Microstructure and Physical Properties of Lightweight Aggregates Produced by Alkali Activation-High Shear Granulation of FBC Recovered Fuel-Biofuel Fly Ash

Abstract

The fluidized-bed combustion (FBC) is an environmentally-friendly way to combust biomass and waste materials. However, as a side-product large amount of ash is created which is difficult to utilize due to its low reactivity and high heavy metal content. Instead, most of the ash ends up in landfills so novel utilization methods are needed. In this study lightweight aggregates (LWAs) were produced from recovered fuel-biofuel fly ash taken from a F boiler by granulating the fly ash with sodium silicate in a high-shear granulator. Blast furnace slag and metakaolin were studied as co-binders. Physical, chemical, and microstructural characterization was conducted for the precursors and LWAs. The crushing strength varied between 4.6 and 14.6 MPa. Addition of co-binders increased strength and produced denser structure. The loose bulk density was <1.08 g/cm3 and dry particle density was <1.77 g/cm3 for all prepared granules so they met the definition of lightweight aggregate in standard SFS-EN 13055-1. LWAs contained X-ray-amorphous material, indicating the formation of aluminosilicates and some crystalline phases, which were derived from the fly ash. This study presents a method of producing LWAs from fly ash that have physical properties comparable to LWAs manufactured by conventional methods without the need for high-temperature sintering or the usage of cement.