Investigation of the Setting Reaction in Magnesium Phosphate Ceramics with Quasielastic Neutron Scattering.

Magnesium phosphate ceramics are a class of acid-base cements for bioengineering and civil engineering applications. We report on quasielastic neutron scattering results focusing on the evolution of the state of water in the system during the setting reaction, to shed light on the reaction mechanisms and the nature of the products. In the first few minutes, a consistent fraction of water molecules appears as immobile, and after a transient time, they start to be progressively bound into a reaction product. The kinetics of this last process has been described with an equation combining an Avrami model and a first-order reaction model with apparent activation energies of 18 and 6 kJ/mol, respectively. The results indicate that during the reaction the water molecules experience confinement effects inside a restricted space. The size of the confining volume decreases as the reaction progresses. It is proposed that an amorphous precursor with high surface area, bonding a relevant fraction of water, but also hosting mobile water, forms first. After an induction period, this phase undergoes further transformation into a product, still amorphous, considered as a further precursor of the final crystalline phase. With the reaction being kinetically driven, nonclassical mechanisms of nucleation and growth may lead to the formation of prenucleation clusters developing the first intermediate compound by coalescence. The mutating pH conditions trigger the transformation of the precursors, which likely contain structural motifs of the crystalline phase, similar to those observed in Ca and Zn, phosphate hydrate systems.