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I had stored a bag of grout in a musty basement. When I went to use it, it had sort of hardened into a big block. I could pulverize it but when I rehydrated it and applied it it just stayed as a powder and didn’t harden into a solid. Is there any way I can get it to solidify again?

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    See also en.wikipedia.org/wiki/Lime_(material)#Cycle Commented Jul 11 at 14:01
  • In short: sure, the damage can be reversed. But doing so would probably need a specialist furnace that you could neither afford nor operate safely, and additives that you'd have to buy by the tonne. Easier to get another bag of grout... Commented Jul 12 at 8:46

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Grout is a cement product. It cures by bonding with water at the molecular level, which yours had done in the bag by absorbing vapor over time. When you pulverized it you did not reverse this process, but simply broke it into smaller pieces. Since the chemical process had already occurred, it wasn't going to occur again when you installed the grout.

There's no real limit to grout storage duration if you keep it in a sealed container. A paper bag isn't that.

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Buy a new fresh bag.

Old grout, mortar, cement if in musty damp conditions is ruined.

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    Agree, just to add, if stored cool and dry, 6 months is about the limit. Commented Jul 9 at 13:21
  • @AloysiusDefenestrate, depends on where you live... It doesn't go bad. It just cures when exposed to moisture and it is hydrophilic (will suck moisture out of the air).. In a dry environment <cough>Arizona<cough> it can last years.
    – Questor
    Commented Jul 9 at 19:16
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    Well, a quick lazy search at Laticrete has them saying that their unopened product is good for 2 years from manufacture. They both gain and lose from this stance so I’m inclined to trust them. But more to the point, labor is expensive and grout is cheap — why take a chance? Commented Jul 9 at 19:50
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    @AloysiusDefenestrate It also depends on the container used for packaging. A sealed plastic bucket will probably last longer than a bag with a thin plastic liner.
    – crip659
    Commented Jul 9 at 20:21
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    @Aloysius Defenestrate "labor is expensive and grout is cheap"... have a relative who recently needed to redo work caused by a bag of old grout; stuff literally didn't set just turned into dry sand/dust. Good grout is too cheap to risk having to pull it all apart, then doing the job twice.
    – GB540
    Commented Jul 9 at 23:05
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To directly answer the question, the grout may be reconstituted by de-hydrating the mix. To do this, you would need to heat the dried pulverized mix to about 150°C to drive off latent moisture. Once the product is completely dried (has a consistent temperature of 150°C), you will then need to cook it to between 1300-1450°C to complete the process of breaking the hydrogen bonds. The drying step is important to prevent BLEVE during the cooking phase. After this is done, you will be left with clinker which can again be pulverized and then used as a new concrete product.

Of course, this will be much more labor intensive and financially expensive than simply buying a new bag of grout, but I figure the question can be answered without resorting to treating concrete products as a magical one time use commodity.

@MLu does bring up a valid point for anyone considering doing this. Many additives to the original mix, (tint, plasticizers, fiber, hardeners, etc.) will likely be destroyed in this process and will need to be accounted for.

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    The question asked is "Is there any way I can get it to solidify again?". I severely doubt OP is going to dry out cement. Assuming OP is a regular person... its not something that they can do.
    – Questor
    Commented Jul 11 at 20:48
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    @Questor That's a poor assumption. You can get close to 2000°C with decent-quality charcoal and a hair-dryer and an old barbecue you don't particularly care about any more. You might need a few more things if you want the barbecue and the hair dryer to survive the process, but nothing that isn't trivially available at the local hardware store. Of course, if you're going there, it's quicker to just buy a new bag of grout... But yeah, this is 2000 year old technology. Trivial to build in your back yard today. Inefficient, but trivial.
    – Perkins
    Commented Jul 11 at 23:23
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    Grout often comes tinted with colour. I'm 100% sure that heating it to 1450°C will break down not only the cement but also the tint. Anyway, it's just a theoretical debate. OP go and buy a new bag of dry grout :)
    – MLu
    Commented Jul 12 at 5:18

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