The large-scale generation of coal gangue in China presents urgent challenges for resource utilization and carbon emission reduction. This study investigates the preparation and performance optimization of a limestone–calcined coal gangue cement (LCC) system under CO₂ mineralization curing. Experimental methods—including X-ray diffraction (XRD) and mercury intrusion porosimetry (MIP)—combined with molecular dynamics (MD) simulations were employed to examine how Ca(OH)₂ content and pore structure influence hydration, carbonation, and strength development. Results showed that moderate incorporation of Ca(OH)₂ (10–20 wt% relative to gangue) under CO₂ curing enhanced 28-day compressive strength by up to 14.9%, owing to the formation of CaCO₃ which densifies the matrix and reduces porosity. However, excessive Ca(OH)₂ caused micropores and strength loss. MIP revealed that refined pore structure, particularly increased gel pores (< 10 nm), was correlated with strength gains. MD simulations further demonstrated that larger pore sizes (e.g., 24 Å) promoted CO₂ diffusion and adsorption, while nanoscale confinement hindered transport due to structured water layers. Overall, the LCC system with optimized Ca(OH)₂ content and pore architecture not only matches the strength of ordinary Portland cement but also enables CO₂ sequestration. This research offers a low-carbon strategy for coal gangue valorization and provides multiscale insights for designing sustainable cementitious materials.