Chemical and Biomolecular Engineering - Theses
Permanent URI for this collection
Search Results
1 - 3 of 3
-
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).
-
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.
-
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.