Understanding geopolymers through synthetic gel systems

© 2016 Dr. Brant WalkleyAlkali-activated materials and geopolymers have received significant academic and commercial interest in recent years due to desirable physical properties, wide-ranging applications and a low CO2 embodiment. Variations in precursor chemical composition can induce significant changes in the structure and physical properties of these materials. In particular, the coexistence of calcium aluminosilicate hydrate (C-A-S-H), calcium (alkali) aluminosilicate hydrate (C-(N)-A-S-H) and sodium aluminosilicate hydrate (N-A-S-H) gel frameworks, which are the main reaction products in alkali-activated materials, results in complex physiochemical interactions which dictate material properties and performance. Despite extensive academic focus on the chemistry of alkali-activated materials and geopolymers, the way in which precursor chemistry controls phase evolution and nanostructural development of these materials is not yet fully understood. The influence of precursor chemistry on gel nanostructure and phase evolution in alkali-activated materials and geopolymers is investigated by application of advanced characterisation techniques, in particular solid state MAS and MQMAS NMR, to a novel class of stoichiometrically controlled, high-purity synthetic precursors and alkali-activated binders. This chemically simplified model system provides a unique tool for fast and reliable synthesis of AAM and geopolymers of a chosen composition without interference of contaminants. Gel formation, phase evolution and nanostructural development of N-A-S-H, C-A-S-H and C-(N)-A-S-H gel frameworks within both single phase and multiphase systems is examined. The interaction of these gel frameworks with each other and with additional reaction products and how these processes are influenced by the chemical composition of the precursor materials used is also investigated. This provides for the first time a process by which the physicochemical interactions occurring during alkali-activation of aluminosilicate-based precursors and subsequent formation of cementitious materials may be accurately simulated by pure systems. Structural analysis of N-A-S-H, C-A-S-H and C-(N)-A-S-H gel frameworks within stoichiometrically controlled synthetic alkali-activated materials reveals a strong dependence of the physiochemical characteristics of the gels on precursor composition. Application of advanced multinuclear solid state multiple quantum MAS NMR to a novel class of isotopically enriched C-A-S-H, C-(N)-A-S-H and N-A-S-H gels identifies six key structural motifs present within N-A-S-H gel frameworks, which are combined to develop a new structural description of the N-A-S-H gel in these systems. The key structural motifs which comprise C-(N)-A-S-H gel frameworks are also identified, and the overall nanostructure of these gels is shown to be significantly altered with changes in bulk composition. The findings presented in this thesis show that even small changes in reaction mixture composition can induce significant changes in phase formation and evolution, dictating the nanostructure of alkali-activated materials and geopolymers. This establishes a new platform of knowledge which is crucial for developing further detailed understanding of the molecular interactions governing phase evolution and nanostructural development of alkali aluminosilicate gels, as well as controlling the material properties and durability of modern cements which gain strength from these gels