Impact of Cl- on the performance of magnesium oxysulfate (MOS) cement

Abstract

As magnesium oxysulfate (MOS) cement finds more applications in the engineering field, the influence of Cl- on the mechanical properties of MOS cement has gained more attention. In this paper, MgCl2·6 H2O served as the source of Cl- to investigate its influence on the evolving properties of MOS cement. The results showed that MgCl2·6 H2O extended the formation time of the 5Mg(OH)2·MgSO4·7 H2O phase, reduced the dispersion of the MOS cement system, and decreased the crystalline size of the 5Mg(OH)2·MgSO4·7 H2O phase (200) crystal plane. The addition of MgCl2·6 H2O had a positive effect on improving flexural strength and increasing the content of gel. During the curing process, the presence of Cl- facilitated the dissolution and recrystallization of Mg(OH)2, altering its growth tendency and promoting the formation of hexagonal crystal Mg(OH)2. During soaking in water, the increased content of MgCl2·6 H2O led to a decrease in the softening coefficient due to the dissolution of excess MgCl2. The structure of the 5Mg(OH)2·MgSO4·7 H2O phase underwent changes as Cl- entered the crystal interior, leading to an increase in the d-spacing of the 5Mg(OH)2·MgSO4·7 H2O phase at the (222) crystal plane. The introduction of Cl- endowed the 5Mg(OH)2·MgSO4·7 H2O phase with the potential to transform into 5Mg(OH)2·MgCl2·8 H2O. Following immersion in water, the modified structure of the 5Mg(OH)2·MgSO4·7 H2O phase retained its relative integrity, resulting in a higher softening coefficient of 0.73 after a 90-day soaking period, attributed to the proper introduction of Cl-.