Rendering a wall

[Corky]

Sorry Doug I judt saw Dave's post and replied,did'nt realise you on it.As you say better not to skim s/c when tiling on.

 

[doug boardley]

So..pva will stop ettringnite reaction/failure between the cement base coat and the gypsum skim/top coat..?...

maybe corky meant sbr?

 

[doug boardley]

So..pva will stop ettringnite reaction/failure between the cement base coat and the gypsum skim/top coat..?...

never pva'd a s/c coat prior to skimming myself, if it's dried to far light sprinkled with water on my brush but never had a failure with s/c and gypsum Dave

 

[DHTiling]

Hi doug..

Cement and gypsum do not mix....have you never heard of Ettringnite failure..?..

Don't want to sound out of term here but plasterers do use pva alot and sometimes it is not suitable for the project in-hand...

 

[White Room]

Gypsum plaster has been put onto s/c backings for years with no problems and the old boys with siriphite and there's never been a problem

 

[doug boardley]

Hi doug..

Cement and gypsum do not mix....have you never heard of Ettringnite failure..?..

Don't want to sound out of term here but plasterers do use pva alot and sometimes it is not suitable for the project in-hand...

I can only go on 27 years plastering and tiling Dave, can only assume ettrignite failure is due to alkali/alkali chrsytallisation (long time since i did my chemistry o'level!) but surely if the s/c has set then it has entered an inert state and the gypsum won't be a catalystic agent, maybe

 

[DHTiling]

Gypsum plaster has been put onto s/c backings for years with no problems and the old boys with siriphite and there's never been a problem

I am just asking...but also when you say no problem....how do peeps know there is a problem unless they get called back..........

It is widely known in the tiling world that cement and gypsum do not mix....can you tell me why this is different for plasterers plz..?

 

[White Room]

Is it the additives in addy that could cause the problem, They have become so high tech nowadays:huh2:

 

[doug boardley]

to be honest I can't Dave, but I've never been called back to a plastering or tiling job ever, only when customer has wanted more work doing, if that flies in the face of conventional thinking I can live with it, but will look into it. off to bed now!!

 

[DHTiling]

The failure is between portland cement and gypsum...now this problem is as we tilers know, is a cement based adhesive onto a gypsum base....

Now the other way around.. a cement based base coat fully cured and then a gypsum top coat...will failure occur....mmm i would like to know this.....

anybody enlighten us as to why not..?...