Preparation of magnesium oxysulfide cement from rare earth smelting magnesium sulfate wastewater: Mechanical properties, microstructure and immobilization mechanism

Abstract

The rare earth smelting magnesium sulfate wastewater (RESW) contains high concentrations of sulfate ions (SO₄^2-), magnesium ions (Mg²⁺), heavy metal ions, and trace amounts of rare earth elements. Conventional water treatment technologies face challenges such as complex processes and high costs. This study aims to explore the application of RESW as a substitute for MgSO₄ in the preparation of magnesium oxysulfide cement (MOSC) to achieve harmless treatment of RESW. The effects of varying concentrations of RESW on the setting time, compressive strength, length variation, water and acid resistance, and leachability of MOSC were investigated. Additionally, the effects and mechanisms of RESW on MOSC were systematically analyzed using infrared spectroscopy, thermogravimetric analysis, nanoindentation, XRD, and SEM-EDS. The results indicated that, under identical preparation conditions, RESW delayed the hydration process of MOSC and contributed to its early-strength characteristics. Although RESW reduced the 28-day compressive strength of MOSC to varying extents, it still reached 55 MPa. MOSC exhibited better volume stability, with length change rates decreased from 0.42 % to 0.21 %. The primary phase composition remained largely unchanged. Additionally, the softening coefficient of MOSC was enhanced, and the leachability of elements from RESW was low, with a solidification rate exceeding 99 %. This study presents an innovative approach to the treatment of RESW and provides a reference for the practical application of MOSC produced from RESW in construction engineering, thereby promoting the high-value utilization of RESW.