Fisher, Robin D. (2011) Conversion of gypsum to phosphate materials for novel composites with enhanced macroscopic properties. PhD thesis, University of Warwick.

Abstract

This thesis describes the synthesis, characterisation and macroscopic properties of phosphate materials made from gypsum, CaSO4⋅2H2O, using inorganic chemical reactions. The aim was to reduce the degradation of gypsum based materials by water exposure, using strategies that included the complete conversion in hydrothermal conditions to a material having a much lower solubility, e.g. hydroxyapatite, Ca5(PO4)3OH, and coating/binding calcium sulfate crystallites with a more water resistant material. The hydrothermal conversion of gypsum to hydroxyapatite was a poorly understood reaction since all previous measurements on the intermediate and final products had been conducted after the reaction had been quenched, which may have resulted in the crystallisation of species that would not usually form. In–situ X-ray measurements described herein have provided new information about the kinetics and mechanism of this conversion. It has been found however, that this reaction is not viable with respect to stabilising gypsum to water degradation after unsuccessful attempts to increase the reaction kinetics. It was discovered in the in-situ measurements that the conversion proceeds from the outside, towards the centre of monoliths of porous vast gypsum samples and seems to be limited by the rate of ionic diffusion. Increases in reaction temperature caused the crystallisation of other species such as calcium sulfate hemihydrate, CaSO4⋅½H2O and monetite, CaHPO4, decreasing the stability of the product. A novel method of processing gypsum has been found, forming true composites of crystal sulfate anhydrite crystallites, bound by a water resistant phosphate coating. These composites which have the microscopic appearance of a ceramic, have been made for the first time and characterised using an array of analytical techniques such as X-ray diffraction, Fourier-transform infrared spectroscopy, scanning electron microscopy and 31P solid-state nuclear magnetic resonance, the latter being extremely informative of binder composition and allowing the identification of an interfacial layer between the anhydrite crystallites and phosphate coating. Physical measurements such as the dissolution kinetics have been made in order to understand the effect of composition on the kinetic stability of the composites to water exposure, related back to the chemical analysis. The phenomenon of thermal diffusion of calcium from anhydrite; thought to be responsible for the composites’ formation, coupled with an apparent lower thermal conductivity, also has exciting implications for other applications of gypsum such as the fireproofing of buildings.

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