pH Dependent Degradation and Characterization of Geopolymer Structures Derived from Fly Ash

Geopolymer structures are a relatively new family of un-fired ceramics that are produced by alkaline slurry of mixed alumina silicate-based materials. One of the most abundant geopolymer-forming powders is the fly ash. Fly ash is a waste product of thermic coal plants, which produce electricity by coal burning. Fly ashes contain mostly Ca, Al, Si and Fe oxide mixtures and compounds formed during the burning process, followed by sedimentation. The standardization is made by SiO₂ content. Class C ash has low silica content (<50 wt.%) and class F ash has a higher silica content (>50 wt.%). The fly ash, studied in this work, was obtained from Kangal Coal Plant. The NaOH was used as the geopolymer binder and curing was performed at 80% relative humidity, at 40°C in sealed plastic molds. The obtained cylinders were aged for one week before releasing from molds. pH dependent degradation was used to determine the ionic exchange of geopolymers into the solution. Afterwards, XRD and SEM were done to detect the structural changes of geopolymers.