Chemical and Biomolecular Engineering - Theses
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ItemThe structure and thermal evolution of metakaolin geopolymers( 2006-02)Geopolymers are a relatively new class of material that has many broad applications, including use as a substitute for Ordinary Portland Cement (OPC), use in soil stabilisation, fire resistant panels, refractory cements, and inorganic adhesives. The synthetic alkali aluminosilicate structure of geopolymer results in a highly versatile material that can be synthesised en masse, cost competitively and from a wide varietyof aluminosilicate bearing raw materials. Despite the commercial promise and technical viability of the technology, the fundamental understanding of the chemical structure and characteristics of geopolymeric materials, and to some degree the academic rigor of some aspects of the science related to geopolymers, leave a lot to be desired. In particular, the understanding of the effects of Si/Al ratio and alkali cation type on the molecular structure of the binder, and how these relate to the microstructure and mechanical and thermal properties are poorly understood. The thesis explores the structure and characteristics of a systematic multi-dimensional matrix of geopolymers derived from metakaolin, a relatively pure aluminosilicate source. The thesis addresses the determination of the core molecular structure of geopolymers by solid-state NMR spectroscopy, and how this is altered by the nominal Si/Al ratio and alkali cation type. The chemical ordering is observed to reduce with Si/Al ratio and with inclusion of potassium over sodium. Most significantly, the presence of Al-O-Al linkages is identified for the first time in specimens with Si/Al ratios close to unity, by the application of 17O NMR techniques on geopolymers. The role of molecular structure and gel chemistry of geopolymers is elucidated, and links are drawn to understand the development of the microstructure and physical properties of the material. The thermal evolution of geopolymeric gels derived from metakaolin is investigated in terms of physical and structural development when exposed to temperatures up to 1000°C. The response of geopolymers to heating is characterised into four regions regardless of the extent of shrinkage or crystallisation. Several critical material performance relationships exist that are related to both the microstructure and chemical composition. The thesis presents an updated structural model of geopolymers to include new insights obtained from application of solid-state NMR techniques and thermal analysis. The improvements in structural understanding described in the thesis have the potential to affect all aspects of geopolymer science.