I recently had a slate tile installed in a new addition. It is a textured surface. After grouting, the general contractor (works alone) had an accident and has not been able to work. The grout or cement has settled into the textured areas leaving it unattractive. I have mopped repeatedly and cannot get the tile grooves clean. Individually scraping each tile will take weeks. What can I use to clean the surface and remove the gray areas from the blue tile?