Chemical and Biomolecular Engineering - Theses

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Author: Keyte, Louise × End date: 2009 × Start date: 2000 × Subject: fly ash × Subject: polymer impregnated cement ×

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    What's wrong with Tarong?: the importance of coal fly ash glass chemistry in inorganic polymer synthesis
    Keyte, Louise ( 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.