As I know, people usually dont change the litter to an especially litter when the cat returns home after spaying. They perhaps change the old semi-used litter into a completely fresh one.
Cats arent so sensitive to bacteria as we humans are...
Thus. Use the litter shehe is used to.
If shehe dont know usage of kitty litter, use dirt. Or if you can, use both...Ie two different litters, one with dirt, one with cat litter. So shehe can choose.
If you have a big litter-box, you can perhaps have half of each? Two halves.
The main point here is not the clinical cleaniness, but so he do dare to make his needs at all... Unnecessary shehe holds on only because of uncertainity.
ps. If hygiene of the dirt bothers you (after all, it DOES contain myriads of microorganisms!), you can always heat the dirt batch in the oven... That should kill them enough for everyday needs.