Surface modification of magnesium oxide and its effect on the performance of magnesium phosphate cement

• Magnesium oxide modified by water glass prolongs the setting time of MKPC. • Modified magnesium oxide can promote the development of later strength of MKPC. • The effect mechanism of surface modification on the setting time of MKPC was studied. In applying magnesium phosphate cement, controlling its setting time is of great importance, and the activity of the main raw material, magnesium oxide, is the key. In this study, dead-burned magnesia was modified with different amounts of water glass and different thermal treatment temperatures, and the effects of MgO on the macroscopic properties and microstructure of magnesium potassium phosphate cement (MKPC) were studied. The results show that the setting time of MKPC prepared with modified magnesia is prolonged, and the increase of the amount of water glass, the longer the setting time is. The setting time of the sample with 15% water glass and 300 °C treatment increased from 4.3 min to 17.5 min, an increase of 307%. The compressive strength results show that the surface modification of MgO decreased MKPC's early strength, which decreased with an increase in the water glass content and a decrease in the thermal treatment temperature. However, when the curing reached 7 days, the samples' compressive strength exceeded that of the reference group, and the strength development was optimum when the content of water glass was 5% with 300 °C treatment. The 28 d strength increased from 65.8 MPa of the reference group to 72.4 MPa, an increase of 10%. Through scanning electron microscopy (SEM), it was found that new phases were formed in the surface modified samples at the later stage of hydration. Combined with the results of energy-dispersive spectroscopy (EDS), it was confirmed that the magnesium silicate hydrate gels (M-S-H) were formed. The mercury intrusion porosimetry (MIP) test results show that these M-S-H gels can refine the pore size of MKPC, and the porosity of the sample with 5% water glass and 300 °C treatment was reduced from 6.6510% to 6.0429%, which explains the improvement in the MKPC compressive strength. Therefore, the surface modification of magnesium oxide can greatly extend its setting time without adversely affecting its later performance.