Blast furnace slag (BFS) is a secondary byproduct of the steel industry. When appropriately tailored, BFS could be an effective functional filler that improves the property profile of widely-applied thermoplastics like polypropylene (PP). This work proposes BFS as tailored filler, which might provide a functional influence on the structure-property profile of PP beyond just being an inexpensive filler material compared to the common commercial fillers in the polymer industry such as calcium carbonate. Furthermore, the possibility to reach promising functional performance for BFS could be a valuable step toward saving significant amounts of energy, that are otherwise devoted to the extraction and processing of commercially utilized mineral fillers such as calcium carbonate. Hence, the main target of the current PhD thesis was to investigate the influence of BFS as a functional filler on the rheological, thermal and mechanical properties of a typical PP copolymer. This research was systematically divided into basic and advanced stages. In the basic stage, the investigation of the influence of BFS filler parameters (distribution, type and loading), kneading process parameters (speed and duration) and compounding technique (kneading versus TSC) on the properties of PP was accomplished. With the evolution of the ‘basic study’ findings, the number of the investigated BFS filled PP compounds was reduced to only two compounds. For each compound, two coupling agents, VES and MPS were compared with respect to the compound properties, where the coupling agent reflecting better compound properties was selected. To produce the final BFS – PP compounds, the better coupling agent, MPS, was hence utilized to modify the BFS for TSC compounding runs followed by subsequent compression-/ injection molding. Based on their properties, the final BFS - PP compounds were investigated for candidacy in automotive interior trim applications. Therefore, the properties of the final BFS - PP compounds were compared with a compression-/ injection molded commercial compound that is industrially applicable for interior trim applications. As a final step, carbon footprint screening analyses were carried out to compare the environmental life cycle impact of the BFS – PP compounds to a conventional limestone – PP one. The investigation of unmodified BFS filled PP compounds showed that their complex shear viscosity and tensile stiffness linearly increased up to 35 and 20 % at BFS loading of 30 wt.-%, respectively. The degree of crystallinity, however, showed a linear decrease up to 40 % as the BFS loading increased. At 20 wt.-% loading, it was noticed that BFS increased the thermal conductivity of PP by 40 – 50 %. The twin-screw compounding and injection molding of the modified-BFS with PP surprisingly increased the strain at break of PP beyond 350 %. That was not possible with unmodified BFS, where premature failure was dominant. After testing their candidacy for interior-trim applications, the BFS compounds achieved 800 % higher strain at break as well as comparable tensile stiffness and toughness levels compared to a commercial mineral filled PP compound that is tailor-made for interior-trim applications. Finally, the carbon footprint screening analyses suggested a ‘best for slag’ scenario, where one ton of unmodified-BFS filled PP compound was shown to generate a little less CO2 than its limestone filled PP counterpart. It is important, however, to emphasize that while limestone was modified and compounding-ready, the BFS was not. Accordingly, the modification process for BFS is expected to emit extra CO2 into the atmosphere, which might move the BFS toward the ‘50,50’ scenario.