The Improvement Effects of NaH 2 PO 4 and KH 2 PO 4 on the Properties of Magnesium Oxysulfate Cement

Sodium dihydrogen phosphate (NaH 2 PO 4 ) and potassium dihydrogen phosphate (KH 2 PO 4 ) were selected as additives for magnesium oxysulfate (MOS) cement. The phase composition and the microstructure of MOS cement were characterized using X-ray diffraction (XRD), thermogravimetric analysis (TG-DSC), Flourier transform infrared spectroscopy (FT-IR), mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM). It is found that both NaH 2 PO 4 and KH 2 PO 4 lead to an increase in the compressive strength and an improvement in the volume stability of MOS cement. The XRD, MIP and SEM results show that the addition of NaH 2 PO 4 or KH 2 PO 4 does not change the phase composition of MOS cement but promotes the formation of strength phase of 5Mg(OH) 2 ·MgSO 4 ·7H 2 O (5·1·7 phase). This phase brings a considerable improvement in the microstructure of MOS cement, which has a positive effect on the properties of MOS cement.