Chemical and Biomolecular Engineering - Theses
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ItemMechanisms and kinetics of gel formation in geopolymers( 2007)Geopolymer chemistry governs the formation of an X-ray amorphous aluminosilicate cement material. Binders form at ambient temperatures from a variety of different raw material sources, including industrial wastes. Early research in this field was based around investigating binder material properties; however, more recently, geopolymer formation chemistry has been intensively studied. Better understanding of the chemical processes governing geopolymer curing reactions will allow a wider variety of waste materials to be utilised and also the tailoring of binder properties for specific applications. (For complete abstract open document).
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ItemDesign and synthesis of star macromolecular architectures with degradable functionality( 2007-10)Polymers with star shaped architectures represent an interesting class of macromolecule. Core cross-linked star (CCS) polymers in particular have shown potential for use in various fields of application including drug delivery, paint additives and membrane formation. The work presented in this thesis is directed towards investigating the synthesis of CCS polymers, looking at various ways of modifying the structural design to further expand the potential range of applications as well as develop a deeper understanding of this unique class of macromolecule. This was achieved through the incorporation of labile functional groups such that specific regions of the CCS polymer could be selectively targeted for degradation, thereby altering the structure and consequentially the properties in a controlled fashion. (For complete abstract open document)
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ItemSolid-gel interactions in geopolymers( 2002)This is partly because the requirements for such an ultimate material change with people’s perception about its properties as well as its environmental impact. Thus, the once-believed ultimate Portland cement binder is now becoming unacceptable for a number of reasons including poor durability as well as severe environmental impact during production. Thus, an improved mineral binder is required by modern society to serve the same purposes as the existing Portland cement binder, as well as to reduce the current environmental impact caused by Portland cement production. Geopolymerisation is such a ‘green’ technology capable of turning both natural ‘virginal’ aluminosilicates and industrial aluminosilicate wastes, such as fly ash and blast furnace slag, into mechanically strong and chemically durable construction materials. However, the source materials for geopolymer synthesis are less reactive than Portland cement clinkers and the chemical compositions of these source materials can vary significantly. Consequently, product quality control is a major engineering challenge for the commercialisation of geopolymers. This thesis is therefore devoted to the mechanistic understanding of the interfacial chemical interactions between a number of natural and industrial aluminosilicates and the various activating solutions, which govern the reactivity of the aluminosilicate source materials. The effects of activating solution alkalinity, soluble silicate dosage and anionic contamination on the reactivity of the aluminosilicate source materials to produce geopolymeric binders, as well as their bonding properties to natural siliceous aggregates for concrete making, are examined. In particular, a new set of novel ‘realistic’ reaction models has been developed for such purposes. These reaction models have been further utilised to develop a novel analytical procedure, which is capable of studying geopolymerisation on ‘real’ geopolymers in situ and in real time. This novel procedure is invaluable for the total understanding of geopolymerisation, which is in turn vital for effective geopolymer mix designs.
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ItemCharacterisation of xanthan based, polymer solutions, physical gels and permanent networks( 2001)Gels and the gel transition are topics that have been the subject of extensive and widespread academic and industrial interest. Polymer gels and particularly those involving biopolymers are extensively applied in the food, pharmaceutical, agricultural, photographic, oil recovery and paper industries. Such widespread commercial application is responsible for the academic and industrial interest in gaining a greater understanding of the intrinsic physics governing the behaviour of these systems. Rheological analysis of biopolymer based gelling systems is an invaluable tool for investigating fundamental properties as well as replicating processing and application conditions. Through combination of careful rheological analysis with techniques capable of probing molecular structure and dynamics, such as static and dynamic light scattering (LS), it is possible to develop structure-function relations that are considered critical in understanding and controlling gels and gelation of biopolymer systems. This thesis utilises the biopolymer xanthan gum, to investigate both rheologically and optically, polymer solutions, physical gels and permanent networks. A physical gel is one in which the interactions between molecules, responsible for the gel like properties, are not permanent, that is they have a finite timescale. Subsequently, on long enough time scales, a physical gel will flow. Alternatively, a permanent network, as the name suggests, is one in which the interactions (or crosslinks) are thermodynamically stable, and the system will therefore never flow. Aqueous xanthan solutions in the presence of mono or divalent cations will produce a solution with 'weak-gel' physical properties. A 'weak-gel' is a term commonly applied to structured fluids that on short time-scales possess properties allowing them to appear more 'gel like', however on longer timescales, they will flow. Conversely, on the addition of trivalent metal ions (and for the purposes of this work, aluminum ions Al(III)) , xanthan will form a strong thermally stable permanent network. Using rheological and light scattering (LS) techniques, this thesis, will consider the similarities and differences of xanthan based physical gels and permanent networks. (From Abstract)
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ItemThe physical and chemical characterisation of fly ash based geopolymers( 2000)The work presented in this thesis represents a first attempt at better quantifying the physical and chemical characteristics of fly ash-based geopolymers while establishing a knowledge base upon which further geopolymer research could be based. Although geopolymers have been around in one form or another for thousands of years, a proper scientific study has only been attempted during the last three decades and even then the research was usually confidential or very narrowly focused. The inroads made by previous researchers have in many cases not been acknowledged by subsequent investigators, mainly as a result of the technology being described by a different terminology depending on the specific field of science that was applied to the research. Secondly, it is also a well known fact that during the last decade a renewed interest has been shown in the development of alternative waste processing and treatment techniques with an emphasis on recycling, waste minimisation and value addition to waste materials. The application of geopolymerisation in the waste processing industry could facilitate in providing an alternative waste processing strategy where value added to waste streams will not only result in new products being produced, but also in minimising the risk to the environment by reducing volumes being dumped and turning a liability into a resource, effectively making waste treatment