A Holistic Framework of Synthesis and Characterization of Waste-based Geopolymers for Civil Engineering Applications

Zhao, Mengxuan

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A Holistic Framework of Synthesis and Characterization of Waste-based Geopolymers for Civil Engineering Applications

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The cement industry is responsible for about 8% of all human-made CO2 emission and 2-3% of energy use in the world, and thus achieving sustainable concrete becomes imperative with the ever-growing worldwide demand for concrete. Geopolymers, as a promising greener alternative to ordinary portland cement (OPC), are investigated in this Ph.D. study, with a focus on the development of a holistic framework for synthesizing and characterizing their mechanical properties, durability, and volume change behavior. Red mud slurry-Class F fly ash-based geopolymers and metakaolin-based geopolymers were synthesized and investigated through an integrated multiscale experimental approach. In the first part of this study, red mud slurry and class F fly ash (RMSFFA) - based geopolymers were successfully synthesized, and the relationship among their synthesis factor-mechanical property-microstructure of RMSFFA geopolymer has first been examined. Then two different aspects of geopolymers were evaluated in the second part of this study: (i) to examine whether the current chemical shrinkage testing procedure of OPC is suitable for geopolymers by using metakaolin as the raw material, and (ii) to characterize the chemical volume change of the more complicated geopolymer system – RMSFFA geopolymers. A comprehensive experimental study was first performed to investigate the influence of different filling solutions (i.e., deionized water-DIW and activator solution-AS) on geopolymerization and the chemical volume change behavior of metakaolin-based geopolymer (MKG). Four stages in chemical volume change of MKGs were confirmed: Shrinkage, Expansion, Shrinkage, and Stable stages. Based on the testing results by Scanning Electron Microscopy-energy Dispersive X-ray (SEM-EDX), Isothermal Conduction Calorimetry (ICC), Fourier Transform Infrared Spectroscopy (FTIR), Brunauer-Emmett-Teller (BET), and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) techniques, AS filling solution generally caused the formation of more geopolymer gels, while the DIW filling solution intervened or dampened the geopolymerization process to some extent, especially on the surface portion. The filling solutions affect the geopolymerization of MKGs and thus their micro-characteristics and chemical volume change, largely through chemical exchanges between the filling solutions and geopolymer slurry, particularly diffusion-based leaching or ingress of Na+ and OH-. Then the chemical volume change behavior of RMSFFA geopolymers was investigated to shed light on the more practical geopolymer system. In the third part of this study, freeze-thaw (F-T) durability of RMSFFA geopolymers was also investigated, with an emphasis on the influence of chemical compositions and curing conditions (i.e., curing time and curing temperature). F-T durability was evaluated on the basis of the sustained mechanical strength of RMSFFA geopolymer specimens after subjected to 50 F-T conditioning cycles. The change in their chemical bonding, mineralogy, and pore characteristics at various F-T conditioning cycles was examined with Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and Brunauer-Emmett-Teller (BET) testing, respectively, to reveal the underlying processes during F-T conditioning. The results of both mechanical strength and microstructural characterization indicate that curing conditions influence F-T resistance of RMSFFA geopolymer samples through affecting their strength development. For 14-day cured RMSFFA samples at 50 °C, they experienced further geopolymerization at the early stage of F-T conditioning but the partial dissolution of geopolymer gels at the later stage of F-T conditioning. For 28-day cured samples at 50 °C, only the partial dissolution of geopolymer gels was likely to occur during the F-T conditioning because the strength development was likely to complete prior to the conditioning. Lastly, the results of RMSFFA geopolymer samples were used to illustrate how a holistic framework can be applied to synthesize and characterize geopolymers and other cementitious materials.

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