ᐅ Black grout is sometimes turning white

Hello everyone!

To match our rather dark 60x60cm (24x24 inches) tiles, we had our bathroom grouted in black.

At first, all the grout lines were nicely black.
However, after some time, some of the grout lines have started turning white.
For example, when they get wet in the shower, the grout turns black again.
When they dry out, they turn white once more.

This effect is consistently reproducible.
What can we do to fix this?

As far as I remember, pre-mixed grout from a bag was used for grouting.

Could possibly be caused by hard water!?

It’s the same for me. It always started with lime deposits, although typical strong acids like vinegar and citric acid don’t help against it...

Since we moved into the house, we have been using a water softener.

Golfi90 schrieb:

Since we moved into the house, we have been using a water softening system.

However, there is still some limescale. A system does not remove everything.

Maybe it’s due to harsh bathroom cleaners?