In this study, tabular alumina and seawater magnesia were used to synthesize in situ Al(2)O(3)-MgO spinel castables. Secar (R) 71 cement was used as a binder. In order to produce in situ formation of Al(2)O(3)-MgO spinel castables, the effect of MgO content on refractory properties was investigated by adding to the mixture at the proportions of 3, 4, 5, 6, 7 wt%. Using 5 wt% content of MgO decreased the bulk density of castables to 3.03 g/cm(3) and increased the porosity to about 17.43%. In situ spinel formation in samples led to the expansion in its lattice as well as an increase in the porosity. On the other hand, 5 wt% MgO had a positive effect on Cold Crushing Strength (CCS) of the castable. SEM investigations of fired samples were carried out to compare the effect of MgO addition and in situ phase formation on the physico-mechanical properties of castables. The effect of MgO content on the mechanism of slag penetration of ultra-low cement castables (ULC) for steel ladles was examined and the penetration layer was chemically analyzed.