The main purpose of this work is to evaluate the thermal stability performance of different aggregates such as; fireclay, bauxite, granite, calcium aluminate clinker, silica sand, furnace slag with calcium aluminate cement(CAC) containing 35%, 40% and 50% alumina under high temperatures. 20 cubic samples are prepared to observe every aggregate with every CAC individually. In fresh concrete, water cement ratio gradient develops around aggregate particles during casting, resulting in an altered microstructure of the surrounding hydrated cement paste. This zone around the aggregate is called interfacial transition zone (ITZ). The quality of ITZ defines the thermal stability performance of the sample. A traditional CAC based castable form ceramic bond between 900-1200°C and then becomes stronger in terms of bond strength. This work is supported with the SEM images of ITZs. Eventually, the bonds are broken and melting is observed at different temperatures for every sample. After 1250°C, the melting point is differentiated according to alumina content and aggregate type. All in all; performance of fireclay samples are the most durable ones with a melting point above 1550°C while the CAC aggregate show the best performance by filling the pores since the partial reaction at the surface of this aggregate also improves the bond between aggregate and cement.