From Newsgroup: uk.d-i-y

Gypsum is the hardened state of plaster of Paris calcium sulfate hemihydrate which is essentially crushed and dehydrated gypsum. The reason it has a use by date is because even if you seal it in a plastic bag moisture condensates. It will naturally soak moisture from the air. This means that even if the bags are in date, say you keep them in the house (lots of moisture in the air) the crystals will begin to slowly hydrate. Unless the bag is new, the bags will set faster and faster with age. It's no joke either, some bags can literally go off in the bucket before you pull out the paddle. Once the calcium sulfate has set out long enough say a few years what you find is that most of the crystals have formed, but not bonded so when you add water some bits might go off, but most of it will never set and create a failed surface, 0 uniformity. Technically you can just stick the plaster in the oven and dehydrate it, but not worth the cost and it does require proper heating. Toss it in the trash, recycle it or bury it, once it has hydrated into gypsum it is inert, but don't put it with organic waste unless you are sure it has fully hydrated, because certain bacteria will convert the sulfate into hydrogen sulfide which smells like a number 2 after Guinness and eggs.
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For full context, visit https://www.homeownershub.com/uk-diy/what-to-do-with-out-of-date-plaster-1431201-.htm

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From Newsgroup: uk.d-i-y

On Tue, 05 May 2026 18:15:02 +0000
Frank <db605e8f71913d1f3966ad908d78b8a8084f5047122037b2b91a7192b598a9ad@example.com> wrote:

Gypsum is the hardened state of plaster of Paris calcium sulfate
hemihydrate which is essentially crushed and dehydrated gypsum. The
reason it has a use by date is because even if you seal it in a
plastic bag moisture condensates. It will naturally soak moisture
from the air. This means that even if the bags are in date, say you
keep them in the house (lots of moisture in the air) the crystals
will begin to slowly hydrate. Unless the bag is new, the bags will
set faster and faster with age. It's no joke either, some bags can
literally go off in the bucket before you pull out the paddle. Once
the calcium sulfate has set out long enough say a few years what you
find is that most of the crystals have formed, but not bonded so when
you add water some bits might go off, but most of it will never set
and create a failed surface, 0 uniformity. Technically you can just
stick the plaster in the oven and dehydrate it, but not worth the
cost and it does require proper heating. Toss it in the trash,
recycle it or bury it, once it has hydrated into gypsum it is inert,
but don't put it with organic waste unless you are sure it has fully hydrated, because certain bacteria will convert the sulfate into
hydrogen sulfide which smells like a number 2 after Guinness and eggs.

From May 15, 2020.
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Davey.
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