Experimental work was conducted on illite-chlorite and kaolinite-carbonate rich clays to investigate effects of pore forming additive materials. Three types of additives were applied: vegetal materials like sawdust, sunflower seeds hull and rice husks; lignite, as high organic content mineral material; fuel-grade coke, as synthetic additive. Raw materials were characterized regarding their composition, thermal behavior, microstructure and chemical composition. Expansion of extruded samples was detected after pressing release. Correlations were determined between the type and cellulose content of vegetal materials and expansion caused. The mineral and synthetic additive does not produce expansion, but lowers the working moisture needed.