In response to the global drive for carbon neutrality, this study investigates whether bentonite can partially replace cement to enhance the mechanical properties and environmental sustainability of cement-stabilized silt. Laboratory tests and field analyses were conducted on waste sludge for subway tunnel engineering. Results show that bentonite significantly improves unconfined compressive strength, with peak performance at an optimal dosage; excessive addition causes strength loss due to swelling and micro-cracks. Bentonite also increases failure strain and reduces brittleness, with the best effect at 9% content. A linear relationship between the deformation modulus (E₅₀) and compressive strength was confirmed. Permeability decreased significantly at 3% bentonite and then stabilized. Strain energy analysis verified improved fracture resistance. Importantly, using bentonite can reduce cement usage by up to 30%, saving about 200,000 kcal of energy per ton of mixture. Field tests further validate its practical effectiveness in subway tunnel applications. Overall, the findings confirm that the study’s goal was achieved, demonstrating both structural and environmental benefits, and providing a practical reference for sustainable construction practices.