What is plaster? Question from a non-plasterer

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johnvaul

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Hi

As a non-plasterer can I ask the following basic question.

Is stuff like Thistle Board Finish and Multi Finish just gypsum or is it gypsum with a light weight aggregate?

And what about backing plasters such as Bonding - are they a mix of aggregate and gypsum?

If they are mixes what aggregate do they use and in what proportions?

Also, why is plaster that pink colour?

John
 
johnvaul said:
Hi

As a non-plasterer can I ask the following basic question.

Is stuff like Thistle Board Finish and Multi Finish just gypsum or is it gypsum with a light weight aggregate?

And what about backing plasters such as Bonding - are they a mix of aggregate and gypsum?

If they are mixes what aggregate do they use and in what proportions?

Also, why is plaster that pink colour?

John
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pretty random question for your third post but a question non the less finish is pretty much gypsum with a retarder added maybe a bit of lime without lookin i think around 1%.
backing plasters depending which kind are pretty much gypsum some mixed with vermiculite or perlite and retarders i think backing plasters years ago wher more sand based.

the plasters vary in colours depending which part of the country they were made :)
 
the pink colour is because theres clay in it... as an impurity.... sometimes its not pink, as flynny says....

knauf finish, made elsewhere in the universe, doesnt have any clay, and is white....

pretty sure this exact question has been dealt with in depth in this very section somewhere? or did it get deleted?
 
did anyone else get renovating that stunk like hippy scent ? :RpS_unsure:


sorry slightly off thread there :RpS_blink:
 
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renovating base coat was the gear i ment, it was just every so often ya would get half a dozen bags that had a very earthy smell,

dont take drugs these days m8 :RpS_thumbup: only fags and viagra :RpS_lol:
 
So pure gypsum is the same as plaster of paris? And gypsum based plasters just have stuff added to them like retarders and perhaps aggregates? And the colour is determined by the colour of the gypsum dug out of the ground?

How long does a gypsum plaster take to go off, approximately?
 
johnvaul said:
So pure gypsum is the same as plaster of paris? And gypsum based plasters just have stuff added to them like retarders and perhaps aggregates? And the colour is determined by the colour of the gypsum dug out of the ground?

How long does a gypsum plaster take to go off, approximately?
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Set and dry are two different things and like i said depends which ones, also depends on the background, thickness, dates of production etc out of interest why are you interested in gypsum
 
he maybe gona make some pirate plasters and knock it out on the markets !:RpS_biggrin:

come on john spill the beans :RpS_thumbup:
 
So pure gypsum is the same as plaster of paris? And gypsum based plasters just have stuff added to them like retarders and perhaps aggregates? And the colour is determined by the colour of the gypsum dug out of the ground?

How long does a gypsum plaster take to go off, approximately?
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yes, yes, yes and not long unless you add retarder which is why plaster of paris sets quickly..

someone post up the chemical thing...

it goes something like gypsum rock/crushed/heated = gypsum powder/mixed with water/generates heat and sets back to a solid lump again...

see? so you start of with a lump of gypsum rock, heat all the water out of it and crush it too powder... then you mix it up with water which starts to reverse the chemical process and it gives off heat and reverts back to its origional state...

calcium carbonate.... go have a googly... :RpS_thumbup:


Set and dry are two different things and like i said depends which ones, also depends on the background, thickness, dates of production etc out of interest why are you interested in gypsum
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well at least I understood the question flynny... :rolleyes)
 
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Chris W said:
yes, yes, yes and not long unless you add retarder which is why plaster of paris sets quickly..

someone post up the chemical thing...

it goes something like gypsum rock/crushed/heated = gypsum powder/mixed with water/generates heat and sets back to a solid lump again...

see? so you start of with a lump of gypsum rock, heat all the water out of it and crush it too powder... then you mix it up with water which starts to reverse the chemical process and it gives off heat and reverts back to its origional state...

calcium carbonate.... go have a googly... :RpS_thumbup:




well at least I understood the question flynny... :rolleyes)
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LOL i was answerin the last bit coz he was repeating what he had already been told ;)

Some people get how long does it take to go off? mixed up with setting and drying which is two different things oh and the formula is CaSO[SUB]4[/SUB]·1/2H[SUB]2[/SUB]O or mixed with water CaSO[SUB]4[/SUB]·2H[SUB]2[/SUB]O
 
Chris W said:
yes, yes, yes and not long unless you add retarder which is why plaster of paris sets quickly..

someone post up the chemical thing...

it goes something like gypsum rock/crushed/heated = gypsum powder/mixed with water/generates heat and sets back to a solid lump again...

see? so you start of with a lump of gypsum rock, heat all the water out of it and crush it too powder... then you mix it up with water which starts to reverse the chemical process and it gives off heat and reverts back to its origional state...

calcium carbonate.... go have a googly... :RpS_thumbup:




well at least I understood the question flynny... :rolleyes)
Click to expand...

LOL i was answerin the last bit coz he was repeating what he had already been told ;)

Some people get how long does it take to go off? mixed up with setting and drying which is two different things oh and the formula is CaSO[SUB]4[/SUB]·1/2H[SUB]2[/SUB]O or mixed with water CaSO[SUB]4[/SUB]·2H[SUB]2[/SUB]O
 
... out of interest why are you interested in gypsum[/QUOTE said:
I've read a fair bit about and played around with lime mortars and plasters but wanted to know a bit about gypsum plasters from working plasterers.

Thanks for the info guys. I think I understand a little bit more now.
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some say its magic, and if you dont mix it for precisely 3 minutes you end up with a bucket full of dogshit....

all we know is....

its called 's**t money for hard graft' :RpS_crying:
 
Right, I've another basic question. (Hope it's OK to put it in this section)

I see a lots of reference to PVA for controlling suction. Why don't you just use water?
 
in some instances you do, for example, rendering an old brick wall with a 10mm thick coat of sand and cement...
when skimming over old plaster however, youre gonna need a bit more than water, something thick enough to block the pores a bit without using too much water...
if you took a hosepipe to old finish plaster the same way you might do to old brickwork on a hot day outside, youre gonna ruin it...
think of what happens to the plaster on a downstairs ceiling after the baths overflowed upstairs.... its pretty well f'cked innit...
 
Right, I get you, gypsum based plasters don't like getting too wet. Our garage wall had gypsum plaster board dot-and-dabbed (is that the right term?) to a solid stone wall that gets damp. I had to pull it off because it failed - went all crumbly and fell to pieces.
 
Oh, yes, that was probably why it failed. But the house is in the hill-side and sits right by the side of the public highway so opportunities for dealing with the source of damp were limited.
 
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