Experimental research on properties of magnesium oxysulfate cement during high temperature exposure

The fire-resistance of magnesium oxysulfate (MOS) cement is important when it is serviced for the structures exposed to high temperatures. This paper investigated the effect of material molar ratios on properties of MOS cement during high temperature exposure. Compressive strength, X-ray diffraction, thermogravimetric analysis and scanning electron microscopy were conducted to study strength development and phase changes of MOS cement. Results showed that thermal stability of MOS cement was mainly attributed to thermal decomposition behavior of 5 Mg(OH)2 center dot MgSO4 center dot 7H2O (517) phase and brucite. For MOS cement with a constant H2O: MgSO4 molar ratio, the increase of a-MgO: MgSO4 molar ratio lowered the proportion of 517 phase but improved compressive strength. Before 200 degrees C, needle-like 517 phases interlaced with each other and worked with healed cracks and pores by thermal expansion to densify the microstructure. MOS specimens all presented a higher compressive strength at 200 degrees C, and the highest strength value was over 150 MPa. After 300 degrees C, the dehydrated 517 phase and brucite resulted in a loose structure as well as a strength reduction.