I bought some Dead Sea bath salts* a few years ago. They were chunky salt crystals, contained in an air-sealed plastic bag within a card box.
I opened the bag and used some of the bath salts shortly after buying them and then, for a few years, the box was stored somewhere dry.
The salt crystals in the box have, during this time, turned into a liquid. What happened there?
Hygroscopy and deliquescence are the concepts that you’re looking for. Some salts can absorb so much water from the environment that they dissolve in it.
Either that or your storage environment exceeds 800°C and the salts melted.
But I’m going to go with Reimann’s explanation as being more likely.
He doesn’t give his location, but if he lives on the surface of Mercury or in Phoenix this could be possible.
It does sound like deliquescence. More from your link:
Does this have anything to do with salt being used to clear snow/ice?
The liquid bath salt has leaked out of the box and it looks like it may have stained the floor, which is a sort of smooth polished concrete. What might the liquid solution be? Any ideas for trying to remove any staining?
If it’s just a stain of the bath salts on the floor, then all you need to clean it up is water.
But you’ve probably already tried that. If that didn’t work, then what you have isn’t a stain, but instead that the leak has corroded away the surface of the concrete. In that case, all you can do is get used to it or cover it over with something else.
The use of salts to clear ice is because salt solutions have a lower freezing temperature than pure water.
If you go deep enough perhaps both properties derive from certain more fundamental properties of ionic compounds, but the ice thing is not directly related to salts absorbing water from humid air (hygroscopy).
I have no expertise, but googling “remove salt stain from concrete” I found…
https://shotblastinc.com/remove-salt-stains-concrete/
…which suggests the use of a stiff brush to scrub, that’s for the penetration, and dilute acidic solution and/or detergent.
And Amazon has a product called “Salt-Away”. Which I’m guessing could be dilute acid with detergent, but who knows.
ETA: if you find something that works better, I think we should market it as “I Can’t Believe There’s No More Salt On My Concrete!”, that should outsell “Salt-Away” any day of the week.
I need to revise this and the edit window is probably too short.
Re: deliquescent substances, it looks like mag chloride might be the culprit. It’s hard to sift through the woo, but looks like that’s a major component of the salt. I’m seeing more analyses of the water, but one cannot assume the relatively concentrations will be the same for both.
I’ll first try soaking the area in some warm water for a while and see if it reabsorbs anything, and then try a salt stain remover spray.
It’s not too bad. If it does leave a mark, at least it may be a holy one, given the salt’s provenance!
Try diluted vinegar (1 part table vinegar, 9 parts water), that is good for clearing magnesium salts (those tend to precipitate as the carbonate, which is dissolved by the acetic acid in vinegar). Put it on the stain, leave for about 10 minutes, rinse away thorougly. “Thoroughly” means you rinse it about three times as many as you think you need.
Thanks. I’ll give it a try.
I’d call it a hypothesis, Reimann’s hypothesis, but yes, that’s likely what happened. Dead sea salt has an odd composition. I’ve had a big chunk of salt from The Great Salt Lake (Utah) sitting on a shelf for years. It’s stayed pleasingly solid, not a speck of drippage or liquifaction.
Fun fact, this is also how they get the caramel in Caramilk bars, I hear. It goes in as a solid and the sugar over time absorbs enough water through the chocolate to become liquid. (And those cherry things and many other tasty liquidy chocolate-coated treats.)
Poor Bernard. Their spelling is terrible, I try (so hard) to get them to spell you’re name right.
Aw, c’mon! That’s a false canard about Phoenix. Yuma, on the other hand…
When he was trying to sell them some forklift batteries, my Dad was treated to a tour of a candy factory. While on the floor, a rack of chocolate-covered cherries rolled by, making exclaim ala Homer Simpson, “Oooh! Cherries!” Invited to sample one, he did and it was an awful, gritty mess in the middle.
The guy giving the tour explained that the cherries are rolled in a sugar and enzyme mixture, enrobed in chocolate, then wheeled into an aging warehouse for a few weeks while the enzyme does its work liquifying* the sugar. “We don’t have to much trouble with theft until they are leaving the warehouse,” the guide said.
*Auto-correct is insisting on “liquidating.” 
Probably invertase
The starting material is solid under standard conditions. The products are not.
Well that’s not really true – they’re way more soluble. They’re not melted.
I have one of those pink salt tealight candle holders, and sometimes it sweats water and has discolored and partly swelled the wooden mantle under it. Oh well, such is a life with a Himalayan salt candle.