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Polyamorphism and frustrated crystallization in the acid-base reaction of magnesium potassium phosphate cements

Abstract

Magnesium potassium phosphate cements are a class of acid−base cements for bioengineering and civil engineering applications. The kinetics of the chemical reaction was investigated in-situ with isothermal conduction calorimetry and with powder X-ray diffraction, quantifying amorphous and crystalline products. The first reaction step, dissolution of MgO, with apparent activation energy of 71 kJ/mol, dictates the time-evolution of two amorphous intermediate precursors and of the crystalline product. The early crystallization of the latter has been described with an Avrami equation with apparent activation energy of 81 kJ/mol, pointing to a mechanism of deceleratory nucleation and growth in one direction, compatible with the acicular crystal habit observed with electron microscopy. The observed polyamorph transformation is controlled by a complex interplay between kinetic and thermodynamic factors, in which the changes in chemical environment (increase in pH) driven by the MgO dissolution, play a crucial role. It is proposed that the onset of the amorphous-amorphous transformation hinders crystallization by decreasing ion mobility, raising the energy barriers to structural reorganization. The rate of MgO dissolution depends on the reactivity of the powder and parameters of the mix (such as the amount of liquid) and influences the reaction pathways, impacting on material performance.

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Publication details

The article was received on 26 Apr 2018, accepted on 26 Jun 2018 and first published on 09 Jul 2018


Article type: Paper
DOI: 10.1039/C8CE00670A
Citation: CrystEngComm, 2018, Accepted Manuscript
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    Polyamorphism and frustrated crystallization in the acid-base reaction of magnesium potassium phosphate cements

    A. Viani and P. Mácová, CrystEngComm, 2018, Accepted Manuscript , DOI: 10.1039/C8CE00670A

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