Thermodynamic modeling of magnesium ammonium phosphate cement and stability of its hydration products

Abstract

Magnesium ammonium phosphate cement (MAPC) is commonly applied as a repair material. However, its hydration mechanism remains unclear. A thermodynamic modeling approach is employed to explore the hydration mechanism of MAPC. The results reveal that the hydration products of MAPC include (NH4)2Mg(PO4)2·4H2O, MgHPO4·3H2O and MgNH4PO4·6H2O at 25 °C, 0.1 MPa, where both (NH4)2Mg(PO4)2·4H2O and MgHPO4·3H2O are intermediate products that can transform into MgNH4PO4·6H2O with an increase in n(MgO)/n(NH4H2PO4) (M/P). The thermodynamic study of borax-modified and boric acid-modified MAPC indicates that the addition of boric acid does not change the assemblage of the hydration products; however, the addition of borax hinders the formation of MgHPO4·3H2O. According to the thermodynamic simulation, the lünebergite (Mg3B2(PO4)2(OH)6·6H2O) phase, which is commonly thought to coat MgO to delay the hydration of modified MAPC, is not predicted to form. The addition of borax and boric acid can improve the final dissolution of MgO, promoting the formation of struvite.