Author Options:

Reusing Plaster of Paris? Answered

...or simply plaster, is a type of building material based on calcium sulfate hemihydrate... It is created by heating gypsum to about 150°C.

...When the dry plaster powder is mixed with water, it re-forms into gypsum...

So...if I heated the gypsum to +302F (150C), it would turn back into plaster of paris, to be reused agian? Of course you would need to powder the used plaster (gypsum), but would it work?


The forums are retiring in 2021 and are now closed for new topics and comments.

13 years ago

HI - cause I live in the middle of NOWHERE - which is miles from Anywhere, and all that I am keen to conserve all my resources...

Yes you can recycle plaster (or gypsum)... and it is done on almost all the excess or recycled plaster board and other plaster products.

I am a little vauge on all the exact chemisty of it.... but basically it amounts to running the plaster through a crusher and then heating it in a ball mill..... to drive out the WATER of HYDRATION.... this is "chemically combined water"... not "soaked in water" like a sponge.

Plaster can more or less be recycled indefinitely......

If I was just experimenting, I'd do it, just to find out...

IF I was using lots of it, regularly, I'd be setting up a small mill to break up / grind and heat the plaster.


Milled Plaster of Paris

name Mike
status Teacher
grade 9-12
location South Africa

Question - Once Plaster of Paris (hemihydrate) has set (as the
dihydrate), can the hemihydrate be obtained again, i.e. by milling the
set materials and dehydrating it by heating? If so, are there any guide
lines as to the suitable fineness (particle size) of the milled
materials and the dehydration conditions to be used? Will the resultant
hemihydrate (if obtained) be as good as the original in using again as
Plaster of Paris, and if so how many times can this recycling and reuse
of Plaster of Paris be continued?>

Hi Mike,

Plaster of Paris (gypsum hemihydrate) is in fact created
industrially by heating fully hydrated gypsum (the
dihydrate). Typically the dihydrate must be heated moderately
above the boiling point of water; usually about 120°C to 170°
for a specific time to drive off excess water of
crystallization and result in the hemihydrate. Since Plaster
of Paris that has "set" is the same thing a gypsum, the
results will be the same, whether heating gypsum, or "set"
Plaster of Paris.... the hemihydrate will result.

So in answer to your question, Plaster of Paris can be
"recycled" endlessly by the above method. As to how finely it
needs to be reground, I suspect that the hemihydrate has
essentially no structural strength, and thus will crumble
easily to powder, so significant effort to regrind it is
probably not needed.


Bob Wilson.





Reply 12 years ago

May I know weather Plaster of Paris can be reused once used? How (Process) ? and how mant times?

John Smith
John Smith

Reply 13 years ago

Thanks! I wasn't expecting such an AWESOME response. Thanks again, that's what I thought about the plaster of paris.


13 years ago

I believe it would, but not practical. It would take several passes through a hot kiln to get all of the water out. Then to powder would take time. Technically you could do, but it's not practical. The same goes for concrete.

John Smith
John Smith

Reply 13 years ago

Ok, thanks. Looking at the low price of it, it wouldn't be practical.