Feasibility Study of the Synergistic Use of Sludge and Coal-Based Solid Waste to Produce Environmentally Friendly Grouting Materials

In this paper, a full solid waste-based grouting material was prepared using three industrial solid wastes, i.e., sludge, coal gangue (CG), and ground granulated blast furnace slag (GGBS), based on the concepts of synergy and complementarity. The effects of the dosage of raw sewage sludge (SS), incineration-activated sewage sludge ash (SSA), and an alkali activator on the fluidity, water separation rate, and mechanical strength of solid waste-based grouting materials were systematically investigated. The mechanism of action was analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Finally, the leaching characteristics and stabilization efficiency of heavy metals in solid-sample-based slurry stones were determined. The experimental results show that the slurry fluidity and water separation proportion are negatively correlated with the amount of sludge. The rate of decline first increases and then decreases. When the SSA content is greater than 10%, the precipitation rate can be controlled to within 5%. The optimal amount of alkaline activator is 8%, and its effect on the mechanical strength is more significant than that of sludge. With an increase in the SSA dosage, the compressive strength first increases and then decreases. At 25%, the strength still reaches 20.8 MPa, and the decrease from 0 to 25% is only 26.2%. On the contrary, the addition of SS continues to decrease the strength of the stone body by 81.9%. The high organic content and low volcanic ash activity in SS hinder the development of hydration cementation. In addition, the comparative analysis demonstrated the contribution of the volcanic ash activity possessed by SSA to the mechanical strength supplement after incineration. The incineration treatment caused the calcite (CaCO3) in SS to decompose at high temperatures, and more Ca sources were introduced in SSA. The incorporation of SSA in the cementation system resulted in higher Ca/Si and Ca/Al ratios, promoting the formation of C-(A)-S-H gel. Moreover, this incorporation enhances the stability of heavy metals within the slurry, reducing the potential environmental risk associated with the leaching of Cr and Ni from the raw materials. Consequently, these findings comply with the leaching requirements specified by the environmental standards. The research provides innovative insights into the synergistic resource utilization of SS and SSA with coal-based solid waste to prepare environmentally friendly, high-performance, and cost-effective grouting materials.