one for ALAN ( AJAX )

[Diamond Pool Finishers]

can you only get ettringnit failure ,with Cement based adhesive on to gypsum floors ?
then why don't plasterers get it using gypsum based plasters on to rendered backing's then ?

 

[DHTiling]

Because the cement has hydrated.

Where as cement onto gypsum is different. 🙂

 

[DHTiling]

Yes it happens walls or floors

 

[Diamond Pool Finishers]

ha ha now you say that it makes perfect sense Dave as the crystals come from the cement lol

 

[White Room]

I asked at the mapei day to one of the tech guys and he said that it was due to an external damp source affecting the render/gypsum...if kept dry there should no problem, just repeating what I was told.

 

[DHTiling]

I asked at the mapei day to one of the tech guys and he said that it was due to an external damp source affecting the render/gypsum...if kept dry there should no problem, just repeating what I was told.

Yep , but with water being added to adhesive , this in turn wets gypsum background and increases the chance , hence priming for a barrier.

 

[Diamond Pool Finishers]

i was just speaking to a plasterer as he had gypsum plaster blowing of a rendered wall, and it was just one of the things ,i mentioned to him along with poor-key in the render, so i have been back and told him to discount that as a possible factor !

 

[Diamond Pool Finishers]

I asked at the mapei day to one of the tech guys and he said that it was due to an external damp source affecting the render/gypsum...if kept dry there should no problem, just repeating what I was told.

they did not cover this on the day i met you mark! did they ?

 

[White Room]

they did not cover this on the day i met you mark! did they ?

I did ask at the time Dave to the guy who was doing the teaching on products and I asked at the time about it and he thought we put a barrier, pva between the two surfaces (gypsum and render undercoats) but I did stay behind to speak to a tech guy.

 

[Ajax123]

The simple answer is you do get ettringite at the interface between render and plaster.

There are various reasons why this might not cause a failure. The first and most important is the level. The ettringite chrystalls are formed when moisture borne sulphate ions come into contact with calcium alluminates in the cement. The level of formation will depend on the amount of sulfate ions present. Now if you think about a wall. Firstly it is generally drier and the general direction of moisture will be from the render into the brickwork. If moisture runs the other way ettringite will form but it will also be self limiting as the water pressure will be away from the render thus forcing sulphates way from the interface. That is untill there is saturation. Secondly the weight of plaster is generally light and so gravity is unlikely to "pull" the plaster away from the render unless there ismsignificant attack which you might see with a risng dampor penetrating damp problem.Thirdly you don't exert impact and dynamic loadings to a wall.

Now let's look to the floor. The direction of moisture movement will be from the gypsum based screed toward the cement thus the concentration of sulphate ions is higher. You walk on the floor thus exerting dynamic and impact loads which will break the weakened bond at the interface. Other influences also come into play such as underfloor heating.

If you want to see an ettringite failure on a wal I shall take a pic of my kitchen wall and post it for you. The plaster is falling off the wall as I have a penetrating damp issue which I need to deal with.

The way to stop ettringite is to eliminate one of the three elements needed. The first being cement, the second being the potential contact between the cement and the third being moisture. These are dealt with by either removing the cement and using non cementitious adhesives whichis the most robust method, removing the contact by priming ideally with an epoxy primer or thirdly remove the water. This last is the hardest to do as there will always be some moisture present.

 

[Diamond Pool Finishers]

ALAN just a thought is there any screeding forum out there !? i think there could be room for one Dan

 

[Ajax123]

ALAN just a thought is there any screeding forum out there !? i think there could be room for one Dan

Never found one so assume not.