Thermodynamical analysis of the impact of chloride-rich environment on the paste mineralogy of magnesium phosphate cement

Magnesium phosphate cement (MPC) is commonly used as rapid repair materials for damaged pavements. MPC together with pavements usually undergoes de-icing salts exposure in winter. However, the deterioration mechanism of MPC under chloride attack remains unclear. A thermodynamic simulation approach was employed to investigate the deterioration mechanism of MPC. The results revealed that compressive strength loss decreases with the increase of the concentration of the NaCl solution. Basic magnesium chlorides cannot be formed in MPC immersed in NaCl solutions. The formed MgNH 4 PO 4 ·6H 2 O, MgKPO 4 ·6H 2 O and part of the residual magnesia can dissolve in NaCl solution, facilitating the precipitation of NH 4 NaMg 2 (PO 4 ) 2 ·14H 2 O, KNaMg 2 (PO 4 ) 2 ·14H 2 O and brucite in MPC. Due to the precipitation of NH 4 NaMg 2 (PO 4 ) 2 ·14H 2 O and KNaMg 2 (PO 4 ) 2 ·14H 2 O, the volume of solid phases increases slightly and then decreases under chloride attack. NH 4 NaMg 2 (PO 4 ) 2 ·14H 2 O and KNaMg 2 (PO 4 ) 2 ·14H 2 O are more soluble in concentrated NaCl solution, leading to the solid volume decrease of the hardened binder phases of MPC.