<p>High-concentration bentonite slurries are widely used in shield tunnelling for ground sealing. However, their high yield stress and viscosity limit penetration into fine rock fissures and increase pumping pressures. This study investigates sodium silicate as an inorganic rheology modifier for bentonite suspensions at concentrations of 3–7.5%, with dosages ranging from 0 to 3% by weight. Flow curves were fitted to four constitutive models: Bingham, Modified Bingham, Herschel–Bulkley, and Sisko. Fresh-state stability, environmental performance, and economic viability were assessed through bleeding tests, performance-normalized sustainability metrics, and Response Surface Methodology, respectively. The results show that 2% sodium silicate reduces the apparent viscosity of a 7.5% bentonite slurry from 283.7 to 14.6&#xa0;mPa·s, while maintaining fresh-state stability below the 5% bleeding threshold. No single rheological model adequately describes all formulations. The optimal model shifts systematically with composition, from Herschel–Bulkley for intact flocculated networks to Modified Bingham for dispersed systems. A composition-dependent model selection framework is proposed to support accurate pumping pressure calculations in the field. Two performance-normalized indices—Carbon Efficiency of Fluidization and Cost Efficiency of Fluidization—are introduced to couple rheological improvement with environmental and economic impact. The 7.5% bentonite formulation modified with 2% sodium silicate achieves a Sustainability Index of 152.18, with an embodied carbon footprint of only 3.63&#xa0;kg CO₂/m³. Multi-objective optimization identifies an optimal composition of 7.38% bentonite and 2.06% sodium silicate, with a composite desirability of 0.912. These findings provide a quantitative framework for designing injectable, low-carbon bentonite slurries for targeted tunnelling applications.</p>