Geopolymer, Next Generation Sustainable Cementitious Material - Synthesis, Characterization and Modeling

Zhang, Mo

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Geopolymer, Next Generation Sustainable Cementitious Material - Synthesis, Characterization and Modeling

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Geopolymers have received increasing attention as a promising sustainable alternative to ordinary Portland cement (OPC). However, the relationship among the synthesis, geopolymerization process, microstructures, molecular strucutres and mechanical properties of geopolymers remains poorly understood. To fill this knowledge gap, this dissertation focuses on the correlation of chemical composition-reaction kinetics-microstructure-mechanical properties of geopolymers. This study also sheds light on the durability, environmental impact and engineering applications of geopolymers from practical perspectives. The first part of this dissertation presents a comprehensive study on red mud-class F fly ash based geopolymers (RFFG). Firstly, RFFG with a high 28-day mechanical strength were successfully synthesized under the ambient condition of 23°C and 40 to 50% relative humidity. A nominal Na/Al molar ratio of 0.6 ~ 0.8 with a Si/Al ratio of 2 was found to be a good starting chemical composition for RFFG synthesis. Secondly, the reaction kinetics and its relation to the mechanical properties of RFFG were investigated by monitoring the development of geopolymer gels, reaction rate, porosity and mechanical properties of RFFG samples cured at room temperature, 50°C and 80°C for up to 120 days. The asymmetric stretching FTIR band of Si-O-T (T is Si or Al) centered around 960-1000 cm-1, which is the characteristic band of geopolymer gels, was observed to shift to a lower wavenumber at the early stage of the synthesis and shift to a higher wavenumber later on during the synthesis. The shift of Si-O-T band indicates that the geopolymerization took place in three stages: dissolution to Al-rich gels at Stage I, Al-rich gels to Si-rich gels at Stage II and Si-rich gels to tectosilicate networks at Stage III. The mechanical strength of RFFG barely increased, increased slowly by a limited amount and developed significantly at these three stages, respectively. An elevated curing temperature enhanced the early strength of RFFG, whereas an excessively high curing temperature resulted in a higher pore volume that offset the early-developed strength. Lastly, the remaining mechanical properties of the RFFG samples after soaking in a pH = 3.0 sulfuric acid solution for up to 120 days and the concentration of heavy metals leached from RFFG samples after the soaking were measured. The RFFG samples’ resistance against sulfuric acid was found to be comparable to that of OPC, and leaching concentrations of heavy metals were much lower than the respective EPA limits for soil contaminations. The degradation in mechanical properties of the RFFG samples during soaking in the acid was attributed primarily to the depolymerization and dealumination of geopolymer gels. The second part of this dissertation is devoted to the investigation of nano-scale mechanical properties and molecular structures of geopolymer gels with grid-nanoindentation and molecular modeling. Four phases (e.g., porous phase, partially developed geopolymer gels, geopolymer gels and unreacted metakaolin or crystals) and their nano-mechanical properties were identified in metakaolin based geopolymers (MKG) with grid-nanoindentation technique. It was found that the proportion of geopolymer gels largely determines the mechanical strength of the resulting geopolymers while other factors (e.g., pores and cracks) also play some roles in macro-scale mechanical strength of geopolymers. The final setting time of the geopolymers increased with the increase in Si/Al ratio and the decrease in Na/Al ratio, while the proportion of geopolymer gels and macro-mechanical strength of geopolymers increased with the increase in both Si/Al and Na/Al molar ratios, within the range of 1.2~1.7 and 0.6~1.0, respectively. In the molecular modeling, a combined density function theory (DFT)-molecular dynamic (MD) modeling simulation was developed to “synthesize” geopolymers. DFT simulation was used to optimize reactive aluminate and silicate monomers, which were subsequently used in reactive MD simulations to model the polymerization process and computationally synthesize geopolymer gels. The influence of Si/Al ratio and simulation temperatures on geopolymerization and resulting molecules of geopolymer gels was also examined. The computationally polymerized molecular structures of geopolymer gels were obtained. The distribution of Si4(mAl) and radial distribution fuctions of Si-O, Al-O, O-O and Na-Al for the models were compared and qualitatively agreed well with the experimental results from nuclear magnetic resonance (NMR) and neutron/X-ray pair distribution function in previous literature. Three polymerization stages: oligomerization, ring formation and condensation, were identified based on the nature of polymerization process, which were found to be affected by the temperature and Si/Al ratio. A higher temperature enhanced the reaction rate while a lower Si/Al ratio resulted in more compact geopolymer networks. The final part of this dissertation presents an experimental feasibility study of using geopolymer in shallow soil stabilization, in which a lean clay was stabilized with MKG at different concentrations. The study confirmed that MKG can be used as a soil stabilizer for clayey soils and the unconfined compressive strength, Young’s modulus and failure strain are comparable to or even better than OPC when the MKG’s concentration is higher than 11%. The binding effect of geopolymer gels on the soil particles was confirmed as the main mechanism for the improvement in mechanical properties of the stabilized soils with the scanning electron microscopy imaging, energy dispersive X-ray spectroscopy analyses and X-ray diffractometry characterization.

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