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Abstract
This research explores the role of water in fly ash-based geopolymers using the less conventional activators; Na2CO3 and Ca(OH)2. It included the determination of evaporable and combined water at ambient, 105°C and 1050 °C, and characterization of the micro-structure using SEM, XRD, IR, TGA and NMR. The results obtained suggested that the resulting geopolymer compounds contain a measurable amount of non-evaporable/combined water (which escapes at and above 105 °C) as distinguished from structural water which is rigidly bound, mainly by hydrogen bonding and constitutes an integral part of the geopolymer matrix in the form of molecular water. The backbone structure of Si-O-T (T=Si, Al) bonds was formed and developed as the reaction proceeds. The majority of the water escapes at below 200 °C, which enables the geopolymer to maintain its structural integrity at 1050 °C. The perception of whether Na-geopolymer is sodium aluminosilicate hydrate (N-A-S-H) was discussed, however, no sufficient evidence from IR, TGA and NMR was found in this research to prove that the geopolymer is a hydrate.
The research also addresses the possibility of using red mud as an activator and a filler for fly ash-based geopolymers. The effect of activators such as Ca(OH)2, waterglass and NaOH on fly ash and red mud-based geopolymers were investigated. The micro-structure of such geopolymers was examined using techniques mentioned above. The results obtained suggested that red mud is not efficient as an activator for fly ash based-geopolymers; NaOH (above 4 M) causes the leaching of sodium resulting in formation of Na2CO3 in red mud-based geopolymers; water glass (mass ratio of SiO2 to Na2O is 2), which is effective for activating fly ash, led to unsatisfactory results with red mud based-geopolymers; 10% addition of Ca(OH)2 proved to be effective for red mud and fly ash based-geopolymers.
This research provides an in-depth investigation on whether geopolymeric product is a hydrate. It also explores the possibility of non-traditional activators, such as red mud, from a novel angle. Further, it investigates the micro-structure of geopolymers using red mud which is a high Fe containing material, as a raw material.