Pore Solution of Stoichiometric MKP Cement Paste: Study of Composition Using Different Extraction Protocols and Thermodynamic Modelling

Magnesium potassium phosphate (MKP) cements are acid-base binders that are attractive for a range of applications.Their interactions with the surrounding environment are largely governed by the chemistry of their pore solution, which remains poorly documented, particularly for near stoichiometric cement pastes (molar ratios Mg/P ≈ 1 and H 2 O/P ≈ 5), due to the limited volume of residual free water in the pore network.This study investigates the pore solution composition of MKP cement pastes through a combined experimental and thermodynamic approach.Several extraction protocols are compared and allow, through the use of correction factors derived from a statistical analysis, a more reliable determination of the pore solution composition at low water content.Experimental results show that the pH increases rapidly during early hydration, and that the solution is highly concentrated in potassium and phosphorus, while the magnesium concentration remains much lower.Thermodynamic simulations reproduce the general trends, with an increase in pH and a decrease in ion concentrations as the water content increases in the paste.However, the predicted concentrations are approximately one order of magnitude lower than the measured values.This discrepancy indicates that the pore solution remains oversaturated with respect to the identified phase assemblage, even after prolonged curing (up to 6 months).Finally, protocols are proposed to prepare synthetic solutions mimicking the pore solution of stoichiometric MKP cement pastes after 28 days of hydration, or at thermodynamic equilibrium.