a profitable business. With the above two objectives in mind, this thesis aims to not only provide a better scientific understanding of geopolymerisation reactions but has at its core the application of this technology to a commonly found and mostly under-utilised waste material, fly ash. The current state of knowledge, as far as geopolymerisation is concerned, is put into perspective in terms of better-studied systems such as silica chemistry and cement chemistry. It is shown that although selected areas of geopolymerisation have been studied, the application of this technology to more heterogeneous systems, such as those containing waste materials, has been largely neglected. The present state of the waste processing industry and specifically the need for new cost-effective technologies is also addressed. It becomes obvious that the volume of waste that could be either utilised or treated with this technology contributes quite substantially to the streams currently being landfilled or otherwise disposed of. The relative lack of large amounts of public domain scientific data on geopolymers could be attributed to not only the confidential nature of some past research projects, but also the highly complex nature and costs associated with solid state analytical techniques needed to properly analyse these structures. The experimental methodology used in this thesis overcame this problem by using combinations of readily available and low-cost analytical techniques and chemical tests such as leaching. The results obtained from the various techniques or procedures were then assimilated into a single semi-quantitative analytical representation and used in further analysis. Apart from establishing that geopolymeric binders could be synthesised using fly ash from various sources, the work presented here also addresses a number of crucial factors that affects almost all aspects of geopolymer formation. The first of these factors to be considered is the effects associated with the specific starting material being used. Past studies mainly concentrated on one starting material and while fly ash was the main topic of discussion in the present study, the effects associated with using fly ash from different sources are shown to be crucial in terms of developing this technology into practical applications. The use of fly ash in combination with other aluminosilicate materials, such as kaolinite, is also considered and again the type of source material, its thermal history and chemical reactivity are shown to be crucial in determining the various physical and chemical properties of the final product. In most cases the effects associated with different source materials can be attributed to incomplete dissolution during synthesis on account of the source materials used. The morphological and crystallographic characterisation of a small number of samples is also performed using a combination of electron microscopy techniques. This study establishes the structural complexity of fly ash-based geopolymers but also proves that a great deal of newly formed phases is in fact non-crystalline and compositionally related to geopolymers studied by past researchers. The type of structural analysis, as performed in this thesis, has not been attempted before and as such proved a valuable and useful tool in identifying, not only the nature of the structural make up, but also the other factors that affect the chemical and physical properties of these binders. One of these factors relates to the type of alkali metal cation used in synthesising the various matrices that were studied. The use of K and/or Na in fly ash-based geopolymers affects not only the properties of mixtures before setting has occurred, but also the setting time as well as all the chemical and physical properties of the final product. As such this factor will remain crucial in any future practical application of this technology. The final part of this thesis establishes that fly ash-based geopolymers could be utilised as solidification and stabilisation agents in the waste treatment industry. The type of toxic metal being immobilised, however, affects the structure of the host matrix both physically and chemically and again this will have to be more closely studied in terms of large-scale future applications. The mechanism of immobilisation also differs depending on the type of metal being immobilised. During acetic acid leaching a fair amount of matrix breakdown can occur although this is not a result of a lack of acid resistance, but rather of physical conditions present during the leaching tests used in this study. The mechanism controlling the early stages of leaching was shown to be governed by diffusion through a hypothetical ash layer while the kinetics of the leaching reaction seems to be extremely complex. After establishing that simple first and second order reaction kinetics could not adequately describe the process, the complex nature of the kinetic reactions present during leaching necessitates a separate study and does not form part of this thesis. The value of the present work lies in the fact that in terms of further study and application of geopolymers, and specifically fly ash-based geopolymers, a ballpark has now been established on which other developments and further research can be based. This has been largely achieved through publication of these results in refereed international journals of good repute and presentations at numerous international conferences and exhibitions. In terms of quantification of all aspects and variables relating to fly ash-based geopolymer synthesis, many years of research are still needed, although the work presented here proves that there is indeed scope for major research and commercial developments in this area.
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ItemCompositional effects and microstructure of fly ash-based geopolymers( 2001)In the context of the concrete industry, geopolymers are an added-value approach to decommodifying the application of Portland cement-based construction materials. In particular, fly ash-based geopolymers are attracting growing interest due to their low cost, environmental advantage and readily available feedstock that allows them to be produced en masse. The development of geopolymers for construction applications, stabilisation technologies, fire resistant panels and a host of other applications, is still relatively new and therefore requires considerably more research. Materials science is one approach which can offer considerable insight into the properties and behaviour of geopolymers as a function of composition, and can also explain the reasons for these properties, based on a vast repository of scientific observations. Thus this approach gives an objective basis for collecting meaningful information on fly ash-based geopolymers which will greatly benefit the manufacturing and process design for any application. This thesis reports the analysis and development of experimental methods typical in the fields of materials science and surface chemistry to describe the microstructure and material properties of fly ash-based geopolymers. An emphasis is placed on describing the microstructure of fly ash-based geopolymers as it remains an ongoing objective of material scientists to link microstructural features to the material properties and macrostructure as a function of composition. More specifically, this thesis explores the positive effect of adding zirconia to fly ash-based geopolymers including the mechanism of incorporating zirconia within the matrix. This thesis also examines the fundamental surface chemistry associated with the inclusion of zirconia. Important processing variables of geopolymer synthesis are highlighted and their influence on the immobilisation of heavy metals is examined. Finally, the advantages of using an alternative alkali activator, sodium aluminate, is reported for both micro and macroscopic properties.