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. [ABSTRACT FROM AUTHOR]