slag–metakaolin grouting material (CSM). Mechanical grinding and Na₂SO₄ addition were first optimized, after which single-factor experiments and response surface methodology were used to evaluate the effects of the water-to-solid ratio, alkali activator dosage, and MCCT content on slurry properties and compressive strength. Grinding for 40 min with 3 wt% Na₂SO₄ relative to the dry CT mass produced the highest activity index of 84.5%. The optimized CSM mixture had a water-to-solid ratio of 0.69, an alkali activator dosage of 6.7 wt%, and an MCCT content of 35.5 wt% of the dry solid precursor. Validation experiments yielded a fluidity of 235 mm, a bleeding rate of 2.8%, a setting time of 165 min, and a 28 d compressive strength of 8.62 MPa, with prediction errors below 5%. Microstructural characterization indicated that mechanochemical activation refined the CT particles, increased their structural disorder, and contributed to the formation of Ca–Si–Al–O-rich binding phases and AFt-like crystals in the hardened matrix. Within the defined preliminary material-level system boundary, the embodied energy, greenhouse gas emissions, and material cost of CSM were estimated to be 67.86%, 82.87%, and 27.28% lower, respectively, than those of the 1 kg ordinary Portland cement (OPC) binder benchmark. These results indicate the potential of CSM as a filling and consolidation grout with reduced material-level environmental burdens and cost within the adopted assessment boundary.