Global warming is strongly affected by the release of various anthropogenic greenhouse gases, in particular CO2, and is still increasing due to the use of fossil fuels, land use, and the consumption and production of goods. Cement production is accountable for approximately 6% of the global anthropogenic CO2 emissions. In this respect, supplementary cementitious materials (SCMs) made from sustainable resources can make a significant contribution to reducing CO2 emissions and mitigating global warming. Biochar typically possesses the necessary properties, such as microstructure, specific surface area and chemical stability, to be used as SCM. The present work reports the reactivity of biochar from pyrolyzed biomass, focusing on screen overflows which consist of green waste, wood waste, and other inorganic waste. Screen overflows from two different composting plants in Germany and different particle size distributions (< 40 μm, 40–125 μm, and 125–250 μm) are investigated using thermogravimetric analysis (TGA) of the R3 test. TGA is conducted after 7, 14, 28, and 56 days. Additionally, the chemical composition is determined using X-ray diffraction (XRD) and particle size distribution. Material from screen overflows was focused purposely in this study due to them being one of the main components being incinerated in waste incineration plants. Therefore, pyrolyzed biochar produced from these screen overflows represents an up-cycling opportunity. Preliminary investigations revealed a moderate reactivity.