Thermally Induced Microstructural Changes in Fly Ash Geopolymers: Experimental Results and Proposed Model

This study presents a proposed model for thermally induced microstructural changes in fly ash geopolymers. Two paste mixes with different as‐cured microstructures are evaluated for thermal resistance. One mix was a highly reacted, high‐strength geopolymer with a compact microstructure and the other mix had higher degree of unreacted fly ash resulting in a low strength, low‐density geopolymer. Changes in the microstructure and bulk properties for each formulation were assessed at 100°C temperature intervals up to 1000°C using SEM, Q‐XRD and physical testing. It was observed that the higher density and apparent lower permeability of the high‐strength geopolymer led to it being more vulnerable to dehydration damage. Dimensional and phase changes also caused further strength losses before sintering at higher temperatures promoted strength gains. The low‐strength geopolymer was not damaged by dehydration and was better able to accommodate volumetric changes; hence it exhibited an increase in strength after thermal exposure due to the sintering. From these results and others in the literature, a model has been proposed for thermally induced changes in fly ash geopolymers.