Hello All! I've noticed a damp on my concrete floor coming from under the front door, so had a look and there is a huge gap between the tiles and the threshold as seen in the pics. Obviously I would need to fill it out with something that is waterproof to stop the damp. What would be the best material to use? Silicone? Mortar? Or something else? I'd really appreciate any product or technique recommendations to rectify this. Thank you!