Magnesium aluminate spinel (MgAl2O4) is an excellent castable refractory product due to its high temperature thermal, chemical and mechanical properties. Alumina spinel castables are produced by addition of synthetic spinel or in situ spinel formation during the firing process. In the first part of the experimental studies, alumina rich MgAl2O4 spinel castable was produced using a solid state reaction technique. Tabular alumina and sea water magnesia (<100 mu m) were used as starting raw materials. In the second part of the experimental studies, commercial synthetic spinel added castables were produced. In order to compare experimental results, both parts of the experimental study involved compositions with the same proportions of MgO. alpha-500 hydratable alumina was used as binder. Castables were sintered at 1500 and 1600 degrees C. Water absorption, apparent porosity, bulk density and cold crushing strength values were considered and the optimum sintering temperature, proportions of synthetic spinel and sea water magnesia were determined. The XRD patterns confirm the phase formation of MgAl2O4. Moreover, the physical properties of the castables were supported by this XRD analysis. Scanning electron microscopy investigations of the fired samples were carried out to compare the effect of synthetic spinel addition and in situ phase formation on the physical properties of the castables. The mechanism of slag penetration to two types of zero cement castables for steel ladles was examined and the penetration layer chemically analysed by energy dispersive X-ray analysis studies.