X-ray computed tomography (XRT) has progressively established itself as a non-destructive and thus reproducible three-dimensional (3D) investigation technique, enabling material- and geometry-independent applications. In this study, XRT provides a better understanding of thermally induced microstructural changes in an andalusite-based refractory, hidden by the limited two-dimensionality of conventional optical investigation techniques. The XRT technique in this study provided a new perspective on the sintering process of andalusite refractories by subjecting an andalusite based sample to an XRT scan after undergoing temperature treatments of T = 110 °C, 800 °C, 1000 °C, 1200 °C and 1400 °C. The XRT investigation focused on the thermally induced defect and crack evolution of the castable as a function of temperature. This includes, in addition to general sintering phenomena, the formation of a capillary network filled with silica-rich glass phases (SiO₂) due to a temperature dependent mullitization process within the andalusite. The results of the XRT analysis indicate the presence of glass bridges within these structures. XRT constitutes a key technique that can be used in tandem with conventional methods, such as scanning electron microscopy (SEM/EDX/EBSD) and X-ray diffraction analysis (XRD), to facilitate comprehensive analysis.