Formation process and thermodynamic mechanism of the crust in magnesite mining areas

Large areas of soils, in the magnesite mining areas of northern China, are covered with a hard-white crust, which affects ecological functions of soil seriously, particularly reduce water penetration rate. To clarify the mechanism of crust preventing water from penetrating into soil, the process and thermodynamic mechanism of crust formation were studied through laboratory simulation experiments. The crust was mainly composed of magnesium oxide (MgO), magnesium carbonate (MgCO3), and magnesium dihydroxide (Mg(OH)2). MgCO3 and Mg(OH)2 were produced in the initial stage of crust formation. The increasing amount of MgCO3 and Mg(OH)2 enhanced the production of 4MgCO3·Mg(OH)2·4H2O. Mg(OH)2·MgSO4·8H2O was formed in the fourth week, which had a greater mechanical strength and became the framework of forming crust. In the latter stages, the increasing amount of 4MgCO3·Mg(OH)2·4H2O and MgSO4·6H2O made the crust to be tighter and harder. The crust formation was a sedimentary process, in which the deposition of dust was accompanied by complex chemical reactions. In addition, the thermodynamic parameters for main chemical reactions indicated that the crust formation was a spontaneous dynamic process. Therefore, the result of this study illustrated the changes of crust formation in different stages from different aspects. The findings of this study provide important information for understanding the formation of crust and developing effective approaches for soil remediation in affected areas.