The sludge on the bottom is just precipitated calcium hydroxide (assuming of course that you have reasonably high quality Kalk.) Just add water and stir well. It should disolve in the water to its saturation point (about 2 tbls per gallon.) You can test the Kalk water using a calcium test kit. If you find that your Ph or calcium levels are gettng too high then you will need to restrict how much Kalk you are adding per gallon of top-off and mixing to saturation will cause problems for you.

PS - The Kalk water should not be forming a crust. If you are not already doing so, Kalk should be stored in an reasonably air-tight conatainer. Interaction with air reduces its efficacy considerably over time. (A rubbermaid brute trash can with a lid is pretty much ideal for Kalk water storage.)

PPS - A good source of Kalk in our area is Fiesta Foodmart, where it is sold as Cal Mexicana. Traditional mexican cooking uses calcium hydroxide to help breakdown the maize corn into a digestible flour for making corn tortillas. It's $.99 for a 4 oz bag.