Date of Award
Open Access Dissertation
Civil and Environmental Engineering
Doctor of Philosophy
Concrete is a major construction material that is largely made up of cement. Unfortunately, the manufacturing process for the cement currently used to produce concrete releases a great deal of CO2, which endangers the environment. Hence, finding alternatives to ordinary Portland cement is of extreme importance. The intellectual contribution of this dissertation is the development of an improved understanding of the geopolymerization process so that the compressive strength of cement can be maximized while the need for external heating is minimized. The intent is to create a substance to replace commonly used cement without sacrificing other beneficial properties. The first study investigated the effects of activating solutions used to create the cement, curing procedure, and source of fly ash on the resulting compressive strength. Results of this experiment indicate that compressive strength is not significantly affected by the curing conditions when silica fume is used in the activating solution in comparison to the use of sodium silicate. Test results further indicate that the resulting concrete has the potential for high compressive strength, and the compressive strength is directly affected by the fly ash source. In the second study the investigation focused on how the ratio of sodium hydroxide ratio, external heat, and the partial replacement of Portland cement affected fly ash-based geopolymer concrete. Experimentation showed that the application of external heat plays a major role in compressive strength development of fly ash-based geopolymer concrete.
Results also show that early and final compressive strength gains can be improved by using Portland cement as a partial replacement for fly ash in the absence of external heat. Scanning Electron Microscopy (SEM) results show that Portland cement utilized the free water, resulted of geopolymerization reaction, reducing microcrack formation and also provided extra alkalinity such as calcium hydroxide. The third study focused on investigating the effects of particle size distribution and varying sources of fly ash on the mechanical and microstructural properties. Test results indicate that, within the range of materials investigated, compressive strength is linearly related to the average particle size distribution and the source of fly ash has a significant effect on the mechanical properties. The last study investigated the geopolymerization process itself. Acoustic emission data was processed through pattern recognition, and two clusters were identified and assigned to specific mechanisms. Results show a significant difference between the twodifferent water/binder weight ratios. Pattern recognition indicated that the geopolymerization mechanisms of dissolution (of Si and Al cations), formation of bubbles, and microcrack initiation, occurred at roughly the same time for the samples of 0.3 water/sold ratio. However, in the 0.35 water/sold ratio of paste samples the mechanisms occurred sequentially. Microcrack initiation classified by pattern recognition coincided with the final setting times.
Assi, L.(2018). Understanding Geopolymerization Process for Enhancement of Mechanical Properties of Fly Ash Based-Geopolymer Concrete. (Doctoral dissertation). Retrieved from https://scholarcommons.sc.edu/etd/4981