I have a metal flange on a pvc pipe. The bottom of the flange is approximately at the level of top of the tile floor. However for about 140 degrees of the flange, the tile is cut at the outside edge of the flange, and there is no concrete immediately surrounding the pvc pipe, for as far down as can be seen. There is about a 1-1.5" concrete gap around the pvc for that 140 degrees. The pvc pipe doesn't move, but at the very least it would be nice to have the pipe surrounded by something underneath the flange to prevent the flange bolts from falling in. My only access to this gap is through the flange bolt openings. What material should I use to fill this gap? Thanks for any suggestions.