Anhydrite adhesive test.

[DHTiling]

Sand and cement render with a gypsum finish

The cement has hydrated so no reaction.. it is putting cement to gypsum 🙂 not gypsum to cement.

 

[White Room]

The gypsum plaster is put on the next day to render backings so some of the moisture is left so not draw moisture out of the finish or it could become friable or get crazing.

Went through this on the mapei day and the guy who was giving the lesson thought that we put a barrier between the finish and render which should'nt be necessary unless the background has been left so long it's so dry it needs a bonding agent/pva.

 

[Protilers]

Well.... thanks for taking time out of your day to educate the masses in you findings........great stuff........thanks

Lee

 

[bugs183]

Tis an honour, it wasn't to scale that you fellas do stuff though!

 

[Andrew]

Ok makes sense! Cheers dave clears that up! I don't claim to know loads about anhydride screeds. Have tiled quite a few. one of the first with builders spec not sanded ardex p51 primer tile fixed with cement based flexi, I was ignorant to the type of screed and didn't question it then. That was few years ago and doing fine! I've returned to it and it's good as new. Unlike the 30 m I had to reset that was fixed with gypsum base. Just leaves me a bit unsure

 

[DHTiling]

Every day can be a learning day... keep tabs on this thread as we are all learning. 🙂

 

[bugs183]

Well thats the plan, it would be great to be able to approach this with confidence.
As i said earlier Sab at Creative Impressions spoke with good authority and approached things from a tilers perspective, we were on the same wavelength, so touch wood in a few weeks i'll be in a good mood using his adhesive!

 

[Gazzer]

As another very small test, while using Gypfix with porcelain tiles I spread the back of a tile and put it to one side. 6 days later the adhesive has stuck extremely well to the tile. This tile was not fixed to the screed so this makes me wonder why ?

 

[Ajax123]

As another very small test, while using Gypfix with porcelain tiles I spread the back of a tile and put it to one side. 6 days later the adhesive has stuck extremely well to the tile. This tile was not fixed to the screed so this makes me wonder why ?

It will most likely be due to strength gain due to moisture loss. Gypsum loses strength when it is wet or rather gains strength by losing moisture. It has free moisture loss from the underside of the adhesive. If it was stuck on the screed this loss of moisture wold be impeded by the fact that it is trapped between the screed and the tile. It would achieve enough strength to stick as the screed would pull the moisture but but as this would dissipate into the top part of the screed the moisture loss therefore strength gain may be slower. The chemical reaction or crystallisation of gypsum in anhydrite screed accounts for about 65% of its overall strength. The rest of its strength is due to friction between the gypsum crystals which increases with drying. i have to make the assumption that gypsum adhesive is similar. If that makes sense.

 

[jquin]

Ok makes sense! Cheers dave clears that up! I don't claim to know loads about anhydride screeds. Have tiled quite a few. one of the first with builders spec not sanded ardex p51 primer tile fixed with cement based flexi, I was ignorant to the type of screed and didn't question it then. That was few years ago and doing fine! I've returned to it and it's good as new. Unlike the 30 m I had to reset that was fixed with gypsum base. Just leaves me a bit unsure

Andrew - might have a job for you if you're interested. Not far from Surrey. Have sent you a PM.