waterproof grout
Tiles can expand or contract due to temperature shifts, humidity levels, and substrate conditions, but this type of grout flexes enough to absorb the tension without breaking apart. This makes it perfect for exterior applications, shower walls, high-humidity spaces, and underfloor heating systems where constant temperature changes occur.
https://s3.dualstack.us-west-1.amazonaws.com/kerakoll.grout/kerakoll-grout-white.html
