Previous work by the authors indicated that the addition of nanomagnesia or colloidal alumina to alumina-magnesia (AM) refractory castables resulted in benefits in terms of reducing the overall linear expansion during heat-up. Considering this aspect, this paper addresses the effects of colloidal alumina sources containing high solid amounts and of an engineered nano-AM powder mixture on the phase formation and microstructure (quantitative X-ray diffraction (XRD) and SEM) of castables containing these nanomaterials. Although the solid content increase of the colloidal alumina was observed, no further benefit was detected in terms of reducing the castable's expansion. The XRD analyses pointed out the major role of magnesia on the spinel formation mechanism, leading to a faster reaction and lower expansion level. In addition, the production of the nanoscaled AM mixture attained the expectation, as it resulted in the lowest overall expansion level among all compositions and speeded up the spinel generation, emerging as a suitable raw material for high-performance AM refractory castables.