Investigation of the Setting Reaction in Magnesium Phosphate Ceramics with Quasielastic Neutron Scattering Alberto Viani,* ,† Mohamed Zbiri, ‡ Heloisa N. Bordallo, §,∥ Alessandro F. Gualtieri, ⊥ and Petra Ma ́ cova ́ † † Institute of Theoretical and Applied Mechanics ASCR, Centre of Excellence Telč , Batelovska ́ 485, CZ- 58856 Telč , Czech Republic ‡ Institute Laue-Langevin, avenue des Martyrs 71, CS 20156, 38042 Grenoble cedex 9, France § Niels Bohr Institute, University of Copenhagen, Copenhagen 2100, Denmark ∥ European Spallation Source ERIC, Tunavä gen 24, 22100 Lund, Sweden ⊥ Dipartimento di Scienze Chimiche e Geologiche, Universita ̀ di Modena e Reggio Emilia, via Universita ̀ 4, I-41121 Modena, Italy * S Supporting Information ABSTRACT: 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. ■ INTRODUCTION Magnesium potassium phosphate ceramics (MPCs), sometimes classified as acid−base cements, find applications in civil engineering as fast repair materials for damaged structures, 1 cements for nuclear waste encapsulation, 2 and bioengineering materials. 3 MPCs set at room temperature through a fast and exothermic reaction between magnesium oxide and an acid phosphate solution, 4 leading to the crystallization of the isomorphous potassium equivalent of mineral struvite (MKP). The reaction is usually written as 5 + + → · MgO KH PO 5H O MgKPO 6H O 2 4 2 4 2 (1) The initial acid environment prompts the fast dissolution of MgO. 6 It has been proposed that Mg 2+ ions are released in solution, forming aqueous metal complexes which eventually condense into a gel by reaction with the phosphate anions. 7 According to this model, nucleation of MKP is delayed in time and occurs at the surface of the unreacted core of MgO grains. The presence of an amorphous phase within the reaction products has been first detected in magnesium ammonium phosphate ceramics (MAPs), 8−10 obtained employing ammo- nium dihydrogen phosphate (NH 4 H 2 PO 4 ) in the place of KH 2 PO 4 (KDP), and, later, in MPCs. 11−14 Recent experimental evidence indicates that the amorphous phase is the precursor of MKP. 11,12,15,16 We have provided quantitative data on the setting reaction of MPCs by the kinetics analysis of X-ray synchrotron diffraction data. 5 A model for the reaction was proposed, in which each one of the two detected events, MgO dissolution and MKP crystallization, was described by two consecutive, partially overlapping processes. First, MgO dissolves in aqueous solution, contributing to the formation of an intermediate amorphous product. In analogy with some results of dissolution experiments of MgO in acidic solutions, 17,18 the first product of MgO dissolution was supposed to be amorphous Mg(OH) 2 . On the other hand, crystallization of MKP was largely described with the Johnson-Mehl-Avrami−Erofe’ev-Kolmogor- ov (JMAEK) model, 19 followed by a first-order equation for its Received: February 13, 2017 Revised: May 6, 2017 Published: May 8, 2017 Article pubs.acs.org/JPCC © XXXX American Chemical Society A DOI: 10.1021/acs.jpcc.7b01396 J. Phys. Chem. C XXXX, XXX, XXX−XXX