Performance of magnesium oxysulfate (MOS) cement prepared by MgCl2·6 H2O replacing MgSO4·7 H2O

Abstract

Despite the influence of Cl- on the properties of cement-based materials, water containing Cl- is more convenient for manufacturing cement-based materials in special areas such as islands and reef construction, compared to fresh water. However, research on the effects of Cl- on the properties of magnesium oxysulfate (MOS) cement was limited. In this paper, MOS cement was prepared by replacing MgSO4·7 H2O with MgCl2·6 H2O to investigate the effect of Cl- on the evolution of MOS cement properties. The mechanical properties, water resistance, phase composition, and microstructure of MOS cement were analyzed. When the replacement rate of MgSO4·7 H2O with MgCl2·6 H2O reached 5 %, MOS cement achieved its highest compressive strength of 77.5 MPa after curing for 90 days, despite a reduction in flexural strength due to MgO hydration. After immersion in water, the trend of compressive strength change in MOS cement remained insensitive to increasing immersion time. During the curing process, Cl- facilitated the formation of hexagonal Mg(OH)2 by participating in the hydration products formation process of MOS cement. Hexagonal Mg(OH)2 contributed to matrix construction and became connected with the 5Mg(OH)2·MgSO4·7 H2O phase. Additionally, Cl- promoted the formation of the modified 5Mg(OH)2·MgSO4·7 H2O phase, known as Mg-S-O-H-Cl phase. After immersion in water, the Mg-S-O-H-Cl phase displayed stability, showcasing their capacity to maintain compressive strength relative stability despite adverse factors such as ions leaching (including Mg2+, SO42-, and Cl-) and the phase transformation of MgO.