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Weirdly enough, almost all calcium compounds are more soluble in cold water than hot. That's why you get lime deposits on the hot water faucets if you live in an area with substantial calcium hardness in your water.
But on the subject of kalk - Jonathon has it correct. Calcium hydroxide (kalk) will spontaneously react with the carbon dioxide in air to form insoluble calcium carbonate, so stored kalk powder that isn't in a perfectly sealed container will gradually convert to a mixture of calcium hydroxide and calcium carbonate.
The same thing happens in a kalkwasser solution - carbon dioxide continuously dissolves in the solution and precipitates as calcium carbonate. That's why it's a good idea to occasionally dump the entire contents of a kalkwasser reactor and restart it with fresh calcium hydroxide/RODI instead of just continuously adding more calcium hydroxide as a top-off.
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