Can stains from a mix of Portland Cement, Morter mix, and Limestone dust be removed from firerglass roofing shingles. Stain came from masons installing stone on fire place.

Masonary stains on shingles in my experience are difficult if not impossible to remove especially if the stain is large, however you may be able to mask them if the stains are small or spotty. I have used a tube of black roofing sealer and spread it with my finger into small stained areas and retrieved a handfull of shingle aggregate from a gutter and worked it into the wet sealer. You could try this in a small spot or inconspicuous place and see what results you get. This would be last result...talk to the mason you hired and ask him how THEY are going to get the stains safely off your roof!