If the stains are etched in then they won't come out, but you can also get a product called MDR (Mineral Deposit Remover)
I imagine it is a similar thing to the Brick acid used on building sites, it is certainly acid based and will disolve cement.
If it's as aggressive as brick acid (wish I hadn't read that post calling it that, for the life of me I can't remember what the bloody stuff is called now! you will need to keep covered up and wear glasses...it burns!!
Regards,
Ian