Synthesis of workable, strong and acid-resistant N-A-S-H geopolymers incorporating dry-water
Creators: Yi, Chaofan
Publisher: name: University of Alberta Library
Year: 2023

<p>The production of cement-based materials releases numerous greenhouse gases into the atmosphere and therefore, accelerates global warming. The search for an alternative to Portland cement systems has thrown up the family of alkali-activated geopolymers as a potential choice. In civil engineering, workability, setting time, strength and durability are the most important demands as they determine the quality of construction, engineering performance and the service life of structures. To boost the widespread application of N-A-S-H geopolymers in civil engineering, developing mature mix design guidelines is strongly necessary. The mechanical performance of N-A-S-H geopolymers has been found to rely on the SiO2/Al2O3, Na2O/Al2O3 and H2O/Na2O ratios. Despite that, there so far has not been a systematic investigation on their attendant role upon workability, setting and durability. How to optimize these properties, without sacrificing the strength remains a challenge that limits the widespread use of locally available precursors for geopolymers in practice. In the meantime, the search for an effective additive that can enhance geopolymerization, without sacrificing the fresh paste rheology or the subsequent setting and durability, also continues into the present study. A preliminary review led to a water-in-air Pickering emulsion, specifically known as dry water, as a potential choice. Its promising characteristic in the present context is that it renders the nano silica as enlarged particles through the physical reorganization, while it retains the amorphous property of SiO2.<br>
Accordingly, this doctoral study aims to explain the mechanisms that underlie the mutual interaction between oxide components and dry water led enhancement in geopolymerization and thus, develop the mix design guidelines for making the workable, strong and durable N-A-S-H geopolymers incorporating dry water.<br>
The results show that an increase in any of SiO2/Al2O3, Na2O/Al2O3, and H2O/Na2O ratios, automatically increased the liquid-to-solid ratio and therefore, raised the flowability of the system. The setting process of N-A-S-H geopolymers was most sensitive to the SiO2/Al2O3 ratio, as this ratio essentially dominated the degree of geopolymerization. By contrast, the other two oxide ratios, i.e., Na2O/Al2O3 and H2O/Na2O, caused the relatively minor influence on setting. A deficient SiO2/Al2O3 ratio and an excessive Na2O/Al2O3 ratio reduced the amorphocity and boosted the significant formation of crystalline zeolite. These, therefore, reduced the mechanical strength and acid resistance. Although a value of H2O/Na2O ratio at the lower end may slightly depress the geopolymerization, the attendant high alkalinity improved the acid resistance. For generally used N-A-S-H geopolymers, a satisfactory combination of compositional ratios to simultaneously achieve the desired workability, final set and strength may fall within SiO2/Al2O3 = 2.8-3.6, Na2O/Al2O3 = 0.75-1.0 and H2O/Na2O = 9-10. When subjected to the acid-rich environment, the SiO2/Al2O3 ratio could be suitably narrowed to 3.1-3.4, and the H2O/Na2O slightly shifts to 8-10.<br>
Moreover, depending on the unique combination of characteristics of size coarsening, micro-filling, supplementary silica source and temporary water encapsulation, the nano silica stabilized dry water resolved the “trade-off” between the fresh and hardened properties. An optimal ratio was found at 15% nanoparticles-to-emulsion ratio to yield the most satisfactory combination of high flow diameter, quick set, high strength and superior acid resistance simultaneously.<br>
Overall, this thesis provides a guideline to synthesize the workable, strong and acid-resistant N-A-S-H geopolymer incorporating dry water. The findings are very promising to boost the widespread application of N-A-S-H geopolymers with locally available precursors, in both the ordinary condition and an acid-rich environment.</p>