Calcium chloride. Same stuff that gets dumped on the roads in winter in some places. Salt = general term for ionic compounds especially stable ones.

And yes some chlorine does gas off especially when interacting with other ions. In my current research I generate 0.6 litres of growth media full of yeast per hour for weeks on end and I need to get rid of it, and we aren't allowed to dump living organisms down the drain into the municipal sewer system, so I have to bleach it first. Enough chlorine comes from the reaction of the bleach with the media that I have to bleach my buckets in a chemical fume hood whenever I'm dealing with more than a litre or two. Learned that the hard way when I did it in the sink the first time. Quite a burning sensation deep down into the lungs...