Influence of Silica Fume on Mechanical Properties and Water Resistance of Magnesium–Ammonium Phosphate Cement

The influence of silica fume (SF) on the strength and water resistance of magnesium–ammonium phosphate cement (MAPC) was investigated, and the improvement mechanism was also studied by employing analysis techniques that included X-ray diffraction (XRD), scanning electron microscopy (SEM), and scanning electron microscopy–energy dispersive spectrometer (SEM-EDS) tests. The addition of SF is observed to incrementally decrease the flowability of MAPC pastes as well as their temperature during hydration processes. The mechanical properties of all MAPC paste mixtures containing SF were higher than those of a basic control group at all curing ages. Moreover, the water resistance of MAPC samples with SF was also enhanced. The pH of the soaking liquids indicates that the precipitate of MAPC is reduced after adding silica fume. XRD spectrums of MAPC pastes indicated the formation of secondary reaction products (MgSiO3), except the main hydration product struvite and residual magnesia. The microscopic morphology and internal action of MgSiO3 were further analyzed by SEM and SEM-EDS.