I was gluing together a 20 x 20 cm ceramic tile (1 cm thick) that had been broken in two for over 3 years. 3/4 of the tile was still cemented and grouted solidly in place, whereas the 1/4 of the tile that had broken off was loose, held in place only by grout and a silicone bead. The broken segment had detached cleanly, so I expected the pieces would align together without any notable gap.
On gluing the tile together using epoxy, I managed to get a perfect flush surface on the exposed side, and the crack aligned as perfectly as possible with the smallest visible gap.
I noted that there was a 0.2 - 0.5 mm step at the edges of the tile where the previously broken sections met. Small, and an aesthetic issue at most, but palpable with a fingernail. Adjusting the tile so the step disappeared seemed to make the crack more visible on the exposed surface.
I was wondering what the cause of this could be, and what could be done to address it?
Could this be cold flow in the tile, given that only one segment was held in place with cement?