Raw materials or chemical compounds containing inorganic contaminants such as heavy metals or metalloids are frequently applied for the industrial production of various goods and consumer products. When these products are discarded at the end of their life cycle, the contained contaminants may pose a problem to various waste treatment options because of existing limit values, guidance values, or quality requirements. This also applies to the co-processing of refuse-derived fuels (RDF) in the cement industry, which is increasingly attractive throughout the world. In several countries, mixed solid wastes are frequently processed to solid recovered fuels (SRF), a quality-assured subgroup of RDF solely produced from non-hazardous solid wastes. SRF producers may need to take measures to ensure that the requirements are met, including measures to decrease contaminant concentrations in SRF. However, this requires prior knowledge of the waste fractions, materials, or products that may contain large amounts of these elements. Besides contaminants, another group of chemical elements plays an essential role in the co-processing of SRF and other RDF, namely ash constituents, which are incorporated into the cement clinker and are, therefore, from a technical perspective, recycled on a material level. This Doctoral Thesis focuses on three different domains concerning contaminants and/or ash constituents: origins, distribution & removal, and fate. An extensive literature review was conducted to identify contaminant carriers and to discuss the occurrence of ash-forming elements in waste fractions. The element distribution, removal options for contaminant carriers and effects on SRF quality were investigated by practical experiments. Special attention was paid to chemically and mineralogically characterizing the fine fractions, which have hardly been investigated before. Concerning the fate of elements, this Thesis focused on ash constituents and determining the material-recyclable share of SRF (R-index) during co-processing, as well as identifying those material or waste fractions that contribute most to the R-index. Consequently, the potential contribution of the cement industry towards reaching European recycling targets was estimated, and the role of the cement industry in a modern circular economy was assessed. The obtained results show that removing the fine fraction can significantly decrease contaminant concentrations in the treated waste stream, especially in combination with near-infrared (NIR) sorters removing PVC. However, the fine fractions also contain the largest share of valuable ash constituents for the cement industry. These materials would be prevented from being recycled when the fraction is removed from the waste during SRF production. This leads to a conflict of interest between resource utilization/conservation and environmental protection. Analyses of conventional SRF showed that 13 to 18 mass-% of the SRF may be considered as recycled on a material level in the cement industry. Therefore, in a modern circular economy, the cement industry may represent an attractive complementary recycling option, offering the material recycling of mixed fractions or sorting residues from established recycling processes in addition to their thermal recovery.