Possible reasons for grout cracking:
- Improperly prepped substrate (ie not stiff enough)
- Grout which doesn't cope with the conditions in which you've used it
a. Not enough flexibility, though the need for that should be countered with proper substrate preparation and reinforcment
b. Grout which doesn't stick well enough to the tiles, ie grout meant for porous tiles, and you've used really dense tiles.
c. Grout which is meant for thicker grout lines than you've made (for example, using grout meant for 4-12mm lines, when you've made the lines 2mm)
d. Grout which is meant for thinner grout lines that you've made (for example using grout meant for 0-3mm when you've made the lines 5mm)
- Faulty batch of grout
- Too much water when mixing the grout
- Too little water when mixing the grout
- Adding water to the mix after the grout has started setting in the bucket
- Exceeding the open time of the grout when grouting (ie mixing too much in one go)
- Contaminated water (which makes the grout cure too fast, or unevenly)
- Too warm envionment while the grout cures (too fast curing)
- Too dry environment while the grout cures (too fast curing)
- Too cold environment while the grout cures (too slow curing)
- Sunlight exposure while curing (uneven and too fast curing)
- Grouting too soon after setting the tiles
- Too much water when washing after after application of grout
- Washing too soon after application of grout
- Walking on the floor too soon after grouting
- Washing with strong detergents too soon after grouting
- Impregnating too soon after grouting.
Etc. Take your pick.
