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