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 (NH 4 H 2 PO 4) (M/P) is less than 1, MgNH 4 PO 4 ·6H 2 O is predicted to form in the temperature range of 0–6.6 °C. The increase in temperature improves the formation of (NH 4) 2 Mg(HPO 4) 2 ·4H 2 O and MgHPO 4 ·3H 2 O. When the temperature reaches 37.2 °C, MgNH 4 PO 4 ·6H 2 O cannot form in the system. The volume of the solid phase expands as the temperature increases. MgNH 4 PO 4 ·6H 2 O is the stable phase within the temperature range from 0 to 41.2 °C when the M/P is higher than 1. MgNH 4 PO 4 ·H 2 O and traces of MgHPO 4 ·3H 2 O precipitate when the temperature is within a range of 41.2 – 70 °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. • The phase assemblages of MAPC are closely related to M/P and temperature. • The volume of the solid phase expands as the temperature increases when M/P is less than 1. • The volume of the solid phases of MAPC with M/P higher than 1 decreases with increasing temperature. [ABSTRACT FROM AUTHOR]