Multicriteria Assessment for Calculating the Optimal Content of Calcium-Rich Fly Ash in Metakaolin-Based Geopolymers

This study examines the impact of calcium-rich fly ash as an additive on metakaolin-based geopolymers. Six types of fly ash (FA1-FA7) from different thermal power plants in the Czech Republic were collected and characterized based on their physical and chemical properties. The addition of fly ash into the geopolymers was evaluated through a multicriteria assessment that focused on density and mechanical properties. By using a multi-criteria approach, the assessment provides a comprehensive and holistic evaluation of the material, allowing for a more informed decision about the optimal addition of additives. This approach helps to minimize any negative impact on the material's properties while maximizing the utilization of the by-product. The result is an optimized geopolymer mixture with improved properties and increased sustainability, as the by-product is used beneficially. Furthermore, calcium content is the key factor that affects the physical properties of geopolymers by accelerating the curing time. This rapid process can result in reduced strength with increasing fly ash content. The multicriteria assessment revealed that the optimal condition is achieved using fly ash (FA2) from the Loucovice thermal power plant (5.2 wt.% Ca) that was treated at a temperature of 615 degrees C. The flexural strength of FA2-based geopolymers increased by 13% compared to concrete (standard). However, the addition of fly ash significantly reduced the compressive strength of geopolymers throughout the range of specimens. The Charpy impact strength of FA2 was higher than the standard due to the presence of unburned biomass solids in the ash structure that can absorb energy easily.