kalk is essentially calcium hydroxide that is used to prepared kalkwasser solution that wil help to maintain calcium concentration and alkalinity or carbonate hardness(kH).
Other Calcium additive might be a mixture of calcium sulphate, chloride and others .And it's main purpose is to raise calcium level in ur tank.
a few advantage of utilising kalk solution is it help to raise ur pH if u drip at night or early in the morning because of the hydroxide ions. At high alkalinity, kalk can precipitate out phosphate which will lower ur nutrient level.
However kalk is a very tricky product and most commercial product will have instruction stated. Follow the instruction STRICTLY. misuse of kalk infact lower ur calcium and alkalinity as carbonate and calcium is being precipitate out.
others calcium additive have higher ease of using.
long term of kalk usage is very economical and effective to maintain calcium and alkalinity but super troublesome.