Carbonation of digested sewage sludge-based sealing material in a final landfill cover system: Role of waterglass and additional industrial by-products

Landfills emit significant greenhouse gases, particularly methane (CH₄) and carbon dioxide (CO₂), yet the interaction between CO₂ and landfill sealing materials remains underexplored. This study investigated the carbonation behavior of sealing materials composed of digested sewage sludge (DSS), waterglass, aggregates, and various industrial by-products as additives—waste incineration fly ash (WIFA), biomass bottom ash (BBA), and aluminum anodizing waste (AAW). An accelerated carbonation process was employed to simulate CO₂ diffusion, and the effects of waterglass and additives on permeability, physicochemical properties, and environmental impacts of the sealing materials with/without carbonation were systematically evaluated. Results show that waterglass improves impermeability by binding particles and filling pores; however, its high alkalinity initially promotes the dissolution of minerals such as boehmite in AAW, increasing porosity. Among the additives, AAW, with its finer particle size, yielded the lowest permeability in uncarbonated samples. Carbonation led to the formation of carbonates and monohydrocalcite, causing phase volume changes that increased porosity and reduced sealing performance. Despite this, overall permeability remained within the acceptable limit (< 6.34 ×10⁻¹⁰ m/s) specified by Dutch regulations, owing to relatively moderate phase transformations. However, carbonation increased the leaching of elements such as Cl and Sb, particularly in DDS-WIFA samples, indicating that carbonation breaks down particles and diminishes physical encapsulation of these toxic elements. This highlights the need for selecting additives with low leachability for environmental compliance. Moreover, a higher dosage of waterglass (2.0 wt%) mitigated impermeability degradation by forming silicate gels and promoting CO₂ adsorption. These findings advance the understanding of carbonation mechanisms in sludge-based sealing materials and inform the selection of feasible industrial by-products for sustainable landfill cover systems.