<p>In this study, uniaxial compression and nanoindentation tests were conducted on cemented broken coal composite (CBC) samples with four different slurries to investigate the mechanical properties and strength enhancement mechanism of broken coal after grouting. The results show that the modulus and width of interfacial transition zones (ITZ) vary with grouting material, while ITZ width is negatively related to the peak strength of CBC samples. Then, a discrete element method (DEM) model, which consists of coal, hardened slurry, and ITZ, was proposed for CBC samples, considering the existence of ITZ. The effectiveness of the proposed approach was validated by comparing the numerical strength and failure mode of CBC samples with those of the experiments. Furthermore, the influence of coal block size on the mechanical properties of CBC samples was investigated using the proposed DEM. Numerical results confirmed that with the increase in coal block size, the total acoustic emission b-value gradually decreased, the failure mode of the sample transitioned from shear failure to tensile failure, and where the failure occurring changed from ITZ region to coal block. Finally, the influence mechanism of coal block size on mechanical characteristics of cemented broken coal composite was discussed. The work provides novel perspectives on the grouting reinforcement mechanism in response to coal block size distribution.</p>