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I just finished grouting my kitchen floor with unsanded dark gray grout (near-black "charcoal" according to the box, also it contains portland cement). Not long after the first section I started noticing some efflorescence forming in a few places. At this point everything is done and it is a day old. Despite some attempts to mitigate, there is still quite a bit of efflorescence (very splotchy, but in many places).

Things I have tried:

  • Sponging sooner and with shorter waits in between, to try and reduce the total amount of time the grout was wet. Did not seem to make a difference.
  • Letting the grout sit and dry out longer in between passes with the sponge (10-15 min). Did not seem to make a difference.
  • Using a brass wire brush after curing. This worked well (aside from some mild depth to the grout lines, but the grout beneath was revealed to be much lighter than the "charcoal" color. A bummer but this got the most uniform result.

So 2 questions:

  • For next time, what stategies are the most effective at mitigating this?

    • Are there types of grout that don't have this problem (or to a lesser degree)?
    • Can I buy some kind of additive?
    • How likely is it that something I am doing during application is the issue? What should I be mindful of?
  • For this time, what can I do now?

    • The wire brush works, but the grout underneath is lighter than the untouched areas without efflorescence (not sure why). Also, if I brush an area without the efflorescence, that is lighter underneath too. I realize I may have to live with that, which is unfortunate but I'm cool with it.
    • Would using muriatic acid get me different results, or would it more or less be similar to the brush method (perhaps without removing as much material)?
  • I think if you worked it a bit two much or it had two much water the cement came to the surface. Without a photo this is a guess and I would let it cure then try a light muriatic rince.Once you get the color you want it should be sealed. – Ed Beal Mar 23 '16 at 0:45
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The efflorescence is just a bit of salts that precipitated at the surface of the grout. Stop wire brushing it and wash it with a scrub brush, with a dilute solution of muriatic acid and warm water, that should take care of the problem.

Next time use a bit less water when mixing your grout, if it is soupy then that is too much water. Always let your grout slake, as (likely) described in the mixing instructions.

You should be sponging as you go, dirty sponge/clean sponge, with well wringed sponges, rinsing and changing bucket water frequently. This helps remove salt-carrying water from the grout surface before it can dry into an efflorescent crust or residue. Always follow up with an acidic "haze" sponging.

  • If your goal is to dissolve the efflorescence but not the cement, wouldn't simple vinegar do the trick? Muriatic acid seems like overkill. – Shimon Rura Mar 23 '16 at 8:05
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    Diluted, it is very common in tile work and it does not remove the cement. We use it on every single tile job, for the final grout haze wash. I guess vinegar would work for DIYr, but compared to muriatic acid the price would be prohibitive for me. – Jimmy Fix-it Mar 24 '16 at 1:13
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You don't mention what type of tile you used in this situation. If it is stone or unglazed tile, consider treating the tile with either tile sealer or grout release before grouting. This will make it much easier to remove the remnants of the grout from the surface of the tile.

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