What could this white substance that appears on our shower grout be? I had cleaned it off less than 12 hours before this picture.

Overnight, we had a space heater on in the bathroom to dry out some shoes, so the temperature was about 35°C.

enter image description here

  • Hard water deposits, soap residue, grout components. What happens if you moisten it?
    – isherwood
    Commented Mar 24, 2016 at 15:33
  • 2
    What does it taste like? (JOKE! that was a joke, don't do it...) Commented Mar 24, 2016 at 15:46
  • 2
    But seriously, do it.
    – isherwood
    Commented Mar 24, 2016 at 15:55
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    Efflorescence maybe? How old is the grout and what exactly is the grout? Do you have particularly hard water in your area?
    – brhans
    Commented Mar 24, 2016 at 18:32
  • you could probably clean it with vinegar and a tooth brush. If that fixes it it probably needs to be sealed. I think Efflorescence or hard water are the culprits.
    – Ed Beal
    Commented Mar 24, 2016 at 20:39

2 Answers 2


That is calcium buildup from hard water. Do you have well or live in the city? Some cities also use well water mind you. Another thing can be if it is city water, high chlorinated water. Ultimately no way to resolve either other than cleaning it with baking soda, vinegar or CLR.


Thank you for the additional information & correction. While I would agree this looks like efflorescence there doesn't seem to be any excess source of water that would be making any salts rise to the surface on a regular basis. Therefore, it has to be that a Polymer Modified grout was used & the Polymer additive was not used or handled correctly.

The Polymers are typically white & will look very much like your situation. Used incorrectly the polymers literally float to the surface if the grout was too wet, sealed too soon, sponged too soon & with too much water or that the polymer was added to the joint separately & topically. The last is very & appears to be your situation. You'll need to remove the grout & fully re-grout in order to address this problem permanently. Sorry for my initial confusion.

  • It's newish shower (less than 3 years old). The grout is underneath the white substance.
    – barber
    Commented Mar 25, 2016 at 4:15
  • Wow, that's a weird setup. Long singular tile strips, but then a big fat solitary grout line? No offense, it just doesn't make sense to me. Thank you, I'll update my answer.
    – Iggy
    Commented Mar 25, 2016 at 4:43

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