I was unaware that cat litter could be used as a calcium source. Over at pinguicula.org they list "marly calcareous detritus" as a soil ingredient for many species but never specify what they use to fill that requirement.
I was going to try kitty litter once upon a time, but the only non-scented, natural stuff I could find was made of a soft clay. After a soak in a dish overnight, it turned to mud by just squeezing it between your fingers. Good test to do with any litter brand if you're unsure whether it's suitable as a soil component or not.