Refractory Castables

I have worked and done several projects in this field for one and half years, almost. I am still confusing theory with practice(I mean the experimental results). Thermal shock resistance, for example, is the most important property to the Al2O3-MgO refracoory castable used for a steel-making ladle. There are several approach, including the grain size distribution, the high alumina content cement, MgO powder, type of alumina powder and its different ratio combination, to enhance the thermal shock resistance of Al2O3-MgO refractory castable. I have been tried the different size distribution of MgO powder, different combined ratio of alumina powder, different high alimuna content cement, and different additives. None of them can give a significant and positive effect to increase its thermal shock resistance. Is there any detail that I do not notice and involve? May I use a wrong test to evaluate the thermal shock resistance? I totally have no idea to face the trouble I am suffering from right now. One of the R & D elders in the refractory industry told me, " It will cost someone who has a strong intention in recfractory three or more then three years to have a clear concept to involve." Alas! I am so green that I can't comprehend the key point of refractory castable. The biggest trouble to me is the intention - I only have a weak intention.