We recently renovated our bathroom (in Sydney, Inner west) and chose a nice grey grout to match the grey floor tiles. But now only a month later the grout is turning white! See the pic for more detail, the floor grout should be the same colour as the wall tile grout (also in the picture)
Does anyone know why this is happening and what we can do to reverse it? If we wanted white grout we would have used it!
Many thanks for your question, I'd be happy to assist. There are soluble salts present in cement-based products like grouts for tiles or mortar between bricks. With moisture, these salts migrate their way to the surface and form a white layering. Calcium in your shower water can also leave white traces on the black grout. This is commonly called efflorescence and is quite common to be seen on new brick houses.
You can try scrubbing it away with a stiff-bristled brush and rinsing with fresh water and over time it will be less visible. As to why the wall tile grout is not similarly effected it might be because the floor grout is in more contact with moisture or it is potentially a different type of grout.
Please let me know if you require further information or had any questions.