Abstract
The chemical interaction between solid and fluid phases often entails the replacement of the solid phase with a product that maintains the volume and external morphology of the parent crystal, sometimes even the finer structural details. Reactions characterized by this feature are named pseudomorphic reactions and are found in a
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wide range of natural and artificial systems, hence the drive to investigate their microstructural and chemical complexities. The development of a transient porosity is one of the key features of pseudomorphic reactions, since it forms void pathways that allow the continuous transport of reactant to the surface of
the unaltered solid phase. The chemical system KBr-KCl-H2O is a binary solid-solution aqueous-solution system which interactions show progressive transformation of both the solid and liquid phases, leading to an alteration product of formula K(Br(x)Cl(1 -x)) with a composition that spans through the whole chemical range comprised within the endmembers KBr and KCl. This system has a simple and relatively well understood chemical evolution that goes through progressive phase transformation with a composition that spans all the intermediate, and the product phase shows retainment of the original volume of the parent crystal, thus it has been employed here and in literature studies to model pseudomorphic reactions. This thesis provides quantitative μCT microstructural in situ data on the early stages of reactions, coupled with qualitative ex situ BSE and EDX analysis to provide an independent comparison of the microstructures and investigate the evolution on the long terms. The in situ data record the temporal and spatial evolution of the transient porosity occurring in a continuously stirring fluid-through apparatus, showing four snapshots taken after 9, 12, 15 and 18 minutes of reaction. The ex situ data were collected on KBr samples that were let react with a KCl-saturated solution in batch reactors for timespans ranging from 30 minutes to 2 weeks. The data provided with this study argue in favour of a replacement reaction that does not follow a continuous chemical and structural progression to produce the pseudomorphic products. Instead, it happens through waves of enhanced permeability alternated to waves of chemical and textural re-equilibration. In the first instance, the connectivity with the outer solution drives the fluids towards the core of the unreplaced crystal, promoting the progress of the reaction. In the second case, the connectivity with the KCl solution is limited and the porosity coarsen
in an attempt to minimize the surface free energy, forming microstructures that are partly inherited in the following step of enhanced permeability. Synchrotron-radiation microcomputed tomography (μCT) has proven to be an invaluable tool, capable of providing simultaneously visual information for qualitative interpretations and insightful quantitative microstructural data.
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