Study on leaching and curing mechanism of heavy metals in magnesium coal based backfill materials

In order to promote the resource utilization of bulk solid waste, it is an effective way to improve the comprehensive utilization level of solid waste and alleviate the environmental pressure caused by storage by using all solid waste as raw material for backfill cementing. In this paper, a magnesium coal based backfill material (MCB) with modified magnesium slag (MMS) and fly ash (FA) as cementing agents, coal gasification coarse slag (CGCS) and coal gangue (CG) as aggregates was proposed. First, the mechanical properties of MCB were studied and then the hydration effect was verified microscopically. Then, the leaching characteristics of raw materials CGCS, CG, and backfill material MCB were characterized. Furthermore, the mechanism of heavy metal leaching and solidifying of MCB was revealed by studying As and Pb. The results show that: (1) The addition of a suitable amount of CGCS with potential pozzolanic activity produces more MCB hydration products such as C-S-H and AFt with a high CG-CGCS ratio. The compressive strength reaches the maximum value of 10.21 MPa when the curing time is 28 days and the CG-CGCS ratio is 3:1. (2) The leaching levels of the target elements (As, Pb, Cr, Mn, Zn, Cu, Cd, Hg, Ag) of CG meet the limits of the class III groundwater quality standard (III-GQSL) in GB/T 14848–2017. The concentration of Pb in CGCS is 12 μg/L, which exceeds the III-GQSL limit of 10 μg/L. The overall leaching risk is CGCS>CG. (3) MCB has a good stabilization/solidification effect on the heavy metal elements Cd, As, and Pb in raw materials. The heavy metal leaching results of MCB with a low CG-CGCS ratio meet the limit of III-GQSL, and the environmental leaching risk is low. (4) In addition to physical encapsulation, the main curing mechanisms of MCB include chemical bonding, alkaline environment, and adsorption. MCB can be used as a potential consolidation agent for the solidification of heavy metal elements. While having good mechanical properties, it can also treat solid waste and effectively stabilize/solidify toxic elements in materials and has the potential for safe application in backfill.