CAUSES

1. When the joint receives heat shock during grouting, cracking may occur (sudden changes in ambient temperature or excessive temperatures in the summer and low in winter).

2. Incorrect grouting (incorrect water and dust ratio, etc.) has occurred, causing the substrate absorbing the dilute grout. Afterwards, the joint seem to be "full" but is actually going through a sedimentation process or it creates cracks.

3. This is how bubbles are created.