Abstract: In a process of safely disposing of waste materials which are solid or contained in aqueous liquids, wherein the waste material is treated with alkaline substances and a solid product is formed, the waste materials which are solid or dissolved or suspended in aqueous liquids are mixed in accordance with the invention with brown coal ash and, if desired, with water, to obtain a homogeneous mixture having the consistency of a pulp or freely flowable slurry and the proportions of the mixture are so selected that it hardens to a material which is virtually impermeable to water. Such materials can be used to dispose of waste materials, such as waste acids, and to seal underground or surface dumps.