Don't use this for chalk paint.  You need food grade calcium carbonate.  requires extensive sanding to get a smooth finish.
This is not the grade of calcium carbonate that you want to use for chalk paint.  It is way to gritty and requires quite a bit of sanding that you shouldn't have to do for a nice smooth finish (and you don't have to do with food grade calcium carbonate or boutique brands).  Buy this grade for gardening or other uses.  The only reason I gave it two stars, it does work with sanding.  I'd give it five stars for gardening.
Rating: 2 stars