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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ItemA conceptual model of geopolymerisation( 2006-10)The discovery of geopolymers is a breakthrough which provides a cleaner and environmentally-friendlier alternative to Ordinary Portland Cement (OPC). Since the pioneering days, the understanding of the chemistry, synthesis, and practical application of geopolymers has improved to the extent that commercialisation of geopolymers on a large scale is possible in the near future. However, the fundamental breakthroughs and understanding to date are based on investigations of ‘pure’ raw materials, like metakaolinite. The utilisation of metakaolinite has been useful in a research setting, but will be impractical for widespread application. Therefore, the thesis attempts to do a more detailed study on geopolymers synthesised from waste materials, such as fly ash. The motivation for using fly ash as the main raw material is driven by various factors: (1) it is cheap and available in bulk quantities, (2) it is currently under-utilised, except for its use as an additive in OPC, (3) it has high workability, and (4) it requires less water (or solution) for activation.
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ItemWhat's wrong with Tarong?: the importance of coal fly ash glass chemistry in inorganic polymer synthesis( 2008)Inorganic polymer cements (IPCs) produced by alkali-activation of Class F coal fly ash commonly display unpredictable mechanical properties, even when ash from a single source is used. Coal fly ash based IPCs may become viable alternatives to traditional Portland cement binders if the reasons for the behaviour of coal fly ash during IPC synthesis are to be understood and the process controlled. Class F coal fly ash is a major by-product of coal-fired power stations and predominantly contains aluminosilicate species. The nature and composition of coal fly ash varies dramatically from source to source and little work has been performed to understand how these variances influence the properties of the IPCs formed from coal fly ashes. Coal fly ash is a complex material displaying inter- and intra-particle heterogeneity, and the nature and morphology of the reactive phases is poorly understood. Five Australian Class F coal fly ashes with different major oxide compositions were examined to observe the differences in the properties of IPCs formed from them and these results were also compared with IPCs formed from synthetic aluminosilicate materials. No correlation could be found between the particle size distribution, surface area, morphology, major oxide composition, nature and composition of the crystalline phases, iron content or silicon to aluminium ratio and the properties of the IPC formed. The natures and compositions of the amorphous phases was then determined by analysing the inorganic matter present in the coal from, which the ash originated. These results were then compared with the crystalline phases formed after the coal fly ash had been devitrified, and it was determined that Class F coal fly ash glass predominantly consisted of two interconnected phases; an amorphous silica rich phase and an amorphous aluminosilicate phase, with composition close to Al6Si2013. Further work found that these phases were expected to dissolve in a similar manner under the conditions prevailing during IPC synthesis and the type and concentration of species expected to dissolve for each material was determined. The mechanical properties of the IPC formed from these coal ashes correlated well with the ratio of silicon to aluminium expected to be present in the new binder phases. This was further confirmed by analysing the new IPC binder phases using devitrification. The phases expected to form during the devitrification of IPCs matched with those predicted, based on understanding the nature and composition of the glass phases present in coal fly ash and how they will behave under alkali-activation. It was determined that the poor performance of Tarong fly ash in IPCs was due to the low aluminium content of the amorphous phases present, resulting in a high silicon to aluminium ratio in the new IPC phase. The other coal fly ashes examined displayed much lower silicon to aluminium ratios, and higher compressive strengths resulted. The silicon to aluminium ratio in the new IPC phase predominantly influenced the compressive strength observed, however, other minor phases present in some of the coal fly ash samples, such as calcium silicates, also influenced mechanical properties.
