<p>Slag has been extensively employed as a supplementary cementitious material in concrete. To promote the resource utilization of slag, cement was partially substituted with slags with three distinct fineness levels. The mechanical properties, hydration heat, hydration products, porosity, and morphology affected by slag fineness were evaluated systematically. The intrinsic hydration behavior of slag and its effect on cement hydration were also investigated using thermodynamic modeling. The results demonstrate that the slag fineness significantly influences both compressive strength development and hydration. Specifically, superfine slag accelerates the early hydration of cement by providing abundant nucleation sites, while refining the pore structure through both a filling effect and pozzolanic reaction. Thermodynamic modeling and experimental analysis reveal a rapid reaction pathway for fine slag involving OH⁻ diffusion, network breakdown, and ion leaching, leading to the formation of C-A-S-H gel and a reduction in Ca(OH)₂ content. Furthermore, the addition of fine slag was found to optimize the pore size distribution by reducing harmful pores, thereby compensating for the dilution effect and enhancing long-term compressive strength. These findings confirm that utilizing superfine slag allows for a higher replacement ratio of cement, effectively reducing carbon emissions and production costs without compromising material performance.</p>