Effect of Silica Fume on the Properties of Magnesium Ammonium Phosphate Cement at Room Temperature and High Temperatures

In this study, magnesium ammonium phosphate cement (MAPC) was prepared with different dosages of silica fume. The physical properties of MAPC were tested and its high-temperature properties were investigated in detail. The results showed that the addition of silica fume decreased the fluidity and setting time of MAPC. The results also showed that the addition of silica fume was beneficial to improving the resistance of MAPC to high temperatures. After exposure to high temperatures (200°C, 600°C, 800°C, 1,000°C, and 1,200°C), the flexural strength of MAPC reached its maximum when the additional level of silica fume was 20%. At 20°C, struvite (${\mathrm{MgNH}}_4{\mathrm{PO}}_4·6{\mathrm{H}}_2\mathrm{O}$) was detected as the main hydration product in MAPC. Heating of MAPC to 800°C resulted in the transformation of ${\mathrm{MgNH}}_4{\mathrm{PO}}_4·6{\mathrm{H}}_2\mathrm{O}$ to anhydrous magnesium orthophosphate [${\mathrm{Mg}}_3({\mathrm{PO}}_4{)}_2$]. After exposure to $\ge \mathrm{1,000}°\mathrm{C}$, the sintering of ${\mathrm{Mg}}_3({\mathrm{PO}}_4{)}_2$ occurred, and meanwhile forsterite (${\mathrm{Mg}}_2{\mathrm{SiO}}_4$) was identified in the X-ray diffraction (XRD) pattern. After adding silica fume, more ${\mathrm{Mg}}_2{\mathrm{SiO}}_4$ with good thermal stability was formed, which made MAPC with better resistance to high temperatures.