Our article shows the results of research on ash-water solidified suspensions heated at high temperatures. The ash-water suspensions are used in underground coal mines to fill the voids and cavings resulting from exploitation. The heating of the solidified samples has been conducted to simulate conditions of the process of self-heating of residual coal substances. Similar conditions may also occur on the surface, e.g. during fires, and knowledge of the influence of the high temperatures on these type of materials may be useful when such substances are used as components of the building materials. For testing purposes we have used the ash from the fluidized bed combustion of lignite with and without the addition of the biomass. The conducted research covered the definition of the high temperature influence on the structure, leachability, phase composition, and mechanical properties of the suspension samples.