I installed new floor tile on our kitchen floor on Sunday; today is Wednesday and there are fine pieces of sand coming off. Not sure if I did not have enough water in the mix - I thought I did. Is my only option to scrape out the existing grout then try again?
Dan, it's possible your grout hasn't totally cured yet which is unusual, but not completely unheard of as there are many factors that can affect curing time. I suggest giving it a few more days to see if the curing process completes. Sanded grout is normally used when installing floor tile, and until it's sealed sand may continue to come off.
My suggestion would be to go ahead with sealing the grout based on the sealant manufacturer's schedule recommendation. I have a feeling that everything should work out okay with a few more days to cure and then doing the sealing. However, if the grout continues to be a problem even after adding the sealant, then I'm afraid your only choice is to remove it completely and start over.
Removing grout can be a very labor intensive and dirty job, and, depending on the room size, might takes a few days. It's important that you take your time as rushing could cause you to chip the floor tile edges. Using a dremel with a carbide grout bit might be the best method, but make sure you wear eye protection as flying pieces of grout or sand could cause serious damage. All of the old grout should be removed to give your new grout the best chance of adhering properly and adding to the structural integrity of the floor.
If you have to remove the old grout, it might be a good idea to consult with a tile contractor or distributor to find out what might have gone wrong with the first batch. If you mixed it properly, there may have been a problem with the grout itself which might entitle you to some free grout or at least a discount on the new bags.