Development and field application of a modified magnesium slag-based mine filling cementitious material

How to make high value use of solid waste is a key issue in achieving sustainable development. In this study, a modified magnesium slag-based cementitious material (M·C) for mine filling was developed using industrial solid waste modified magnesium slag (MMS), fly ash (FA), and desulfurization gypsum (DG); the mechanical properties, stability, hydration heat and synergistic reaction mechanism of M·C were investigated; and the M·C with optimized ratios was used to prepare cemented aeolian sand/coal gangue backfill material for industrial mine backfill tests. The results revealed that (1) the uniaxial compressive strength (UCS) of M·C at 28 d was 5.60–19.30 MPa, and the FA and DG contributed significantly to the development of the UCS of M·C. A combination of 65–90% MMS, 2.5–7.5% DG and 5–35% FA was determined to be optimum for the preparation of M·C. (2) The expansion rate (Ex) of M·C increased rapidly in the early stage, increased slowly after 28 d, and stabilized after 90 d. E28 of M·C was 0.107%–0.188%, which satisfied the requirement of Ex ≤ 0.6% in GB/T 2938. (3) The hydrated exothermic process of M·C was divided into initial dissolution, induction, acceleration, deceleration and slow reaction stages. DG can effectively promote the early hydration process of M·C, while the contribution of FA is not noticeable, and even reduces the hydration heat of M·C. (4) The hydration of MMS generates C-S-H gels and CH to increase the strength and form an alkaline environment. DG promotes the dissolution and polymerization of MMS and FA into C-S(A)-H gels and ettringite, a process that consumes Ca2+ and OH− and promotes the hydration of MMS. (5) The UCS of the 28 d downhole cores from the Changxing and Mahuangliang coal mines were 7.73 and 12.62 MPa, respectively, and the leaching indexes met the requirements stated in GB/T 14848 (Ⅲ), both of which reach the requisites for safe underground backfill. The results of this study demonstrate the feasibility and advancements of the new all-solid waste M·C replacement cement and its backfill application, and also standardizes the production and filling application route of M·C and accelerates the industrialization process of this technology.