I had a plasterer come in to plaster a wall today. the wall varied from 'down to brickwork (Looked damp if I'm honest) to new plasterboard. my plasterer said that a handful of cement speeds up the drying time of (bonding) plaster!! Anyway, first coat of about 10/12mm didn't look as if it was drying (about an hour) so he put another 10/12mm on and left it. Still not setting when I left later on. Checked the 'use by dates', bonding ok but cement well out' Will I be hacking it off tomorrow do you think lads?

can take a while for plaster to set at best of times!

id leave all windows open get plenty of air to it n see what its like in the morning!

hacking it off might be bit rash, although u said brickwork looked damp before, which aint a great sign!

I'd never add cement to gypsum. Aside from that, it's the same old story that when you want it to go off, it won't and vice versa

Originally Posted by doug boardley

I'd never add cement to gypsum. Aside from that, it's the same old story that when you want it to go off, it won't and vice versa

wasn't in a position to argue as he's been in the game for 30yrs

Never seen that done before.

as the spread says it should speed the set, but not a good practice!!! the same as peeing in the mix, or dirty water!!!it all weakens the final streanth!!

Originally Posted by doug boardley

I'd never add cement to gypsum. Aside from that, it's the same old story that when you want it to go off, it won't and vice versa

With Doug on this, the thickness of plaster will take time even longer if applied a pva to the wall before plastering

Most gypsum plasters (anhydrite screed included in this) will tolerate a small quantity of clean ordinary portland cement. Different manufacturers argue on the percentage but vary between 3 and 5%. This can actually be beneficial in terms of surface hardness as the cement hydrates relatively quickly. It does have the disadvantage of reducing the flexural strength of the material. In fact most Hemi Hydrate Screed materials have around 4% cement in them.

If however, as is more likely these days, he has used a modern bagged cement, many of which contain plasticisers (e.g. Multicem and Mastercrete) these can severely retard the setting of the gypsum as they impede the chrystalisation process.

I occasionally see issues where a concrete mixer has failed to wash out thoroughly and the anhydrite gets contaminated with polycarboxylate plasticiser and it can take several days for the screed to go hard. Unless there has been a significant level of plas added though it will go hard it could just take several days. The dampness in the wall will not have helped and ought to have been dealt with first as it could affect the long term bond of the plaster to the wall itself.

In fact when I have wanted to slow down the setting time of out of date plaster I have added a couple of cap fuls of detol (acid retarder) and have been able to work with it. Mind you the smell was horrid.

If it was a really modern cement which is blended with GGBS (a cement replacement) then you might as well take it off the wall now. This latter is unlikely. I am not sure phoenix (trade name for Blue Circle) is available in bagged form. GGBS and Gypsum are very poor bedfellows and the latter leads to poly sulphide attack in the Gypsum core causing aggresively expansive degeneration of the gypsum.

The shelf life of cement is to do with the removal a few years ago of Chromium 6 from the cement. This acts as a stabilising agent to prevent the cement hydrating whilst in storage. Hexavalent Chromium is probably the best material for this purpose but unfortunately it is also responsible for the the dermatitis people got when using cement. It was replaced predominantly with Ferrous Oxide which is less effective so manufacturers now put a 3 to 6 month shelf life on cement but at least it does not give you cancer any more.

I think realistically it is most likey the result of the addition of plasticised cement and whilst it might take longer than it ought to go hard there is little need to worry.

Originally Posted by Ajax123

Most gypsum plasters (anhydrite screed included in this) will tolerate a small quantity of clean ordinary portland cement. Different manufacturers argue on the percentage but vary between 3 and 5%. This can actually be beneficial in terms of surface hardness as the cement hydrates relatively quickly. It does have the disadvantage of reducing the flexural strength of the material. In fact most Hemi Hydrate Screed materials have around 4% cement in them.

If however, as is more likely these days, he has used a modern bagged cement, many of which contain plasticisers (e.g. Multicem and Mastercrete) these can severely retard the setting of the gypsum as they impede the chrystalisation process.

I occasionally see issues where a concrete mixer has failed to wash out thoroughly and the anhydrite gets contaminated with polycarboxylate plasticiser and it can take several days for the screed to go hard. Unless there has been a significant level of plas added though it will go hard it could just take several days. The dampness in the wall will not have helped and ought to have been dealt with first as it could affect the long term bond of the plaster to the wall itself.

In fact when I have wanted to slow down the setting time of out of date plaster I have added a couple of cap fuls of detol (acid retarder) and have been able to work with it. Mind you the smell was horrid.

If it was a really modern cement which is blended with GGBS (a cement replacement) then you might as well take it off the wall now. This latter is unlikely. I am not sure phoenix (trade name for Blue Circle) is available in bagged form. GGBS and Gypsum are very poor bedfellows and the latter leads to poly sulphide attack in the Gypsum core causing aggresively expansive degeneration of the gypsum.

The shelf life of cement is to do with the removal a few years ago of Chromium 6 from the cement. This acts as a stabilising agent to prevent the cement hydrating whilst in storage. Hexavalent Chromium is probably the best material for this purpose but unfortunately it is also responsible for the the dermatitis people got when using cement. It was replaced predominantly with Ferrous Oxide which is less effective so manufacturers now put a 3 to 6 month shelf life on cement but at least it does not give you cancer any more.

I think realistically it is most likey the result of the addition of plasticised cement and whilst it might take longer than it ought to go hard there is little need to worry.

