A lot of people say that they're the same as the "calcium blocks" they sell at pet stores to add to a turtle tank... if those blocks are actually effective, in a turtle tank you'd need a HEAP of them; turtle tanks are big.
The plaster of paris; ive heard it before, would never trust it. wouldn't advise it; just feed your turts with a high calcium diet; lots of freshwater animals such as shrimp and fish, keep the diet as natural as possible. shortnecks also love freshwater aquatic plants when they get a bit older. UVB is also essential.