Abstract: Mineral resources, as the foundation of modern industrial society, continuously ensure the supply of raw materials essential for socioeconomic development. Prolonged intensive mining has resulted in the gradual depletion of high-quality shallow resources, thus rendering the development of deep mineral resources an inevitable measure for the sustainable development of the global mining industry and a critical strategy for safeguarding national resource security. Backfilling mining, which combines rock-pressure control, rock-movement suppression, and solid-waste resource utilization, has become the core extraction technique for high-risk deep ore bodies. Engineering practice indicates that backfill slurry typically fails to achieve effective roof contact within a mining area because of objective conditions such as bleeding and settlement, material segregation, and irregularities in the mining roof. The quality of the backfill roof contact is a core technical indicator for evaluating mine backfilling effectiveness, thus directly affecting the stability of deep-mining operations. To achieve reliable roof contact in deep backfilling, mining scholars have conducted extensive experiments and developed multiple techniques such as multiple backfilling cycles, roof caving, slurry pressurization, and gravity-assisted roof contact. However, these methods—categorized under forced roof closure—involve complex construction processes and yield suboptimal outcomes. Moreover, considering the varying engineering conditions in each mining area, relying solely on experience-based forced closure techniques typically results in significant uncertainties. In recent years, expansion filling has emerged as a novel roof-closure method in the mining industry. Its advantages, including high water-retention capacity, rapid setting, and active roof closure, have expanded its adoption in production practices. The author believes that suitable materials for expansive fill bodies should exhibit rapid expansion, high expansion ratios, and high early strength. Gas-phase expansive agents can only afford significant volumetric expansion, which significantly affects the strength of the fill material. Moreover, the rapid reaction of gas-phase expansive agents is detrimental to the pipeline transportation of the slurry. Therefore, the author proposes a novel “active roof contact” filling concept. By introducing solid-phase expansion components, the filling material achieves controlled expansion through hydration reactions, thus enabling the active filling of uncontacted roof areas. More importantly, the sustained pressure generated during expansion exerts a counteracting force on the rock-filling interface, thereby forming a beneficial “prestress field.” This effectively mitigates the secondary stress concentration in the surrounding rock, thus ultimately enhancing the synergistic load-bearing capacity of the backfill-rock system under subsequent mining disturbances. This study focuses on fully tailing-based waste-rock cemented backfill by analyzing their strength, flowability, and expansion capacity under different expansive agent types and dosages. The optimal expansive agent and dosage suitable for mine backfill are identified using the grey target decision-making method. The results indicate that all expansive agents degrade the mechanical properties of the filling material to some extent. The effect of expansive-agent type on slurry flowability is ordered as follows: CaO-based > ettringite-based > MgO-based. At the same dosage, samples with CaO-based expansive agents exhibit higher vertical expansion rates and expansion stresses. Compared with MgO- and calcium aluminate-based expansive agents, their expansion rate within 14 d is 49.9 and 1.27 times higher, respectively (using 10% dosage as an example). Considering downward-filling requirements, the grey target decision optimization yields 10% calcium aluminate-based expansive agent as the optimal solution. This formulation affords a 14-d vertical expansion rate of 5.88% and an expansion stress of 0.81 MPa.