Usually grout crumbles because either the underlayment is incorrect, the wrong grout was used or the grout wasn't prepared properly.
When the underlayment is incorrect, that means the structure the tiles rest on is flexing. Sometimes only the grout will crack, and sometimes the tile too. If this is the case, there is no fixing it other than to redo the whole thing.
If its the wrong grout or if the grout wasn't properly prepared....that's an easy fix with the drill brush mentioned 3 posts above. You have to remove a lot of the grout, not just the offending pieces, to get a good bond. The grout should be the 'sanded' type for strength.
As far as cleaning goes, you can do it yourself inexpensively. This is from the 'Bob Vila' website:
For Stain Removal:– Mix equal parts water and hydrogen peroxide in a spray bottle and spray onto any stains. Let it sit for five or ten minutes, then scrub with a soft scrub pad (not steel wool) or a soft-bristled brush. (Be careful if you have a slate floor with colored grout, as this bleach solution could damage the color of that grout.)
– For tougher stains, mix together peroxide and baking soda into a bubbly paste. After the bubbles die down, apply the paste to the stain and let it set. Then wipe the slate clean with a damp, warm towel.
After cleaning, they recommended polishing with teak oil (for teak furniture)
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the windows and cabinets
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