Study on performance optimization of gasification slag based paste filling materials in Ningdong mining area

As one of the nine coal bases at hundred million ton level in Yellow River Basin, Ningdong Mining Area has nearly 2×10⁸ t of coal-based solid waste produced annually, and faces to the problems of large stock of gasification slag, difficulty in scaled utilization, and limited space for simple landfill. Filling mining could solve the problem of stockpiling, of which the cost is high and the performance needs to be optimized. According to the response surface method, 17 groups of central composite experiments were designed for the three factors at three levels: the mixing amount of gasification slag in solid (A), the mass ratio of gasification slag to cement (B), and the slurry concentration (C). Meanwhile, comparative optimization research was conducted for the slump, expansion, 7 d and 14 d uniaxial compressive strength and other properties of gasification slag based paste filling materials. Before the experiment, the composition and micromorphology of the raw materials were observed and analyzed by XRD and SEM. Besides, the test block was observed by SEM and had its hydration characteristics analyzed after uniaxial compression, thereby revealing the strength formation mechanism. By integrating the strength and fluidity, the following optimum mixing ratio and performance characteristics of the filling material were obtained: 48% for A, 3 for B and 80% for C. Meanwhile, the desulfurized gypsum, coal gangue and bottom slag should be mixed at the mass ratio of 2∶1∶1 to reach the 7 d and 14 d strengths of 1.15 MPa and 2.41 MPa respectively, the slump of 133 mm, the expansion of 325.5 mm, and the slump to expansion ratio of 0.41. According to the further analysis based on response surface method, the single influencing factors of 7 d and 14 d strength are B>C=A and B>A>C, the interaction influencing factors of 7 d and 14 d strength are BC>AB>AC and AB>AC>BC, and the single influencing factors of slump and expansion are C>B>A and C>A>B respectively according to the significance. On this basis, the proportioning scheme and parameters were optimized for the 3 different functional requirements, including the strict control of surface settlement, rapid filling to reduce pipe plugging, and control of strength and cost. Generally, the research results provide important basic parameters and optimization direction for the ecological protection and low damage coal mining in Yellow River Basin.