What IS plaster?

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Ok, 'Plaster', the substance you cover walls or ceilings with can be divided into two main categories.
Finish plaster and
Backing plaster..
There are other types of plaster available, such as one coat plaster, designed to do both jobs in one coat, monocouche renders (one coat external plaster), venitian polished plaster and decorative renders but for now we'll stick to the basics.


Finish plaster, what exactly is it?
'Finish' as its known in the trade is the top and final coating for a wall or ceiling, the one you apply your paint or wallpaper to. Usually pink in colour it should be finished to a high standard without bumps, ripples, scratches or otherwise unsightly defects (within reason, its not supposed to be a nasa laboratory)
The main finish plasters in use today consist of gypsum. A rock, found in the ground.
Thats it, just rock, maybe a few additives to slightly change its behaviour as its setting to suit different backgrounds but essentially its the same stuff.
So, you take your gypsum, you crush it and you heat it.
The heating process removes any moisture in the rock so that the end product is a pinkish, fine powder, the stuff you find in the bag.
There is a chemical formula and process formula but no plasterer on gods earth has ever had to refer to it while skimming a wall so we'll leave that alone..
what happens is this...
You take your fine powder, and add it to water. This makes it spreadable.
It also starts it along its process to revert back to its origional state..rock.
You spread it on your wall and it starts to set quite quickly.
It sets because the water is a) is being sucked into the substrate, b) evaporating into the room and c) causing a chemical reaction with the gypsum so it reverses the manufacturing process and gives off heat.
Not a lot but if you feel the bottom of a bucket of mixed plaster after an hour you'll feel it.
The end result is what was once a lump of rock in the ground is now a flat smooth rock about 3mm thick which is bonded to your surface.
so thats it.
plaster is rock. Gypsum to be precise.


there are many types of backing plaster gypsum based ones are
bonding coat- this is for low suction backgrounds i.e p/board or pretreated surfaces with gypbond/pva .....it should be applied to a maximum thickness of 8mm per coat
hardwall- medium to high suction backgrounds applied to 11mm maximum thickness
toughcoat- medium to high suction backgrounds but is better for high suction backgrounds ........dont know why ask someone else :)
limelight- a renovating plaster with breathable properties i.e if its stone or brick with a single skin the wall needs to allow the moisture in and evaporate ...not as strong as sand cement (contains waterproofing additives, used a lot on damp proofing jobs)
sand/cement- applied in various gauges dependant on background


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cas042h20 is the formula when its mixed and cas042h(half)20 when its dried or the other way round...........:)
also just to add it is crushed then heated to 195 degrees..............


when heated it evaporates all the moisture ie h20 out of the gypsum, and when adding water back to it...it returns itself to its former state...with the help from the extra additives.....its why u shouldn't re add water after the mix ...as this will weaken end product...


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Its heated to remove 75% of the water christals and keratin is added as a retarder also gypsum is added to cement as a retarder.
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