<p>The process of Electro-Slag Remelting (ESR) is commonly used for alloyed steels intended for critical applications like marine turbine components, supercritical power plants, and rocket motor casings. Most of the conventional ESR fluxes contain high concentrations of CaF₂, along with Al₂O₃, CaO, SiO₂, MgO, etc., in various proportions. However, the emission of fluoride vapor from the molten slag is known to cause several environmental and health problems, triggering a need to reduce fluoride concentration in ESR fluxes. The productivity and energy efficiency of the ESR process depend on the combination of electrical conductivity, viscosity, and liquidus temperature of the molten slag, which, in turn, is dependent on its ionic structure. The present study investigates the effect of changing SiO₂ concentration on the structural features and melting characteristics of the modified flux. The observations have been compared with the simulations carried out using the FactSage™ (version 8.3) software. An increase in SiO₂ concentration resulted in enhanced polymerization levels in the ESR slag through the formation of complex alumino-silicate linkages in the melt. This manifested in the form of a higher degree of supercooling during solidification, while carrying out thermal analysis of the slag samples. Simulations using FactSage™, too showed an increase in the relative proportion of bridging oxygen along with a reduction in that of non-bridging oxygen. However, the liquidus temperature of all the modified slag samples was observed to remain within a range acceptable for electroslag refining. Combining the observations on liquidus temperature and structural features promises to help in identifying the flux composition window suitable for adoption in the industrial ESR process.</p> Graphical Abstract <p></p>