plaster,
fast setting and slow setting
A common scenario for fast or slow setting plaster.
you turn up to carry out some plastering and you've 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out of date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Out of date plaster is best disposed of, as it either sets very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 doesn’t 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 Old out of date plaster would appear to have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, don’t 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 don’t 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 don’t 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 http://www.goldtrowel.org/skimming.htm
2) you can buy the advanced plastering skimming techniques
films http://www.goldtrowel.org/films_dvd.htm
When you’ve finished mixing and you want to
store materials you should be rolling the bag(s) over at the
top for opened bags
back to goldtrowel
plastering organisation
back
to the plastering forum
|
