Here is a very good article on bladder stones and sludge…worth reading if you have not:
http://rabbit.org/bladder-stones-and-bladder-sludge-in-rabbits/
This is in particular in the article talks about diet:
Diet high in digestible calcium?For many years, veterinarians have believed that dietary calcium content was important in the development of bladder stones and sludge. For this reason, and because so much about this condition is not known, we have emphasized dietary changes directed toward reducing calcium intake for affected rabbits. Based on what we now know, it can be stated that dietary changes alone will NOT reduce the formation of stones or sludge in rabbits prone to this condition. Several scientific studies in which rabbits were fed huge amounts of digestible calcium for months (far more than a pet rabbit would ever get even on a completely commercial diet) have demonstrated that these rabbits failed to develop stones or sludge. So I don’t believe that diet ALONE is ever the entire problem.