Study on the Reaction Stability of Fly Ash-Based Geopolymers

Fly ash-based geopolymers’ quality varies based on their constituent materials’ quantity and reactivity. Determination of the ratio of fly ash as a base material reacted with the ratio and quantity of the alkaline activator as a reagent is a challenge in producing a stable geopolymer reaction. This study aimed to provide insight into the stability of the mixture of fly ash-based geopolymers. Variations in the ratio and amount of alkaline activator to fly ash in a wide range of quantities could provide an overview of the stability of the geopolymer reaction. The fundamental indicators of the stability characteristics of geopolymers are their physical stability and changes in strength. Evaluation of the stability of the geopolymer mortar was observed from the development of compressive strength, visual inspection of efflorescence during partial immersion, and the strength changes after a heating treatment of 150°C for 4 hours. The results showed that the more alkaline activator content in the mortar, the stability of the resulting geopolymer is reduced, as seen from the appearance of efflorescence on the specimen surface, expansion cracks, and a decrease in compressive strength. The ratio of alkaline activator also affected the compressive strength resulting in a range of optimum ratios. Meanwhile, depending on the calcium content in the fly ash, a secondary pozzolanic reaction could also occur in the geopolymer concrete at a later age.