In this paper, the Scheil-Gulliver approach on solidification modeling is applied to slag solidification. The Scheil-Gulliver model assumes no diffusion in the solid phases, infinitely rapid diffusion in the liquid phase, and local equilibrium at the solid/liquid interface. For two distinct CaO-MgO-SiO2 slags, solidification is simulated with the Scheil-Gulliver model and with the commonly used thermodynamic equilibrium model. The simulations show that in the Scheil-Gulliver model, as opposed to the equilibrium model, compositional gradients in the solid phases can exist and peritectic reactions do not occur. Solidification experiments are performed under laboratory conditions to validate the simulation results. The experimental results show a better correspondence with the Scheil-Gulliver model than with the equilibrium model. However, quantitative differences in mineralogy persist.