Upcycling municipal solid waste incineration bottom ash in clay-bonded bricks

Abstract

There is an increasing concern about the management of municipal solid waste incineration bottom ash (BA), of which approximately 300,000 tons are generated annually in Finland. As an alternative to the landfilling of this waste, which is the most common practice in the industry, this study investigates the feasibility of upcycling BA for clay brick production. Here, kaolinitic and illitic clays were selected as the precursors. Kaolinitic or illitic clays containing 10, 20, and 30 wt% BA and their counterparts without BA were fired at 1000 °C. The materials and prepared bricks were characterized using X-ray diffraction, thermogravimetry, scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy, mass loss, visual appearance, linear shrinkage, water absorption, apparent density, compressive strength, freeze–thaw, and leaching tests. The experimental results show that the incorporation of BA as an admixture positively influenced the properties of illitic clay-based bricks, which exhibited a reduction in cracks, mass loss, and water absorption while having higher apparent density, compressive strength, and residual strength than kaolinitic clay-based bricks. This is mainly ascribed to the difference in their sintering behavior and chemical and mineralogical composition; more notably, it is ascribed to the higher content of alkali oxides (K₂O and Na₂O) in the illitic clay than in the kaolinitic clay. The water absorption of the bricks varied from 4 % to 25 %, and the compressive strength varied from 12 to 19 MPa depending on the BA content and clay type. Nevertheless, the compressive strength of all bricks satisfied the ASTM C62 standard for bricks subjected to negligible weathering, and the residual strength of some samples after exposure to 50 freeze–thaw cycles demonstrated their resistance to severe weathering. The dual effects of sintering and BA addition resulted in more stable crystalline phases and a densified microstructure. Moreover, the heavy metals in BA were encapsulated in kaolinitic and illitic clay bricks and satisfied EU regulations for inert and nonhazardous materials.