Doesn't matter what kind of sand, sand is sand and it can lead to impaction (amongst other problems). Calci-sand is particularily bad for them because it has a deyhdrating effect for some reason
Your best bet is to go with something like shredded aspen, beta chip, and though I haven't heard of anyone using it with sandboas before, Carefresh would probably work as well. Beta chip is best for feeding on since it is in chip form, it doesn't stick to the prey as much as shredded aspen does. Hope this helped