As a base for the stove, I plan to lay old marble slabs as a mosaic, not square pieces but of "naturally" uneven and varying size and shape, all being 20 mm thick. Perhaps an average of 10x10. Therefore, the joints will be quite wide and varied.
What would you use for fixing and/or grouting? It will be laid on a concrete slab.
Throwing in an extra question; what would you use to plaster on a new 50mm silicate board to make it resemble the adjacent whitewashed chimney?
Thanks for the input!