Ceramic Corner Shelf installation question

sukhenkoi

I am putting wall tiles and need to attach two ceramic corner shelves to Durock Cement Board walls in my bathroom.
What is better to use: the same adhesive I use for tiles or 100% silicone?
One small problem I have is that one of two corners I am planning to use for shelves is not exactly 90 degree, but a little bigger so the shelf is not perfectly fit in the corner.

Thanks a lot.