This paper deals with a series of experiments dedicated to the analysis of the time evolution of a mushy zone in a fixed temperature gradient, carried out on the BM05 beamline at the European Synchrotron Radiation Facility (ESRF) on Al-Cu alloy. Because most of phenomena involved in this evolution are dynamic, in situ and real time investigation is essential for conducting a thorough analysis as a function of time. Synchrotron X-ray radiography is a non-invasive visualization technique, perfectly suited to such a study since it is able to reveal the microstructural changes of the mushy zone during the holding stage, In addition, we extended our analysis by performing advanced image processing of synchrotron X-ray radiographs to characterize the solute distribution in the liquid phase. These measurements gave us crucial information for understanding the competition between the diffusion processes in the bulk and the mushy zone. Moreover, combining these data with mass balance equations at the two boundaries of the mushy zone enables us to demonstrate the major role of solute diffusion in the dynamics of the mushy zone, from the early instants to the final state of the holding stage.