Try using a grout haze remover which is available at tile stores.
Clean grout from slate floor tiles.
Cleaning the slate is easy but now you have to clean that grout.
Wring thoroughly as too much water can damage.
It s not always effective on slate however.
If you fail to thoroughly clean the slate tile surface after grouting the grout can.
Spray the solution on the grout and let it soak for about ten minutes.
Remove grout haze with water and cheesecloth.
Protect your hands with rubber gloves and dip your cheesecloth or terry cloth in water.
Mix equal parts water and hydrogen peroxide in a spray bottle and spray onto any stains.
Don t worry you can bring back the brightness of the grout by using a 50 50 solution of water and hydrogen peroxide.
Grout cleaning is the toughest part of cleaning any tile floor whether you have mexican tile slate ceramic travertine granite porcelain or terrazzo getting that grout clean this is because the dirt in there is usually untouched by mops and household scrubber vacuums.
Porous tiles especially a stone such as slate need up to two or three applications of a special grout release.
Your grout is probably discolored by a gradual buildup of dirt and grime.
Grout is worked into the space with a damp sponge and the remainder of the grout is cleaned from the surface.