[Fred Hueston]

This is a classic case of efflorescence, the pitting and spalling is the result of soluble salts depositing in the pores of the stone.

There a number of reasons for this as follows:

The salts can come from the setting mortar, the grout joints and/or the water.

At this point there is not a lot that can be done but there is lots that can be done to prevent this from happen if it is replaced.

I write specs for stone and tile all over the US. I would be glad to help if you decieded to replace.