Clumping clay litter forms into a hard cement-like material when exposed to liquid.--edit--Actually, it is the active compound, sodium bentonite, that causes the super hard clumping, but all clumping clay litters use it. Also, the mining of the clay is a very evironmentally harmful process, and not at all renewable likt the recycled paper, wood or corn litters-- Think about what happens if your kitty eats some, or even just ingests it by grooming. There are reported cases of death due to blockage of intestines, lungs and nasal passages, mostly in kittens.
I use SWheat scoopable, because it is both clumping and flushable without containing clay. I love it! I have two kittens and get no smell, pee or poo, unless they have used the box less than maybe 2 minutes previous. They like it, had no problems adjusting to it, and I think it smells kind of nice, like bread. I scoop once or twice a day. I found that after a month a complete change of litter was needed.