The expansion due to air moisture is a phenomenon produced in manufactured baked clay pieces. The cited phenomenon lies in the environmental moisture absorption by the materials that constitute the ceramics pieces. This absorption causes a slight dimensional increase of a ceramics piece that it could generate several pathologies in the buildings. Moisture expansion effects (e.g. crackings, fissures, etc) appear especially in the hollow clay blocks for floor and in the bricks for façades . The mechanism by which ceramic materials expand when absorbing moisture is not perfectly known yet. It is supposed that porosity and proportion between amorphous and crystalline phases of the ceramic piece are the most important variables involved. The goals of research are the study of inclusion of special additives, in order to modify the internal structure of ceramics piece and the influence of baking parameters.