thesis
Crystal growth and dissolution of gypsum and analogous materials : a multi-scale approach
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Abstract
This thesis is concerned with the growth and dissolution of gypsum and
analogous crystalline materials, with the aim of understanding the kinetic and
mechanistic processes at the mineral-solution interface. The research
conducted was a collaborative project sponsored by Saint-Gobain Gypsum.
First, an image processing (IP) software package was developed to meet highly
specialised IP needs and expedite the extraction of vital surface information
from images produced in the growth and dissolution studies carried out in this
thesis.
A simple but powerful morphological analysis of characteristic etch pit features
formed on the basal plane of gypsum was proposed, to aid the determination of
intrinsic dissolution kinetics. Limiting the study to short times produced
microscopic active features, which exhibited high and quantitative mass
transport rates. At early times, the reaction was surface controlled, with the
edge planes dominating the process, revealing anisotropic step propagation
kinetics. With time, an increased contribution from mass transport was
observed, suggesting that at later times, the basal plane dominated reaction
kinetics. Common ion effects indicated a greater impact of Ca2+ than SO42- in
reducing dissolution rates while inert ions enhanced dissolution in a directionspecific
way. With this approach, microscopic phenomena were related to
macroscopic measurements thus reconciling experimental length scales.
Dissolution of the basal (010) and edge (001) surfaces of gypsum and
polycrystalline anhydrite, were probed at the bulk scale by coupling the channel
flow cell (CFC) technique which displays high mass transport rates, with off-line
spectrometric measurements of dissolved Ca2+. Quantitative modelling of the
diffusion-reaction within the CFC yielded a linear rate law for the dissolution
process. Rates from the basal plane and anhydrite were found to be consistent
with other bulk measurements, while the highly reactive edge plane exhibited high rates indicating a transport-limited process. Sodium trimetaphosphate, a
common humid-creep inhibitor was found to significantly retard basal plane
dissolution rates. Further CFC studies were carried out on industrially-relevant,
chemically modified CaSO4 based materials, using a simple flux approach. It was
found that models proposing a dissolution-precipitation pathway as the mode
of action of humid-creep inhibitors were less plausible than those proposing a
surface binding pathway.
Finally, the influence of solution stoichiometry, r = (aCa2+ / aSO42-) on the growth kinetics of microscopic gypsum crystals was determined at a constant
supersaturation. Crystal growth was found to be entirely controlled by surface
kinetics over the range of r, with the edge planes dominating the process. The
highest lateral rates were found at r = 1, diminishing sharply at r ≠ 1, and
indicating strong plane-specific dependence on Ca2+ and SO42- availability.
Additionally, dramatic changes in the morphology of grown crystals were
observed. Propagation of steps on the basal face revealed a complex
polynuclear layer-by-layer growth process for this surface. Macroscopic growth
rates compared well to previous bulk measurements indicating that the
approach used provided a comprehensive multi-scale view of gypsum growth
processes