Physicochemical and Mineral Characteristics of Soil Materials Developed Naturally on Two ∼ 50 Years Old Coal Combustion Residue Disposal Sites in Croatia

•Weathered coal ash landfills exhibit pH decrease and formation of secondary minerals.•High sulfur content in CCR impedes pedogenesis and limits plant growth.•The presence of montmorillonite accelerates pedogenesis.•Particle properties such as SSA and CEC change with pedogenesis in coal ash landfills.•13C NMR spectra point to the unburned coal particles as the main origin of organic matter.