Nanoscale insight on the initial hydration mechanism of magnesium phosphate cement

Abstract

The hydration rate, microstructure, mechanical properties and durability of Magnesium phosphate cement (MPC) prepared by different kinds of phosphates are quite different. These differences in macroscopic properties are essentially determined by microscopic chemical reactions. Therefore, in this paper, the adsorption mechanism of MgO interface to water and ions in the initial hydration process of MPC and the hydration structure of different ions were discussed at the molecular level. Herein, the structure and dynamics properties of water, NH4+, Na+, K+ and Cl− ions at MgO interface were analyzed by molecular dynamics method. The results show that the water molecules near the MgO interface can be connected to the matrix oxygen through hydrogen bonding and have a Coulomb interaction with the Mg on the interface. Therefore, the water molecules accumulate and stratify at the interface and the density distribution curve of water molecules forms a peak near the interface. This explains the stage of MgO adsorption of water molecules during the initial hydration of MPC. Besides, a large amount of ammonium ion, sodium ion and potassium ion are adsorbed on the MgO surface, and the cation density distribution curve also forms a peak near the interface. In addition, the radius of sodium ion is smaller and the Na-Os bond is stronger than that of NH4+ and K+ (Os represents the oxygen atom in MgO). Hence, the number of sodium ions adsorbed to the interface is the largest, the radial distribution function (RDF) of Na-Os forms a high-strength and sharp peak, The Os coordination number of Na+ is more than that of NH4+ and K+. This also explains the experimental phenomenon that sodium magnesium phosphate cement has a faster hydration rate and shorter setting time than ammonium magnesium phosphate cement and potassium magnesium phosphate cement. Due to the adsorption of the interface, the mean square displacement of cations has decreased to some extent compared with that in the corresponding pure solution models. This study provides a basic understanding of the initial hydration mechanism of MPC at the molecular level.