How to Repair Non-Sanded Grout That Has Shrunk on Marble Mosaic

Scrape the old grout out of the joint using a grout removal tool, which resembles a screwdriver with a flat triangular tip. Avoid scraping the tile surface with the tool because this can cause permanent damage to the marble's finish.

Vacuum over the surface of the tile to remove any grout dust left behind from removal. Wash the marble surface with warm water and wipe dry.

Mix powdered unsanded grout with water, using the water-to-grout ratio recommended on the package. The prepared grout has the texture of a dough when properly mixed.

Smooth the grout over the tile joints with a grout float. Hold the float at a 45-degree angle to the tile surface so the edge doesn't catch in the tile joints. Use the float to push the grout into the joints.

Turn the float so it sits perpendicular to the tile. Scrape it gently over the tile surface to remove the majority of the excess grout. Allow the grout to set for 20 minutes.

Moisten a large, clean sponge in a bucket of cool water. Wipe the excess grout from the tile surface, rinsing the sponge out regularly. If the grout smears from the joint lines, allow it to set for an additional five minutes then resume cleanup.

Mist the grout with water twice daily for three days following the repair. Grout requires moisture as it cures to set fully.