Granite is primarily mica, feldspar, and quartz (a few other things, but those 3 are the vast majority of the crystals/minerals present).
mica decomposes into clay first, and then feldspar will also decompose into clay. Quartz will eventually break down into smaller pieces (depending on how big the crystals were to begin with), but won't change into anything else.
depending on how decomposed it is, you probably either have clay mixed with feldspar and quartz, or clay mixed with quartz. No clue whether they do any processing like crushing/grinding, sifting, etc.
I can't see anything harmful, but I don't think there would be all too much benefit either...