My guess would be, it would be just fine. Because your 'tufa/concrete sphere is not air tight! Unless you really used some admix to seal it, but even then, concrete breaths some. However, an air escape hole would not hurt a thing. I put 'tufa over a gourd, and the gourd absorbed the water from the mix and expanded and cracked the piece while it was still damp. I should have soaked the gourd first, until it was fully expanded from water before I covered it with 'tufa. Lessons we learn.