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A common scenario for fast or slow setting plaster.
you turn up to carry out some plastering and youve been told the plastering materials are already on site
you check the use by date on the bags and oops the plaster is out of date
the nearest builders merchants is an hour away and its rush hour.
Do you risk using the plaster?
To know why your plaster sets too fast or not at all you will need to understand the fundamental basics of plaster and plastering conditions
In some parts of the world the Term Plaster is used to describe sand and cement render, but not here in the UK
we call sand and cement render (sand and cement render)
Plaster is calcium sulphate hemihydrate.
Gypsum is the dihydrate
Gypsum is heated to extract the water of crystallization and ground to a powder.
Some plasters contain additives to improve the texture and or adhesion etc
Out of date plaster is best disposed of, as it either sets very fast (Flash sets) or doesnt set at all.
2CaS04.2H2O (heat)??? 2(CaSO4).H2O + 3H2O
The reaction operates in reverse when water is added and the plaster sets. Old out of date plaster would appear to have taken atmospheric moisture in and the traces of the dihydrate seem to act as seed crystals, causing flash set.
Another reason for this is badly stored materials, your plaster should be stored in a completely dry environment off the ground.
The RHL (relative humidity level) for the storage room(s) should be checked with a hygrometer if you are in doubt as to the moisture level present in the room.
The conditions for plastering on site should be controlled by you as much as is possible, this we call controlling your environment
If your working in domestic properties you should be aware that people in the winter do like to have the heating up as high as possible
Turn off the heating, dont be bullied into working in a sweat shop
If the property is warm or hot when you turn up then open the windows and release the heat.
When its cooled off enough you should close the windows and or doors as a breeze coming through the room will help set off the plaster faster.
Dirty water is another way to speed the set of plaster, some plasterers deliberately do this so they dont have to wait around too long for a small job
The backing plaster or surface is also another reason for plasters to set fast or at a controlled rate
When you carry out a glue and re-skim you may notice the plaster sets fast on the first coat, high suction backgrounds are obviously going to set the plaster fast, some people use pva to seal the high suction.
by doing this you break down the chemical bond between the backing plaster and the finishing plaster.
If you need to control a high suction backing plaster because you never got round to float and set the same day then you should only be using clean water to dampen down the surface prior to skimming
Lime based plasters dont have a chemical set like gypsum plasters, they dry by carbonisation, there are ways to speed up the set by the addition of pozzolanic additives such as brick dust or cement dust, reactive silicates and aluminates in the presence of water,
To Slow the set of Gypsum based plasters some plasterers have doctored the gypsum based plaster with a handful of lime putty to slow the set for big hits.
Be warned that this can have an adverse effect on the finished plaster with the impact strength being reduced dramatically, Truth is that the plaster companies have enough time money and experience with plasters to slow the set but feel that an approximate 20 minutes of usability should be enough time. If the hit is too big, then you need more hands on deck.
If the plaster hit is not too big and you have done everything correctly by controlling your working environment and used the correct materials at the right consistencies and thickness yet still find the plaster sets too fast for you then you have a few options that we can help with.
1) you can book the advanced plastering course with goldtrowel
2) you can buy the advanced plastering skimming techniques films
When youve finished mixing and you want to store materials you should be rolling the bag(s) over at the top for opened bags