Abstract:
Geopolymer can be synthesized using industrial wastes such as fly ash, making it a promising material to replace Ordinary Portland Cement (OPC) as a green binder for concretes. In this study, four locally available fly ashes were used to synthesize geopolymers. These fly ashes were carefully selected to include both the high calcium and low calcium fly ashes. Two of the fly ashes have very similar chemical compositions. Comprehensive experimental programs have been carried out to characterize these fly ashes, to examine the critical factors affecting the mechanical properties of geopolymers, and to evaluate the potentials of these fly ashes as source materials for geopolymers. Testing results show all these fly ashes can be used to synthesize geopolymer. Among them, the fly ash from Gaston was found to be the most suitable source material for geopolymer. Fracture behavior of the produced geopolymer mortar was studied using splitting wedge testing together with digital image correlation technique. The measured full-field displacement clearly shows that a fracture process zone (FPZ) exists ahead of the major crack, suggesting that the fracture of the geopolymer mortar follows a nonlinear behavior. The bi-linear constitutive law of the FPZ was then obtained three an inverse analysis. To gain insight into the geopolymerization process, the electrical properties of the produced geopolymers were measured. It has been found that both the magnitude of the electric impedance or the electric resistance can be used as an indicator of the degree of geopolymerization. The bulk resistance of all geopolymers increases with age over one year, suggesting that the polymerization is a very long process. Finally, to explore the possibility to reinforce the brittle matrix such as geopolymer with Carbon Nanotubes, a model of shear force transfer from CNTs to the matrix is developed to understand and optimize the reinforcing effect of CNTs.
