It was established that pore-forming effect of both fuel ash and ashy microspheres on clay rocks compounds is caused both by the structural features (the presence of the spherical shaped hollow particles), the ashy component composition, and the physical and chemical processes taking place in the clay ash and clay ashy microspheres systems, the latter processes preventing the shrinkage on sintering (owing to the anorthite and mullite synthesis occurring with molar volume increasing). Ill. 6. Ref. 14. Tab. 2.