The paper concerns testing and treatment of post-process waste from waste incineration plants with alkaline activation. The paper presents research results on ash and slag from thermal waste treatment processes, which were exposed to alkaline solutions. The results of the research of waste which solidified by geopolymerization and the research results of the hydrothermal activation in order to obtain the zeolites, are presented. For ash and slag treated with geopolymerization we present the results of compressive strength and selected results of leaching. For the waste which were treated with hydrothermal synthesis, we present the particle morphology (SEM) of the obtained zeolites. The research confirmed the possibility of immobilization of ash and slag in geopolymers and the possibility of obtaining zeolites from certain types of waste generated in waste incineration plants