A combustion front quenching technique is used to investigate the separation of reaction products during a fast self-propagating high-temperature synthesis (SHS) thermite reaction, which is applied to the aluminothermic welding of rails. Quenching of the reaction zone with a melt temperature at approximately 2400 K enables the possibility of analyzing intermediate stages during exothermic SHS type reactions and the prediction of a time duration for a complete separation of the products Fe, Al2O3, and intermetallics. An average separation time is then used to validate a 3D phase field solver phaseFieldFoam and solution parameters through application of the diffusive Cahn-Hilliard equation, also known as model B.