A study is made of the effect of additions of clay and Y 2 O 3 and the temperature difference in the range 1000/20 -1500/20°C on the growth of the crystalline phases in ceramic specimens obtained by the conventional sintering of component mixtures at 1300 and 1500°C. The effects of these additions and the temperature difference on the dimensions of the crystals, elastic modulus, ultimate flexural strength s bnd , and the linear correlation between elastic modulus and s bnd are also examined. The rate of crystallization of the mullite phase and tetragonal Zr 2 O 3 and the associated increase in crystal size are greatest in specimens sintered at 1300°C and subjected to thermal shock with temperature differences of 1400/20 and 1500/20°C. Such specimens have the highest values for elastic modulus and s bnd and are characterized by the existence of a linear correlation between elastic modulus and s bnd .

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New dense and porous ceramics are developed using two formulation approaches. These are ceramics with remarkable content of mullite, corundum and ZrO2 - tetragonal and monoclinic in crystalline phase and porous (average size of pores up to 150-350μm) spinel-enstatite/forsterite ceramics. To promote the phase development by high-temperature synthesis of these ceramics the mineral raw materials – illite clay and illite clay together with dolomite are used. Bulk density for mullite (corundum) – zirconia ceramics is increased by equal addition of illite clay (8.2 wt.%) as well as ZrO2 and especially ZrO2 +Y2O3 to starting mullite mixtures. Contrary the addition of different kind and amount of illite clay together with dolomite trends to formation of spinel – enstatite (forsterite) ceramics and decrease the bulk density to the value up to 0.35 g/cm3. The XRD results indicate that ZrO2 monoclinic/tetragonal incorporation in mullite (corundum) structure realizes through the liquid phase. Development of the spinel MgAl2O4 and enstatite MgSiO3 crystalline phases along with the sufficient amount of glassy phase in microstructure and even distributed pores are proper for spinel-enstatite (forsterite) ceramic samples.