I have done this before and it definitely does raise the pH of the water after a while.
It shouldn't be too much of a problem if your water is acidic though (below 7.0).
People actually recommended buying coral chips or putting shells in tanks as a solution to very acidic water.
If you do put the shell in your tank you will have to do regular pH tests to make sure the pH doesn't get too high
Make sure you rinse the shell well before you put it in and sterelise it with boiling water.
But if your water parameters are fine atm then I probably wouldn't put the shell in the tank as it could change them and cause a lot of problems.