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In-situ synchrotron study of the kinetics, thermodynamics, and reaction mechanisms of the hydrothermal crystallization of gyrolite, Ca₁₆Si₂₄O₆₀(OH)₈·14H₂O

Samuel Shaw^*, C. Michael B. Henderson and Simon M. Clark

Daresbury Laboratory, CLRC, Warrington WA4 4AD and Department of Earth Sciences, University of Manchester, Manchester M13 9PL, U.K.

The hydrothermal crystallization of gyrolite was studied dynamically at 190–240 °C using synchrotron-based in situ Energy Dispersive Powder Diffraction (EDPD). The reaction mechanism involves the initial crystallization of a calcium silicate hydrate (C-S-H) gel, which has a sheet structure with well ordered Ca(O,OH) layers and disordered silicate layers. This is followed by the intermediate formation of Z-phase which finally transforms to gyrolite. This process involves ordering of the silicate layers and an increase in the order along c.

Kinetics data for all stages of the crystallization process were determined by analyzing the growth and decline of various diffraction peaks with time. The activation energy (E_a) (nucleation) for Z-phase is ~39 kJ/mol while that for gyrolite is ~56 kJ/mol. E_a (crystallization) of gyrolite is higher at ~80 kJ/mol. The reaction occurs via a two-dimensional, diffusion-controlled mechanism and is a continuous process that suggests that Z-phase is an unstable, transient phase.

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