Thermal conductivity of fired clays: Effects of mineralogical and physical properties of the raw materials

The physical, chemical and mineralogical properties of representative commercial Italian clays were investigated by X-ray powder diffraction and Rietveld refinements, laser granulometry, X-ray fluorescence spectroscopy and calcimetry. The clays were used to prepare bricks by both extrusion and uniaxial pressing. The effective thermal conductivity of the fired bricks was determined and correlated with physical and mineralogical properties of the raw materials. Unfortunately, the complex nature of the system with many influencing parameters and interactions did not allow linear correlations with single parameters. Hence, a multiple linear regression approach was attempted and a statistically valid model was built for extruded samples. Although the model cannot be regarded as conclusive, due to the system complexity and the limited number of observations, the results gave some indications regarding the role played by the raw materials properties on the effective thermal conductivity of the bricks. The pore forming effect of organic material decreases the thermal conductivity of the bricks. On the contrary, the thermal conductivity increases with decreasing particle size, possibly due to an increased sintering rate and/or improved particle packing.