Ductile cast iron, also known as nodular cast iron, is a type of graphite-rich cast iron with high impact and fatigue resistance, due to its nodular graphite inclusions. Ductile cast iron is produced by incorporating additives (mainly FeSi alloys) to the iron base metal at different production steps to obtain the desired graphite shape. A crucial step is the addition of Magnesium to promote the spheroidization of the graphite. The most common method is by adding by adding crushed and sized Ferro-Silicon-Magnesium (FSM). The alloy composition, microstructure, and sizing are assumed to affect the key parameters of this reaction, namely, reactivity, recovery, and slag formation. Therefore, the study of the solidification of FSM is important to understand and predict its performance in the foundries. The present work aims at understanding and predicting numerically the formation of the major phases during the solidification process. Two approaches have been used: phase field modelling using MICRESS and thermodynamic calculations through Thermo-Calc solver. The models have been calibrated by comparison with advanced statistical characterization of the microstructure. The results indicate a competitive growth of the major phases and transformation of phases in solid state that can be emulated by the model.