I have been asked to polish a newly installed marble floor and the grout is cracking. What causes this and can it be repaired?

The are several reasons why the grout may have cracked. The installer may have used too much water when he mixed the grout causing it to crack or there may be movement in the floor causing the cracking. To repair the grout you can overlay a thin coat of new grout over the existing cracks or if its severe you may want to remove and replace the grout.

TIP To remove the gout use a small grout saw. It will not be necessary to remove all the grout. You will need about 1/4 inch to overlay additional grout. Use only non-sanded grout.