The Concerted European Action on Sustainable Application of REFractories (CESAREF) is a consortium created to drive sustainable refractory materials and processes in steel production. This project, which runs from 2022-2025, seeks to improve the microstructure for increased sustainability and thermo-mechanical performances of refractory castables. In this work, different formulations of alumina-spinel refractory castables are considered. The main objective is to propose a new design for the microstructure of refractory materials with improved thermo-mechanical properties by considering: •The nature of aggregates (chemistry, crystallinity, physical properties…) •The arrangement of the calcium aluminate phases network(formation temperatures,unique formation mechanisms,location and morphology).