Determining the Optimal Gradation of Mixed Recycled Aggregate Containing Coal Gangue Brick Aggregate

Aggregate grading design is the foundation of concrete mix proportion design. Existing aggregate grading design methods are based on the tightest accumulation theory of homogeneous natural aggregates. However, this method has not been applied to mixed recycled aggregate (MRA) containing coal gangue brick aggregate (CGBA) (hereinafter referred to as MRA) due to significant differences in physical and mechanical properties. In this study, we subjected five types of MRA (initial particle size: 19–26.5 mm) to confined crushing, sieving, and grading composition analysis, then configured recycled concrete. We investigated the determination method of the optimal gradation of MRA, summarized the composition, particle distribution characteristics, and fractal characterization of the optimal gradation. This study focused on analyzing the factors that affect the compressive strength of recycled concrete, as well as examining the mix proportion design method. The results reveal that the optimal gradation of MRA can be determined using the five-stage loading method for approach minimum porosity. Our analysis of MRA gradation suggests that the mix proportion design of recycled concrete should be determined by considering the porosity of optimal gradation and the surplus volume coefficient of slurry. Finally, we determined that the water-binder (W/B) ratio is the primary factor that affects the compressive strength, while the CGBA’s intrinsic strength has a minimal impact. The replacement ratio of CGBA primarily affects the porosity of MRA and the W/B ratio, indirectly reducing recycled concrete’s compressive strength. Based on our findings, we recommend that the replacement ratio of CGBA should not exceed 50%. This study can promote the recycling of recycled concrete containing CGBA.