We have been getting the salt at Home Depot in 40 lb bags. It is identified as water softener salt and is labeled as pure salt but apparenty that is not the case! We now know to sweep to dissolve the salt more quickly but don't know how to deal with the stain already there. I found a previous post (through Google) by waterbear identifying the stain as iron. It also talked about adding "sequestrant" but I do not know what "sequestrant" is!