I just installed new grout in a granite floor and after 24 hours it is all turning powdery. What can I do to fix it?

The are many reasons why the grout becomes powdery. If the tile is very absorbent it will soak up most of the moisture from the grout preventing it from curing properly. Too much water added to the grout mix can also make the grout powdery. To repair the grout you should replace it with new grout. This time wet the floor before installing, this will prevent the grout from drying too fast. Also I would use a latex additive in the grout mix.

TIP If this problem continues you may want to use an epoxy grout.