Further study of the erosion mechanism of calcium salt on 517 phase

Abstract

As the main hydration product of magnesium oxysulfate (MOS) cement, 517 phase (5Mg(OH)₂·MgSO₄·7H₂O) is one of the main sources of MOS cement strength. At present, the experimental results show that calcium ions have a great influence on the stability of 517 phase, so this paper mainly explores the erosion process and mechanism of calcium ions on 517 phase. The 517 phase with 94.56 % purity was synthesized in this paper, and the erosion effect of different calcium salts on 517 phase was also characterized, where thermogravimetric-differential scanning calorimetry (TG-DSC), scanning electron microscope (SEM), transmission electron microscope (TEM), selected area electron diffraction (SAED), X-ray diffraction (XRD) were used to analyze the performance changes of 517 phase under different calcium salt erosion. The results show that in the solution, Ca²⁺ combined with SO₄²⁺ in 517 phase, resulting in an increase in the spacing between the MgO₆ octahedron layers in 517 phase thus destroying the structure of 517 phase. Moreover, when ions diffused, MgO₆ octahedron and Cl²⁺ recombined into 518 phase (5Mg(OH)₂·MgCl₂·8H₂O). In addition, insoluble weddellite was also found to erode 517 phase to a certain extent, while the 517 phase blended with gypsum was stable. The simulation results of molecular dynamics also showed that Ca²⁺ had better adsorption ability for SO₄²⁺ and Cl²⁺ than Mg²⁺. When it was in contact with 517 phase, Ca²⁺ was absorbed near SO₄²⁺ and Cl²⁺ was brought into the system.