Effect of Steel Slag Coarse Aggregate Particle Size and Replacement Ratio on Concrete Mechanical Properties and Mesoscale Structure

This study investigates the impact of steel slag coarse aggregate (SSA) particle size on the macroscopic mechanical properties of concrete. Considering that the macroscopic behavior of concrete is significantly influenced by its mesoscale structural characteristics, and that coarse aggregate particle size is a key factor determining these features, uniaxial compression experiments together with mesoscale simulations were carried out to develop a model linking the mesoscale structure to the mechanical response of steel slag coarse aggregate concrete (SSAC). The results show that SSAC exhibits a failure pattern comparable to that of natural aggregate concrete (NAC), but its stress–strain curve exhibits a steeper ascending branch and higher peak stress. With the increasing SSA replacement ratio, the peak stress continuously increases; within the same particle size range, the elastic modulus shows an initial increase followed by a subsequent decrease, reaching its maximum at a 50% replacement ratio. Expanding the particle size range changes the peak strain response from approximately linear to rapidly increasing; smaller particle sizes result in a gentler post-peak drop, whereas higher replacement ratios produce a steeper decline. The mesoscale model further shows that for SSA particle sizes of 5–20 mm, 5–15 mm, and 5–10 mm, the cohesive strength of the interfacial transition zone (ITZ) increases by 75%, 106%, and 92%, respectively, compared with NAC. Increasing the coarse aggregate volume fraction further enhances the ITZ strength improvement. This study offers valuable insights for improving the mixture design and performance of SSAC.