Abstract: Cut-and-fill mining is the core mining method for deep, high-risk ore bodies, but filling slurries often fail to achieve effective roof contact due to issues such as bleeding and settlement. Existing forced roof contact technologies usually have problems like complex processes and poor performance. To address this, the authors propose a novel "active roof-contact" backfill concept. By introducing solid-phase expansive components, controlled expansion of the filling material is induced through hydration reactions, enabling active compensation for unroofed areas. This study focuses on full-tailings and waste rock cemented filling bodies as the main research object. It analyzes the strength, fluidity, expansibility, and hydration behavior under different types and dosages of expansive agents, and optimizes the suitable expansive agent and its optimal dosage for mine filling based on the grey target decision-making method. The results show that all types of expansive agents cause certain deterioration to the mechanical properties of the filling material. The order of influence of expansive agent types on slurry fluidity is: CaO-based > ettringite-based > MgO-based. At the same dosage, samples with a CaO-based expansive agent exhibit a higher vertical expansion rate and expansion stress. Taking a 10% dosage as an example, its 14-day expansion rate is 49.9 times and 1.27 times that of MgO-based and ettringite-based expansive agents, respectively. Considering the requirements of downward filling, the optimal scheme obtained through grey target decision-making optimization is ettringite-based expansive agent with a 10% dosage, which achieves a 14-day vertical expansion rate of 5.88% and an expansion stress of 0.81 MPa.