In high temperature processes, such as the steel or cement industry, refractory materials are required to withstand chemical, thermal and mechanical loads. A rapid change in temperature, a so-called thermal shock, often leads to tensile stresses that cause crack initiation and / or further crack propagation. Consequently the requirements of the used materials with respect to temperature resistance, resistance against crack initiation and propagation have to be fulfilled in the best possible way. These properties can be described by figures of merit that enable a comparison between various refractory ceramics. An important representative is the brittleness number B and the characteristic length lch of the material. These figures of merit are dependent on material’s strength, Young’s Modulus and specific fracture energy. In the present work, the brittleness reducing property of micro-cracks in a magnesia spinel structure is investigated. For this purpose, calculations are performed to simulate the formation of these cracks during the production process. The finite element model describes the crack initiation in a microstructure due to unequal coefficient of thermal expansion between magnesia and spinel. The resulting micro-crack network causes a reduction in strength, an increase in specific fracture energy and thereby a reduced brittleness. Compared with the starting materials properties, the resistance against crack growth and the thermal shock resistance are increased. Another part focuses on the correct representation of the material microstructure by a representative volume element in the FE analysis. Several aspects and their influence, among others the spinel content of the model, are investigated. Also the influence of different thermal expansion coefficients on the resulting mechanical properties is examined. By varying the contact properties between the two materials and by the addition of a third material, the importance of the contact can be shown.