This study concerns the chemical reactions involved and the phases formed during penetration of slags of variable composition into porous plaster structure of tundish. Tundish plaster is mainly composed of MgO with minor amounts of SiO2 and impurities, with a grain size of less than 1 mm, inorganic or organic fibers in order to decrease density and provide porosity for insulation, plasticizers and stiffening agents and some other additions. Tundish slag analysis for different grades of steel (7176D and 1191D, according to DIN standard) at different sequences, indicated a very variable composition of CaO (11-42%), SiO2 (28-46%), Al2O3 (6-12%), MgO (11-20%), MnO (0-13%) and some minor variations of Fe, FeO and TiO2. Experimental work indicated that slag when penetrated into pores of plaster, develop the phases of Monticellite (CaO center dot MgO center dot SiO2) and Merwinite (CaO1.5 center dot MgO0.5 center dot SiO2) around MgO particles and decrease the liquidus temperature from 2,800 degrees C to about 1,500 degrees C and provide dissolution of MgO grains in steel making process. Calculation based on two kinetic equations developed for diffusion controlled dissolution, indicated that the dissolution of MgO in tundish plaster is not a diffusion controlled process and is affected by turbulent flow parameters. Phase diagram of CaO-SiO2-MgO indicates that decreasing SiO2 to below 20% and increasing CaO content to as high as possible, increases the liquidus temperature to above 2,000 degrees C. Sources of SiO2 in the process are the rice husk addition, which is used as an insulating material on top of the melt, and the slag flux addition. These sources should be reduced to as low a level as possible. This fact does not affect the fluidity of slag which is required for inclusion removal. Fluidity of slag comes from low melting point eutectics in CaO-Fe2O3 and CaO-FeO (about 1,200 degrees C) due to iron oxide on top of the steel melt.