Effects of Alumina as an Effective Constituent of Metakaolin on Properties of Magnesium Phosphate Cements

The main interest of this research investigation is to explore the effects of the Al2O3 proportion present in metakaolin (MK) on the mechanical properties of magnesium phosphate cement (MPC). Different contents of MK and Al2O3 were used to replace magnesium to prepare magnesium phosphate cement and some properties, such as setting time, compressive strength, flexural strength, and bonding strength, were tested in this paper. Experimental results showed that the presence of MK and alumina could decrease the exothermic reaction intensity of MPC, and resulted in extending setting time. The addition of MK can improve the compressive strength and flexural strength of MPC; meanwhile, the rate of flexural strength increasing with MK addition was lower than that of the compressive strength. It was also found that the bonding strength of MPC-containing MK and alumina was superior to the control mortar mixture. In addition, the improvement mechanism was discussed based on the microanalysis of X-ray diffraction (XRD), scanning electron microscope (SEM), along with energy dispersive spectrometer (EDS). The microstructure results showed that a homogeneous and amorphous substance (AlPO4) as a secondary hydration product was distributed widely, which modified the mechanical properties of MK-MPCs.