Influence of calcium mass fraction of magnesite tailings on cementing properties of magnesium oxysulfate cement

To investigate the influence of impurity factors(calcium) in tailings on the magnesium oxysulfate cement system, magnesium oxysulfate cement prepared from magnesite tailings was used as the research object. Firstly, based on magnesite tailings, dolomite was added to alter the calcium content in the tailings. Secondly, the setting time and uniaxial compressive strength test of magnesium oxysulfate cement were conducted. Finally, the influence of calcium mass fraction on the hydration products, mechanical properties, setting time, and microstructure of magnesium oxysulfate cement was studied using analysis methods such as thermogravimetric analysis, differential thermal analysis, X−ray diffraction, and scanning electron microscopy. The results show that magnesium oxysulfate cement is mainly composed of unreacted magnesium oxide(MgO) and hydration products 3Mg(OH)2·MgSO4·8H2O(318 phase), magnesium hydroxide(Mg(OH)2), calcium sulfate dihydrate(CaSO4·2H2O), and magnesium carbonate(MgCO3). The increase of calcium mass fraction in magnesite tailings gradually reduces the 318 phase crystals in the hydration products of magnesium oxysulfate cement, while the Mg(OH)2 and CaSO4· 2H2O crystals gradually increase, resulting in a gradual decrease in the mechanical strength of magnesium oxysulfate cement specimens. The increase of the mass fraction of calcium in magnesite tailings will lead to the decrease in the mass fraction and activity of active MgO in the calcined product, and to the increase in the mass fraction of free calcium oxide. The increase of calcium mass fraction also significantly increases the hydration reaction rate of magnesium oxysulfate cement, shortens the setting time significantly, and leads to incomplete hydration reaction of magnesium oxysulfate cement, resulting in a decrease in the generation of hydration products. © 2024 Central South University. All rights reserved.