colloidal silica and hydratable alumina can be pointed out as important substitutes to the calcium aluminate cements as calcium-free binders for refractory castables. based on the advantages and drawbacks of these binders, it is reasonable to consider that their combination could maximize the advantages and reduce the negative aspects observed when they are individually used. the high permeability level generated by the colloidal silica would reduce the risks of explosive spalling of hydratable alumina containing castables during first heat-up. additionally, the total porosity of the colloidal silica containing systems could be decreased due to the hydration of the ha. in the present work, colloidal silica and hydratable alumina were combined in a new calcium-free binding system. their rheological behavior, permeability level, apparent porosity, mechanical strength and drying rate profile were compared with the performance of each binder individually.