## 1 Introduction  
Coal gangue (CG) is solid waste generated during coal mining and washing, comprising about 10-25% of total coal output. In China, CG production ranges from 600 to 800 million tons annually, with a utilization rate below 70%. Currently, CG stockpiles exceed 7 billion tons, increasing by 300-350 million tons each year. This accumulation leads to land wastage, severe air and water pollution, and potential environmental disasters like landslides and spontaneous combustion [1]. Therefore, it is crucial to explore effective utilization strategies. Over the past two decades, technologies have emerged for CG disposal in various sectors, such as power generation, concrete production, and road construction. However, due to the complex mineral composition and transport costs, only limited disposal options are viable. Recently, cemented backfills--composed of cementitious materials and mining waste--have gained popularity for stabilizing mine goals. This method does not require specific mineral content and allows for on-site use of CG, making it a convenient and cost-effective solution [2]. CG cemented materials (CGCMs) are vital for supporting surrounding rock, helping to maintain stability in underground spaces. However, their high brittleness makes concrete susceptible to cracking over time due to challenging environmental conditions. If issues like micro-cracks are not quickly resolved, the lifespan of CGCM structures can be greatly diminished, leading to severe consequences. Incorporating additives like fibers and nanoparticles is considered an effective method to improve the mechanical properties of CGCMs. There are many studies focusing on additive materials for multifunctional concrete. Among them, carbon fibers (CFs), steel fibers (SFs), and carbon black (CB) are widely used because of their high performance and lower price. [PERSON] et al. [3] evaluated the mechanical properties of concrete with SFs and found that the addition of fibers to self-compacting concrete improved its mechanical behavior after cracking load. [PERSON] et al. [4] studied the flexural behavior of CF-reinforced concrete beams under impact loading, and their test results showed the addition of CFs improved concrete's ability to absorb impact energy. [PERSON] et al. [5] analyzed the self-sensing capability of concrete incorporated with CFs or CB, and they found that carbon-based materials improved the mechanical properties of concrete, and CFs were the best for improving the self-sensing capacity of concrete. [PERSON] et al. [6] studied the tensile self-sensing and mechanics of SF-reinforced concrete. Their research showed that SFs produced a significant enhancement of both self-sensing capacity and tensile strength. [PERSON] et al. [7] compared the electrical and mechanical properties of concrete with different CB, and they found that a CB particle with high structure and surface area significantly enhanced electrical properties of concrete but had a harmful impact on its compressive strength. [PERSON] et al. [8] studied the mechanical properties and damage evolution of waste tire SF-reinforced CGCM. Their results demonstrated a significant enhancement in both the compressive strength and peak strain of the CGCM. [PERSON] et al. [9] found that incorporating CB significantly improved the mechanical strength and workability of cemented materials. A similar finding was reported in the study by [PERSON] et al. [10], which showed that CB achieved comparable compressive strength due to its internal curing, interlocking, and bridging effects. The literature mentioned above focuses mostly on macro-mechanical properties of concrete with different admixtures. The macro-mechanics of concrete is the result of deformation and damage of micro- or meso-structure, so it is important to study the meso-mechanical characteristics of multifunctional concrete. However, literature about this field is rare.