I have a slate tile floor inside the front entrance to my house. Some mortar against the doorstep and sidelight has crumbled and is easily removed, but it's such a small amount that I'm hoping I can use one of the products I have on hand to replace it. The area missing the grout is 1/4" w x 1/4" deep (the depth of the slate tiles) x 8' long. I have some basic Quikrete Mortar Mix and also some Special Mix Poly Modified Stone Veneer Mortar. I have some sifted sandbox sand and some sandblaster sand. The subfloor is plywood. The existing mortar is light grey, but it is probably too much to expect new mortar to match and I can live with that. Even though this is at the front door, the entrance is rarely used. Any suggestions about the best way to do this repair would be greatly appreciated.