A solid state NMR and in-situ infrared spectroscopy study on the setting reaction of magnesium sodium phosphate cement

In the present work, the setting reaction of magnesium sodium phosphate cement was followed in time with in-situ attenuated total reflectance Fourier-transform infrared spectroscopy and solid state nuclear magnetic resonance spectroscopy, in order to gain insights into the products and phase transformations. The results indicated that, during the progress of the reaction, amorphous phases containing the H2PO4− structural group converted into phases containing HPO42− and finally PO43−. The increase in pH triggered by the MgO dissolution was considered as the driving force for the transformations. This is supposed to promote the establishment of a high degree of supersaturation close to the surface of MgO grains, resulting in kinetically driven transformations and favouring the amorphous nature of the products. It is suggested that in the later stages of the reaction, two orthophosphate amorphous phases, hosting bound water molecules, coexisted, with the one showing a relatively more ordered 31P local environment, converting into a second with a more disordered phosphorous environment. The densification of the ceramic can be considered as a structural reorganization encompassing bonding of water molecules into a pervasive amorphous phase, containing magnesium-phosphate structural units and involving limited structural rearrangements of the local environment of P and, especially, of Na.