Those particles could be the result of your box/bag/bucket of salt having been exposed to humidity, any time salt mixes are allowed to get moist, (or sit open) the concentrated "mix" will allow reactions to occur that will basicaly form calcium carbonate and a few other compounds. Which is why it is important to not add water to the salt mix, but to add the salt to the amount of water you are preparing to avoid any unwanted reactions. The idea is to avoid a salt paste or slurry as you are mixing it.
Chuck
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