Filling g an exterior to Interior hole

I would either make hole wider to in corporate brick or block keeping with original or if stone work keeping with original and using same cement as original so if sand lime ie older buildings keep with original and keep the exterior always looking the same so wouldn't stand out

I would use acrylic blocks giving light to the environment and innovation

Filling a large hole from an old air outlet, especially one that transitions from an exterior to an interior wall, requires a material that is suitable for both indoor and outdoor environments. Using concrete mixed with small stones might not be the best choice in this situation due to the potential issues it could present, such as cracking, difficulty in achieving a smooth finish, and compatibility with interior wall finishes. Here's a more suitable approach to fill the hole effectively: 1. **Patch Material Selection**: Opt for a high-quality patching compound designed for both interior and exterior use. Look for products labeled as suitable for repairing holes in masonry, concrete, plaster, and drywall. This ensures that the material will adhere well and won't cause compatibility issues with different wall types. 2. **Prepare the Hole**: Clean the hole thoroughly to remove any debris, dust, or loose particles. Ensure the area is dry and free from any contaminants that may affect adhesion. Use a wire brush, vacuum, or compressed air to clean out the hole. 3. **Fill the Hole**: Follow the manufacturer's instructions to mix the patching compound. Fill the hole completely with the compound, slightly overfilling it to allow for shrinkage as it dries. Use a putty knife or trowel to smooth the surface and level it with the surrounding wall. 4. **Allow for Drying and Curing**: Give the patching compound adequate time to dry and cure. Follow the recommended drying time provided by the product manufacturer before sanding or painting over the patched area. 5. **Finishing Touches**: Once the compound is fully cured, sand the patched area lightly to smooth out any unevenness. You can then proceed to paint or finish the area to match the surrounding wall for a seamless look. 6. **Inspect and Touch-Up**: After finishing, inspect the patched area to ensure it blends well with the rest of the wall. If needed, apply additional coats of paint or texture to achieve a uniform appearance. By using a suitable patching compound designed for indoor and outdoor applications, you can effectively fill the hole left by the old air outlet and ensure a durable, long-lasting repair that seamlessly integrates with your wall surfaces.