Thermodynamic modeling of the effect of temperature on the phase assemblage of magnesium ammonium phosphate cement

Magnesium ammonium phosphate cement (MAPC) is commonly used in the rehabilitation of infrastructure and waste stabilization/solidification. Few studies have investigated the influence of temperature on the phase assemblages of MAPC. A thermodynamic modeling approach is employed to investigate the phase evolution of MAPC at slightly elevated temperatures. The results reveal that when n(MgO)/n(NH4H2PO4) (M/P) is less than 1, MgNH4PO4.6H2O is predicted to form in the temperature range of 0-6.6 degrees C. The increase in temperature improves the formation of (NH4)2Mg(HPO4)2.4H2O and MgHPO4.3H2O. When the temperature reaches 37.2 degrees C, MgNH4PO4.6H2O cannot form in the system. The volume of the solid phase expands as the temperature increases. MgNH4PO4.6H2O is the stable phase within the temperature range from 0 to 41.2 degrees C when the M/P is higher than 1. MgNH4PO4.H2O and traces of MgHPO4.3H2O precipitate when the temperature is within a range of 41.2 - 70 degrees C. The unreacted MgO is thermodynamically stable and not involved in the reaction when the temperature changes. The volume of the solid phases of MAPC with mix proportions of M/P higher than 1 decreases with increasing temperature.