In accordance with the requirements of sustainable development, the lack of natural raw materials and increase of waste, a search for innovative, alternative solutions and development of new skills in the processing and use of waste instead of natural raw materials is required. Among the possibilities of processing alternative materials, slag is an interesting byproduct of the use of coal in thermal power plants. In my thesis I explored the possibility and permissibility of using slag, obtained in Thermal power plant Trbovlje, in the construction industry, from the environmental perspective. The aim of the research, which the thesis is based on, is to obtain the results of slag leaching in Thermal power plant Trbovlje. The first part of my research was carried out by a percolation test (an up-flow percolation test - under specified conditions), with a precise regard to the guidelines, defined in SIST-TS CEN/TS 14405:2004 standard. The second part of the research consisted of monitoring pH and electrical conductivity in ten different mixtures of slag and water. By comparing both studies we determined the slag’s impact on the environment.