Grout will not stick due to a variety of reasons.
Grouting ceramic tile and natural stone tile installations requires a variety of preparatory steps to ensure the quality and stability of the finished product. If there are any issues during the grouting process, such as when the grout won't stick to the tile or stay in the joints, long-term issues can become a serious hindrance. If your grout will not stick, there are a few things you can look at to fix the problem.
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While pre-sealing natural stone tiles is a good idea if you want to make the grouting process easier due to the fact that natural stone is porous, it can also be detrimental if special care is not taken. Overspray into the grout joints as well as overspill onto the edges of the tile can keep the grout from sticking in the joints as well as against the edges of the tile. If you are pre-sealing your tiles, cover the face of the tiles, not the actual joints.
Dirt and debris is another issue for grouting projects. For this reason, grout the installations as soon as possible after the thinset mortar has had time to cure. The longer you wait, the more time dirt has to build up on the surface of the tile as well as in the joints themselves, creating a layer that will impede the bond of the cement within the grout.
A layer of water will keep the cement in the grout from bonding with the cement of the thinset, in addition to ruining the consistency and strength of the grout. General moisture from humidity is normally not an issue, but any standing water that is allowed to collect in the joints prior to grouting the tile installation will make grouting an impossibility.
While it is a rarity, from time to time you can come across a bad batch of grout. If so, it will lack sufficient cement or polymers that aid in the adhesion to the sides of the tiles as well as the thinset at the bottom of the grout joint. While mixing the entire batch of grout is important, nothing can save you from the reality that human error will eventually play a part in the grout manufacturing process.