Comparative Stabilization of TSC Composite Soil Using Cement, Lime, Solid-Waste-Based Binders, and MICP: Mechanical Performance, Microstructure, and Carbon Emissions

To promote the resource utilization of industrial by-product titanium gypsum while improving its engineering performance, a TSC composite soil containing titanium gypsum and clay at a mass ratio of 60:40 was investigated. Four stabilization methods, namely cement, solid-waste-based cementitious binder, lime, and microbially induced calcium carbonate precipitation (MICP), were systematically compared. Mechanical and durability performance was evaluated through unconfined compressive strength, water-stability, and freeze–thaw tests at different curing ages, while the stabilization mechanisms were characterized using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM-EDS), and nitrogen adsorption–desorption analysis (BET). Carbon emissions were further evaluated using a life-cycle-based emission-factor approach. The comparative framework integrating engineering performance, microstructural mechanisms, and environmental impacts constitutes the main contribution of this study. At 28 days, cement-stabilized soil exhibited the highest UCS of approximately 2.6 MPa, followed by the solid-waste-based binder, lime, and MICP systems. MICP produced CaCO3 precipitates that filled pores and formed interparticle bridging structures, thereby improving water stability and structural integrity despite its relatively lower short-term strength. The estimated net carbon emission of the MICP system was 4.3 kg CO2/t of stabilized soil, approximately 86% lower than that of cement stabilization. These results demonstrate a clear trade-off between mechanical performance and environmental impact among the four stabilization approaches and indicate that MICP is a promising low-carbon option for titanium-gypsum-based composite soils, although further field-scale and long-term validation is required.