Impact of Biomass Fly Ash on the Performance of Diatomite and Iron Dust Powder-Based Alkali-Activated Binder

This study investigates the influence of biomass fly ash (BFA) produced from high-temperature combustion of woody biomass fuels typical of Lithuanian energy plants on the mechanical performance, microstructure, and reactivity of alkali-activated binders based on diatomite and iron dust. Diatomite was used as a reactive silica source, while iron dust served as a matrix-modifying component enhancing binder density and strength. The role of BFA (10-30%) was evaluated in terms of compressive strength, softening factor, water resistance, and structural development using XRD and FTIR analyses. The results show that the formation of a compact geopolymer gel is the key factor controlling strength development. The highest compressive strength (53 MPa) was obtained at 10% BFA; however, it decreased to 33 MPa after thermal treatment at 200 °C, indicating limited structural stability. Increasing the BFA content to 20% and 30% improved the softening factor and water resistance but significantly reduced compressive strength to below 20 MPa and 10 MPa, respectively, demonstrating a trade-off between strength and durability. XRD analysis confirmed similar mineralogical compositions in all samples, dominated by largely unreacted quartz and magnetite, while minor amounts of andradite formed after thermal treatment. FTIR results revealed increased polymerisation with higher BFA content, reflected by the shift of the Si-O-T band (~966-985 cm^-1 to ~988-995 cm^-1), although the presence of Ca-rich and partially unreacted phases led to a less efficient geopolymeric network. Overall, the performance of the studied systems is governed by the balance between gel formation, phase composition, and microstructural integrity, with optimal properties achieved at moderate BFA content rather than at extreme compositions.