This is completely normal. Tortoises kidneys are built differently. They lack what is called the "loop of Henle". The means they do not concentrate uric acid into urine as we do. Urine is too water soluble and since it dissolves in water so easily, it requires you pass most of the liquid in the bladder to get rid of it. Uric acid is not very soluble in water and can precipitate out and create a semi-solid mass of urates. This can be passed without the need to void much water. So it is an ingenious design tortoises have that allow them to store water for long periods of time.
When water is available, they will normally go ahead a pass a lot of the water in their system along with urates and immediately drink and replenish new, fresh stores of water. Urates can also be passed along with feces, so you can sometimes see white urates in the "poop".
This is how their metabolism works. As mentioned above, if it does get gritty, that is cause for concern. Normally a sign they are holding water too long in their system. Also the amount of urates they pass is obviously tied to the amount of protein in the diet. Uric acid is a byproduct of protein metabolism.