With fine powders, and quite likely absorbent granules like kitty litter, you may need to add a surfacant to allow the water/acrylic emulsion to mix without being repelled. Also, in the case of absorbent clays, like kitty litter, it may be beneficial to use a more viscious medium, like heavy gel or modeling paste, with a generous amount of extra water. The added water will be soaked up by the granules anyway, dehydrating as the paint drys normally, and the heavy gel will tend to stay between the grains rather than flow out the bottom.
Don't take my word for this, though. I haven't yet tried this myself, but I'll report back when I do.