I was gonna say that too

I knew that

good point there ajax, about the plasticised cement i being an old boy only ever use OPC for rendering/ screeding and then put my own aditives in the mix, and another old one is sugar in the sand/cement mix will slow/kill the set ,lol

Never heard the sugar thing

ajax123 you the man

Originally Posted by Ajax123

Most gypsum plasters (anhydrite screed included in this) will tolerate a small quantity of clean ordinary portland cement. Different manufacturers argue on the percentage but vary between 3 and 5%. This can actually be beneficial in terms of surface hardness as the cement hydrates relatively quickly. It does have the disadvantage of reducing the flexural strength of the material. In fact most Hemi Hydrate Screed materials have around 4% cement in them.

If however, as is more likely these days, he has used a modern bagged cement, many of which contain plasticisers (e.g. Multicem and Mastercrete) these can severely retard the setting of the gypsum as they impede the chrystalisation process.

I occasionally see issues where a concrete mixer has failed to wash out thoroughly and the anhydrite gets contaminated with polycarboxylate plasticiser and it can take several days for the screed to go hard. Unless there has been a significant level of plas added though it will go hard it could just take several days. The dampness in the wall will not have helped and ought to have been dealt with first as it could affect the long term bond of the plaster to the wall itself.

In fact when I have wanted to slow down the setting time of out of date plaster I have added a couple of cap fuls of detol (acid retarder) and have been able to work with it. Mind you the smell was horrid.

If it was a really modern cement which is blended with GGBS (a cement replacement) then you might as well take it off the wall now. This latter is unlikely. I am not sure phoenix (trade name for Blue Circle) is available in bagged form. GGBS and Gypsum are very poor bedfellows and the latter leads to poly sulphide attack in the Gypsum core causing aggresively expansive degeneration of the gypsum.

The shelf life of cement is to do with the removal a few years ago of Chromium 6 from the cement. This acts as a stabilising agent to prevent the cement hydrating whilst in storage. Hexavalent Chromium is probably the best material for this purpose but unfortunately it is also responsible for the the dermatitis people got when using cement. It was replaced predominantly with Ferrous Oxide which is less effective so manufacturers now put a 3 to 6 month shelf life on cement but at least it does not give you cancer any more.

I think realistically it is most likey the result of the addition of plasticised cement and whilst it might take longer than it ought to go hard there is little need to worry.

^^^ WOW! He knows his stuff huh! I bet that wasnt in the Marks and Spencer book!

On a serious note i have seen sparkys put bonding and cement together regularly to set pattresses in the wall. It goes off real quick. I have used it one or twice round the edge of a bath to speed the drying time before a top coat of hardwall. Never had a problem with it but then its only been in small quantities

Originally Posted by gooner59

good point there ajax, about the plasticised cement i being an old boy only ever use OPC for rendering/ screeding and then put my own aditives in the mix, and another old one is sugar in the sand/cement mix will slow/kill the set ,lol

We used to give each concrete driver a couple of bags of sugar when we were pouring rapid setting concretes in the summer up to just a few years ago. If they got delayed or held up on site they could chuck it into the mixer to stop the concrete going off. We did have one vindictive blighter who was sacked for stealing and so put his sugar in the deisel tanks of 3 lorries. stops that working as well.

I too much prefer the old fashioned OP cement and add my own retarders, plasticisers etc but it is getting more and more difficult to get it. It will only be a matter of time before it is "outlawed" cos we all got to save the planet.

they also put gypusm into the cement as a setting agent. If you ever feel really bored try putting high allumina cement and gypsum finish plaster together and then try mixing it up with an alluminium shovel. Bet you cant get it mixed before it goes hard. When it has gone hard I bet you can't get it off the shovel......

Fun days with nothing much to do in the technical lab......

i tried to put brick acid in an alluminium bucket , wow!!!! it whent mad!!!!

Originally Posted by gooner59

i tried to put brick acid in an alluminium bucket , wow!!!! it whent mad!!!!

produces Hydrogen Gas - another reason not to smoke......BOOOM!

gonna re-title this thread "How to be a terrorist"

This thread is more like a chemistry lesson

List for shopping tomorrow, ally bucket and brick acid

Originally Posted by doug boardley

gonna re-title this thread "How to be a terrorist"

time for a slurp.... Molotov ****tail anyone?

Hope big brother is'nt listening in

Originally Posted by Scottley

This thread is more like a chemistry lesson

List for shopping tomorrow, ally bucket and brick acid

when you get your brick acid get some pottasium permanganate and you can make incendairies as well........

Perhaps I should stop now before I get banned or a visit from the anti terrorists squad ,....... I don't even like fireworks

Originally Posted by Ajax123

when you get your brick acid get some pottasium permanganate and you can make incendairies as well........

Perhaps I should stop now before I get banned or a visit from the anti terrorists squad ,....... I don't even like fireworks

I remember the potasium from school days along with the magnesium ribbon.

My other half is a science teacher too!

BOOM!

ooooh! Magnesium Ribbon. Pretty Fire.......safety goggles and gloves required for this next bit. Wrap one glass mable up in magnesium ribbon. Place in fume cabinet and close door. light the ribbon with a match and stand well back - very quickly - and watch the marble magically disappear - and if you use too much ribbon watch most of the fume cabinet magically disappear as well. I almost got the sack for that one....... I won't even let my kids use matches nowadays - honest.....

Pottasium is even more fun when you put it in water. You can watch it dance about like a flaming fairy. You have to store it in Oil to stop it from blowing up.

[ame=http://www.youtube.com/watch?v=OFG4Yr7lQzw]YouTube - Potassium and Water[/ame]

in the swimming pool plant room you never want to mix chlorine/acid =mustard gas as used in the first WW,

cough cough!!

how did we get here from plaster not going off...... could be my fault I guess