The confinement of anisotropic particles can lead to entropy-driven structural phases not observed in bulk systems. Using a density functional theory framework, we explore the rich phase diagram of hard cylindrical disks confined within a slit pore. Our investigation reveals complex structural phenomena driven by the interplay of particle geometry and density. A key finding is a concentration-driven reentrant anchoring transition for thin disks ($0.3\le \frac{L}{D}<0.5$), where the system switches from a homeotropic (face-on) to a planar (edge-on) alignment and back to a homeotropic alignment with increasing packing fraction. Furthermore, at high densities and sufficient pore widths, we identify a stable hybrid T-type structure, where layers with orthogonal particle orientations coexist. The phase diagrams are also characterized by sequences of first-order layering transitions and the formation of biaxial planar phases for thicker disks. Our results demonstrate that geometric constraints in athermal colloidal systems produce a diverse range of structural phases.

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• Applications of soft matter
• Confinement
• Liquid crystal phase transitions