[hellohello]

I mixed up some 40-minute fast-setting joint compound (aka hot mud) for a basement wall I built, and it took forever to dry in this one spot. It was probably 1-4"-1/2" thick x 4" wide of mud (a double outside corner) right above the concrete floor, and it took over a day to dry! Six hours after mixing/applying it, it was literally still as wet as the fresh mix.

I mean, that stuff normally dries solid for me in under an hour, no matter how thick it is. Once it gets unworkable in my mud pan, I put it in the trash, and it hardens completely soon after that. So why would 1/4" thick mud have taken so long to dry/harden? This is the first time this has happened for me. Could it be because I used extra cold water to mix it. Perhaps because it was so close to the cold concrete floor? It's not like it was extremely cold in this basement (probably a comfortable 60 degrees farenheit) and using cold water hasn't been an issue in the past.

So what could've caused this?

 

[oh'mike]

Either to cold--or a bad batch---powdered mud can get funny if it is old---

 

[Nestor_Kelebay]

Could it be because I used extra cold water to mix it.

There's some of that...

Perhaps because it was so close to the cold concrete floor?

And a lot of that...

It's not like it was extremely cold in this basement (probably a comfortable 60 degrees farenheit) and using cold water hasn't been an issue in the past.

You might not have felt cold in the basement, but the concrete could have been considerably colder than you think. Cold water might not have been an issue in the past, but that's what "experience" is all about. The more times you wonder "WTF? That was never a problem UNTIL NOW!", the more experience you gain. Now, you'll now to mix your mud with warm water if it's going to be cooled by a nearby concrete floor.

Next time, just scrape the stiff mud smooth with a paint scraper, and put on another coat if needed, and let that dry overnight.

 

[Colbyt]

A simple box fan for air circulation also is a great aid in achieving a 'set'.

 

[MrBryan]

The "40 minutes" more accurately describes how long you have before it starts to set up...not how long it takes to dry completely.
Sounds like kind of a thick layer too, so that will add to the dry time.

 

[ToolSeeker]

Cold water will definitely slow it down just as hot water will turn 40 minute to 20 minute. and if it was mixed too thin it will slow it down a lot. As stated an old bag that has been opened could well affect the time.