Use warm water and wipe it over the surface of the tiles with a sponge. Then using some fine wire wool, rub gently over the grout which will begin to dissolve and rub off at the same time. Once the tile is clean and has no more hardened grout, wipe over with a damp cloth and leave to dry.
How is it bad? If it's on the tiles Alice has a good idea, although I would be hesitant to use a wire brush. The wire brush could ruin the surface of ceramic, marble etc if it gets too close. I prefer to use a razor blade and scrape very carefully to remove the grout on the tile surface.
If the bad grout is in the gaps you'll need to get a grout removal knife. The grout lines should have been sealed after it dried. That sealer may make it difficult to get new grout to stick. Carefully scrape out some of the grout to make the gap bigger. Hopefully you have the same grout to replace the old. If not you may want to remove all the grout and replace it. No matter what you decide be sure to dampen the gaps before you put in the new grout. The old grout may absorb some of the moisture and cause the new grout to dry too fast, you want it to cure. Once it has cured (manufacturers recommendations) be sure to seal all the grout lines.
Bad portion of the original grout evidently didn't have enough liquid, consequently it has turned to powder in some places. Sounds like it will need to be cleaned out and new properly mixed grout installed.??? Thanks for your comments.
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Use warm water and wipe it over the surface of the tiles with a sponge. Then using some fine wire wool, rub gently over the grout which will begin to dissolve and rub off at the same time. Once the tile is clean and has no more hardened grout, wipe over with a damp cloth and leave to dry.
How is it bad? If it's on the tiles Alice has a good idea, although I would be hesitant to use a wire brush. The wire brush could ruin the surface of ceramic, marble etc if it gets too close. I prefer to use a razor blade and scrape very carefully to remove the grout on the tile surface.
If the bad grout is in the gaps you'll need to get a grout removal knife. The grout lines should have been sealed after it dried. That sealer may make it difficult to get new grout to stick. Carefully scrape out some of the grout to make the gap bigger. Hopefully you have the same grout to replace the old. If not you may want to remove all the grout and replace it. No matter what you decide be sure to dampen the gaps before you put in the new grout. The old grout may absorb some of the moisture and cause the new grout to dry too fast, you want it to cure. Once it has cured (manufacturers recommendations) be sure to seal all the grout lines.