A multi-scale material model for the estimation of the transversely isotropic thermal conductivity tensor of fired clay bricks

Fired clay bricks are employed in increasingly demanding application domains, such as multi-storey buildings and facades allowing only minimal heat loss. The latter requirement is often met by the use of pore forming agents. Then, pore size and morphology, as well as the thermal conductivity of the solid constituents of fired clay, govern the material's overall thermal conductivity tensor. We here quantify corresponding structure- property relations in the framework of random homogenization theory, introducing ellipsoidal shape and orientation distributions of material phases (including pores) at three different organizational levels. The model is validated by experimental techniques, such as scanning thermal microscopy, micro-computed tomography, and scanning electron microscopy. A final sensitivity analysis reveals that four design parameters heavily contribute to the overall thermal conductivity of fired clay bricks; namely the thermal conductivity of the fired clay matrix as well as the porosity of each of the three organizational levels.