Formation of pore structure during the sintering process is determined by thermochemical processes in the illite type clay. High compressive strength and remarkable porosity characterises sintered illite clay ceramic. These properties privilege illite clays for production of powerful light weight ceramic building blocks. Admixture of dolomite in the natural quaternary clay and extra additive of dolomite can rise or keep high mechanical strength of porous ceramic material due to high temperature interaction of dolomite and illite at temperature 10000C. The porosity of ceramic produced from quaternary Latvian clay is possible to increase with organic combustible additives. This impact finds expression not only by burning and gas forming, but also by chemical impact between carbon containing compositions and clay minerals with pore formation in the nanosize range. Additive of more pore forming components together at quaternary clays are powerful to decrease coefficient of thermal conductivity. This combined additive contains mineral component such as milled dolomite and various organic combustible matter.