Factors influencing the performance of biomass fly ash-based geopolymers

Biomass fly ash, a by-product of biomass power generation, often contains high levels of unburned carbon and irregular particle morphology, which can influence its suitability for geopolymer applications. In this study, biomass fly ash was modified through flotation method using a kerosene to oil system, which effectively reduced its loss on ignition, cleaned the particle surfaces, and increased specific surface area. The modified biomass fly ash was then used to partially replace coal fly ash in geopolymer mortar, with ground granulated blast furnace slag as precursors.

To evaluate the effects of modified biomass fly ash on geopolymer mortar, three curing conditions (air, water, and high-temperature), two alkaline activators with different SiO2/Na2O ratios (2.31 and 0.86), and modified biomass fly ash replacement levels (0%, 20%, 40%, and 60%) were investigated. Results showed that modified biomass fly ash reduced mortar flowability due to higher surface area and water absorption. The highest compressive strength (63.10 MPa) was achieved with 40% modified biomass fly ash under water curing and a high SiO2/Na2O ratio. In contrast, air and heat curing resulted in decreased strength and coarser pore structures. Low SiO2/Na2O ratio further increased microporosity and reduced strength. Modified biomass fly ash addition also led to higher drying shrinkage, especially in the early 15 days.

Correlation analysis confirmed that SiO2/Na2O contents were positively associated with strength and negatively with total porosity. Overall, modified biomass fly ash can serve as a viable alternative to coal fly ash in geopolymer mortar, provided appropriate activator composition and curing conditions are applied.