Metal-organic framework (MOF) materials combined with aluminum saline slag wastes represent a relatively novel research direction. This innovative approach not only leverages industrial waste for the development of advanced materials but also promotes a circular economy and reduces the ecological footprint of aluminum production. Furthermore, it addresses the inherent stability challenges of pure MOFs. Notably, this study is the first to propose that this industrial by-product can serve as a novel, cost-effective, and environmentally friendly modulator, enabling the selective transition between the MIL-96(Al) and MIL-110(Al) phases. For the first time, novel composites of MOFs and the saline slag waste resulting from the initial aluminum extraction in an acidic medium (RW1*), have been developed. These composites, designated as MOF@RW1*-n (n = 25, 50, 75), were synthesized via the in-situ preparation of the MOF on RW1* using a hydrothermal treatment method and were subsequently evaluated as CO2 adsorbents. The optimized content of RW1* in the composite significantly affect its role as a modulator in the formation of specific crystalline structures, whether MIL-96(Al) or MIL-110(Al), leading to hybrid materials with excellent textural properties and thermal stability.