QUESTION

We remodelled our kitchen in early December and the tile was replaced along with new cabinets and quartz countertops. There was tile in this space previously. They put the tile over a cement subfloor.

We are having 3 tiles/ 2 areas where the grout is cracking and falling out in certain spaces. It sometimes extends to the tile next to it. This happened in one area after the cabinets went in (installed in late december and then happened in another area once the quartz was installed in mid january)

The contractor initially used a mix of sanded and unsanded grout as per the recommendation of the tile store.
He came back to replace the grout in these problem areas once with unsanded grout and the same thing began to happen after a few days of walking on it. They also used thin grout lines about 1/8th on a large square tile during the installation.

(Maybe relevant: About 3-4 weeks after installation we had some extreme temperature shift. The temperatures went from 29 F to -7.6 F or -2 Celsius to -22 Celsius and some of our pipes froze close to underneath one of the problem areas.)

I am worried this will happen / start spreading all over the kitchen or that the tiles in question will crack.
What will fix the problem? Should we inject glue underneath or use caulking grout? Both?

Thank you for your advice! I am expecting in a few weeks and this has been causing a lot of stress.

ANSWER

ANSWER - Grout can crack for a varied of reasons, and it might be because of a combination of different deficiencies.

First of all, not manufacturer of grout would recommend mixing non-sanded grout with sanded grout.  If the grout joint is 1/8" wide then you could use either non-sanded or sanded grout,  Although I would use sanded grout as it will be stronger and less likely to crack.

Since the cracked grout was replaced and cracked again, it suggests there could be too much deflection in the floor.  Although if the tile was installed over a concrete slab, it shouldn't have deflection.  Or perhaps they installed a mortar bed or a cementitious backer board over a wood subfloor that has too much deflection?

If the grout was mixed with too much water or cleaned with too much water, it could weaken it and make it more susceptible to cracking.  If the installer did not fully fill the grout joints full with grout that could cause grout to crack.   If the tiles adjacent to the cracked grout were loose, then that could cause the grout to crack.

It should be repairable; unless there is too much deflection in the floor.  You just have to verify that tile is bonded well.  That there isn't excessive deflection in the floor. And that when the grout is replaced it is done properly.