Date of Award
Doctor of Philosophy
Civil, Construction, and Environmental Engineering
It is well known that the use of Portland cement (PC) in concrete construction is causing severe environmental issues primarily due to vast quantity of carbon dioxide released to the atmosphere during the manufacture of PC. On the other hand, disposal of industrial solid wastes such as fly ash and slag in landfills is creating another threat to the environment. The development of a fly ash geopolymer binder, produced from the reaction of fly ash and alkaline solution, may replace Portland cement as a construction material and at the same, reduce the disposal of fly ash in landfills.
This dissertation reports the efforts in optimizing mix proportion, predictive modeling on early age properties, shrinkage control and mechanical performance of an engineered composite made with fly ash-based geopolymer. This dissrtation consists of four papers: (1) Optimization of Mix Design Parameters on Thermal, Setting and Stiffening Behaviors of High Calcium Fly Ash Geopolymer; (2) Prediction of Strength, Setting Time and Heat Generation of Fly Ash Geopolymer Using Artificial Neural Network; (3) The Effects of Activator and Shrinkage Reducing Admixture on Shrinkage Behavior of Fly Ash Geopolymer, and (4) The Effect of Slag on Mechanical Properties of Engineered Geopolymer Composite.
Due to the lack of knowledge to optimize the mix proportion of fly ash based geopolymer in the published literature, Paper 1 is focused on the effects of design parameters including SiO2/Na2O mole ratio (Module), solute (NaOH and Na2SiO3) mass concentration on the fresh and hardened properties (i.e., setting time, compressive strength and heat of hydration). The knowledge gained from this study is expected to assist in the optimization of the mix proportions for thefly ash geopolymer. Results from Paper 1 have shown that modules less than 1.5, concentrations between 40% and 50%, L/F ratios less than 0.40, and higher curing temperature, such as 50oC, were preferred to synthesize a geopolymer system using high calcium fly ash.
In Paper 2, an artificial neural network (ANN) approach was applied to analyze the complexity between geopolymer properties and various parameters forgeopolymer mix proportion design. The predictive models for setting time and compressive strength of geopolymer were established for the ease of mix design. Paper 2 concluded that ANN was an effective tool for parametric study of the properties of fly ash geopolymer. The effects of geopolymer mix design parameters on setting time, compressive strength and heat generation were discussed in accordance with the prediction profiler generated by the ANN models. The proposed model can be used as a guidance for high calcium fly ash geopolymer mix design in the future.
Shrinkage of cement-based materials is a major cause of cracking. The work discussed in Paper 3 was to characterize the shrinkage behavior (e.g., free drying shrinkage and restrained ring shrinkage) of fly ash-based geopolymer in comparison with that of PC paste. The effects of activator (Module and Concentration) and shrinkage reducing admixture (SR) on the shrinkage behavior of fly ash-based geopolymer have been explored. In addition, the flowability of the geopolymer using a mini slump test and compressive strength test were also carried out. The results indicate that the fly ash geopolymer has comparable flowability properties as compared to that of PC. SR slightly decreased flowability of PC and fly ash geopolymer. It was also found that the drying shrinkage of fly ash geopolymer was of similar magnitude to that of PC, but was not due to mass loss for fly ash geopolymer. The SR significantly reduced the drying shrinkage of fly ash geopolymer up to 52% as well as in PC. The SR decreased the restrained shrinkage up to 16%, delayed the cracking time, reduced the crack width and lowered the cracking potential for both PC and fly ash geopolymer. The fly ash geopolymer mixtures had lower cracking potential than PC. The effects of Module and Concentration on drying shrinkage and restrained ring shrinkage were also concluded.
The last paper (Paper 4) investigated the mechanical performance of fly ash-based geopolymer in a fiber reinforced composite, namely an engineered geopolymer composite (EGC). Fly ash was replaced with slag in the geopolymer. The physical and chemical interactions of these two cementitious materials have resulted in a high strength (up to 110 MPa) and workable EGC. The mechanical properties including compressive strength, tensile strength, tensile strain capacity, toughness, elasticity, flexural bending strength, ductility and pullout bond strength were assessed. Experimental results in Paper 4 revealed that all EGCs exhibited strain hardening behavior. Twenty percent slag addition improved the engineering strength most. However, as slag addition increased, the tensile strain capacity, ultimate deflections, toughness and ductility decreased. In addition, bond strength can be estimated precisely based on the compressive strength of EGCs.
Ling, Yifeng, "Proportion and performance evaluation of fly ash-based geopolymer and its application in engineered composites" (2018). Graduate Theses and Dissertations. 16398.