pH-controlled dual-phase formation in a magnesium phosphate-silicate composite cement revealed by thermodynamic modeling

Magnesium silicon potassium phosphate cement (MSPPC) is a ternary MgO-K2HPO4-SiO2 cement system prepared using dipotassium hydrogen phosphate (K2HPO4), which differs from conventional magnesium phosphate cement based on acidic phosphate KH2PO4. The alkaline nature of K2HPO4 significantly alters the chemical environment of hydration and enables the formation of both magnesium phosphate and magnesium silicate phases. In this study, thermodynamic modeling combined with experimental validation was employed to investigate the hydration mechanism of MSPPC. The results show that the hydration process is governed by the evolution of pH, which regulates the thermodynamic stability of hydration products. Initially, dissolution of K2HPO4 produces a pore solution with a pH of approximately 8.7. As MgO hydrates and the Mg(OH)2 dissolution–precipitation equilibrium is established, OH− becomes increasingly available in the pore solution, increasing the pH rapidly to about 12.4 to 12.5. The highly alkaline environment promotes silica dissolution and shifts phosphate speciation toward PO43−, thereby regulating the ionic activities and thermodynamic stability of hydration phases. Thermodynamic simulation indicates that Mg(OH)2 forms first during early hydration. With increasing M/P molar ratio, the intermediate phosphate phase Mg2KH(PO4)2·15H2O becomes temporarily stable and subsequently transforms toward MgKPO4·6H2O (MKP). At higher M/P molar ratios, the intermediate phase becomes unstable, and MKP becomes the dominant stable phosphate hydration product. Meanwhile, dissolution of silica under highly alkaline conditions leads to the formation of magnesium silicate hydrate (M-S-H). The final stable hydration products consist of MKP and M-S-H, indicating that MSPPC is a magnesium phosphate–magnesium silicate composite cement formed within a pH-regulated thermodynamic stability window, rather than a simple superposition of independent magnesium phosphate and magnesium silicate systems.