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

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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.

Item Type:	Thesis (PhD)
Subjects:	Q Science > QD Chemistry
Library of Congress Subject Headings (LCSH):	Gypsum, Phosphates -- Analysis
Official Date:	May 2011
Dates:	Date Event May 2011 Submitted
Institution:	University of Warwick
Theses Department:	Department of Chemistry
Thesis Type:	PhD
Publication Status:	Unpublished
Supervisor(s)/Advisor:	Walton, Richard I.
Sponsors:	British Gypsum Limited ; Engineering and Physical Sciences Research Council (EPSRC)
Extent:	247 leaves : ill., charts
Language:	eng

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