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ItemThe durability of inorganic polymer cements( 2008)Inorganic polymer cements have the potential to provide a substantial reduction in global greenhouse gas emissions by replacing Portland cement in concrete. Several decades of research has advanced the field to the point where commercial implementation of the technology is imminent. In order for this to proceed, some understanding of the ability of inorganic polymer cements to provide durable, long lasting materials must be obtained. The objective of this thesis is to examine some aspects of the durability of IPC. Previous studies on various aspects of the durability of IPC are examined, with reference to known durability issues in Portland cement concrete. From this, some specific aspects in need of investigation are identified, including the importance of microstructure and porosity. The microstructure of IPC synthesised from fly ash and mixtures of fly ash and blast-furnace slag is examined, and the fate of various components of the ash and slag determined. The effect of the composition of the activating solution on microstructural development is also examined, and a mechanism describing the effect of soluble silicate on microstructural development proposed. The porosity and size distribution of the pores in IPC are determined, and linked to compositional factors. Examination of the distribution and morphology of pores is achieved by intrusion of Wood's metal. This technique is applied to IPC for the first time, and a novel method for achieving high-pressure intrusion is described. The functional aspects of the pore system are also determined for the first time, using a leaching test to determine the ionic permeability of the IPC paste. From this, important compositional factors are identified which will help produce IPC capable of protecting embedded steel reinforcement from corrosion. The effect of acids on IPC is determined, and it is demonstrated that some test methods currently in use provide highly misleading results. One corollary of this is that IPC is not as resistant to acidic environments as is frequently claimed. A new model for the process of acidic corrosion of IPC is presented, which helps to explain the discrepancy between apparent and actual performance of IPC binders in various tests. The effect of a broad range of mix design and processing parameters on acid corrosion of IPC at a pH of 1 is examined, and the factors which contribute most to acid resistance are identified. This will provide a basis for future endeavours to produce acid resistant concrete products. The conversion of amorphous metakaolin-based IPC into crystalline zeolitic phases, accompanied by dramatic strength loss, is identified for the first time. The process of strength loss is found to be mechanistically similar to that of conversion in calcium aluminate cements, i.e. formation of a metastable phase, which converts to a denser, more stable phase with time. The densification procedure is accompanied by a decrease in the volume of the reaction products, an increase in the volume of large pores and a significant reduction in strength. A recommendation is made against the use of metakaolin-based IPC because of the deleterious effect of crystallisation. Crystallisation is also examined in fly ash-based and mixed fly ash and blast-furnace slag-based IPC. In fly ash based systems crystallisation does occur, but to a much lower degree than in metakaolin-based systems. Crystallisation is not accompanied by a major loss of strength. The presence of blast-furnace slag prevents crystallisation in IPC systems. Finally, some conclusions regarding the durability of IPC are made. The importance of testing the durability of IPC formulations is highlighted, as is the need to apply accelerated tests carefully and interpret results with consideration of the differences in chemistry and microstructure of Portland and inorganic polymer cements.
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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.
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ItemShrinkage behaviour of geopolymer( 2009)Geopolymer cements offer an alternative to, and potential replacement for, ordinary Portland cement (OPC). Geopolymer technology also has the potential to reduce global greenhouse emissions caused by OPC production. There is already a considerable amount of work and research conducted on geopolymers in the past decades, and it is now possible to implement this technology commercially. However, to ensure that geopolymer becomes commercially available and able to be used in the world, further understanding of its ability to provide durable and long lasting materials is required. One main property which is still relatively unexplored compared to other properties is its shrinkage properties. The objective of this thesis is therefore to examine the shrinkage of geopolymers and factors which might influence it. The factors which influence geopolymer strength were investigated as being the factors which may influence shrinkage. The selection of the activating solution is an important factor in forming the final product of a geopolymer. Activating solution SiO2/Na2O ratio is determined to be an important influence on the shrinkage of geopolymer. SEM images of the samples enable observation of the sample topology and microstructure. An important observation was the existence of a ‘knee point’ which also occurs in OPC shrinkage. The ‘knee point’ is the point where the shrinkage goes from rapid shrinkage to slow shrinkage. From SEMs it is noted that the samples past the knee point are shown to have a smoother topology which means it is more reacted. Autogenous shrinkage is an important issue for OPC containing a high amount of silica, and is also a key factor in geopolymer shrinkage. Autogenous shrinkage is tested by keeping samples in a sealed environment where water lost to drying is kept to a minimum. It is noted that sealing and bagging the samples reduces the shrinkage considerably. The water to cement ratio, which is an important factor in OPC shrinkage, is also explored for the case of geopolymers. Water content plays an important role in determining early stage shrinkage, and has little to no effect on the later stage shrinkage. The water loss from the samples during drying on exposure to environment is noted and compared. The addition of more water did not necessary means that more water was lost. Addition of slag is known to be beneficial to geopolymers by giving early structural strength and faster setting time. Commercial geopolymer concrete will also include the use of slag. However, the addition of slag up to a certain extent gives a deleterious affect on shrinkage. A different type of Class F fly ash source with different composition data was used to see its effect on shrinkage, with only a slight influence observed between the two ashes tested. Fly ash was also ground for different lengths of time before use in geopolymerization, with grinding for less than 12 hours giving higher shrinkage than an unground sample, but shrinkage the decreasing with grinding for 18 or 24 hours. This initial higher shrinkage has been attributed to the mechanism of grinding which resulted in unevenly shaped fly ash particles taking up a larger initial volume resulting in higher shrinkage. The sample grinded for 24 hours showed higher shrinkage due to the particle size to be so fine that agglomerates may have form during mixing which would result in a lower reaction rate which increases the shrinkage. Elevated curing temperatures also reduce geopolymer shrinkage. Thus, it is clear that the shrinkage of geopolymers is influenced by a wide range of variables, and more notably by a few important variables: activating solution ratio, addition of water, grinding and bagging. The shrinkage of geopolymers can be correlated to the strength to a certain extent. However, the understanding of the shrinkage of geopolymers is still at a very initial phase, and further research is required.