Y2O3 and CaO are ideal ceramic materials for precision casting of TiAl alloy, overcoming challenges associated with intricate component removal, such as turbine blades, leveraging the stability of Y2O3 and the water solubility of CaO. To address these challenges, we employed Isobam104 in a novel gel casting method, serving as both a dispersant and a gelling agent. This method enabled us to create a stable Y2O3–CaCO3 slurry with high solid content (up to 55 vol%) and low viscosity (101.51 mPa·s at a shear rate of 10 s−1). Following sintering at 1500 °C for 2 h, Y2O3–CaO composite ceramics were produced. Surprisingly, two distinct characteristics of CaO were noted. One showed a rod-shaped morphology with high chemical reactivity crucial for water solubility, while the other displayed a flat-like feature, forming a well-maintained interface due to its coherent or semi-coherent matching relationship with Y2O3, thereby providing excellent hydration resistance. Ultimately, we acquired composite ceramics showcasing a 30-day “shelf life" alongside exceptional water solubility. This establishes the theoretical groundwork for storing and employing soluble ceramic cores in the precision casting of TiAl alloy.