Effects of 5•1•7 Phase Seed Crystal on Performance of Magnesium Oxysulfate Cement

Magnesium oxysulfate (MOS) cement is lightweight, offers fire resistance, and has good aesthetics. Although the addition of citric acid improves the cement’s late strength, the concomitant retardation of hydration of MOS by citric acid results in slow setting and low early strength. This may in return decrease the production efficiency of MOS cement products. In this study, 5•1•7 phase (5Mg(OH)2•MgSO4•7H2O) seed crystal (SC) is used as a modifier that works as a seed nucleus and accelerates the hydration of MOS cement in the early stages, in the absence or presence of citric acid. The effect of SC on compressive strength, setting time, and volume change is investigated in detail. X-ray diffraction, scanning electronic microscopy, hydration-heat release rate, and mercury intrusion porosimeter are performed to explain the influence mechanism of SC on the performance of the MOS cement. The results revealed that the in situ formation and growth of the new 5•1•7 phase crystals on the SC surface by the addition of SC can significantly accelerate the setting, improve the 12 h and 28 d compressive strengths, and reduce shrinkage of MOS cement without or with 0.05% citric acid.