Mystery bathroom tile corroding - what is it and can it be repolished?

[d2a]

Hi there - I've attached photos of the tiles in question. They are 305x305mm and 10mm thick, light grey and have a smooth matt finish. They were in my bathroom when I moved in and I have no idea what they are or how to clean them. One photo shows the corrosion - the smooth surface has been eaten away and the surface below is pitted, soft and almost pumice-like. My guess is that at some time a limescale remover or similar product has been used on the bath or tile and has corroded the seal and surface of the tile. I'd like to repair or replace it so any clues as to what they are would be a great help.

Of course Sod's Law says it's not available any more - bathroom was done up in 2007 afaik...

Thanks

 

[d2a]

Re: Mystery bathroom tile corroding - what is it and can it be repolis

Hopefully the tiles are still available somewhere.