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

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. This paper mainly explores the erosion process and mechanism of calcium ions in the 517 phase. The 517 phase with 94.56% purity was synthesised and the erosion effect of different calcium salts on the 517 phase was characterised. Results show that, in the solution, Ca²⁺ combined with SO₄²⁻ in the 517 phase, resulting in an increase in the spacing between the MgO₆ octahedron layers in the 517 phase, thus destroying the structure of the 517 phase. Moreover, when ions diffused, the MgO₆ octahedron and Cl⁻ recombined into the 518 phase (5Mg(OH)₂·MgCl₂·8H₂O). In addition, insoluble weddellite was found to erode the 517 phase to a certain extent, whereas 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 Cl⁻ than Mg²⁺. When it was in contact with the 517 phase, Ca²⁺ was absorbed near SO₄²⁻ and Cl⁻ was brought into the system.