4 Year Old Gyvlon (anhydrite) preparation advice

[Algy]

We had Lafarge gyvlon screed laid 4 years ago and are only now in a position to get it tiled. Up to now it was covered with temporary carpet.

As far as I can remember nothing has been done in prep (I know we should have removed the laitance a couple weeks after laying, but to be fair it looked sound and tiling was a long way in the future).

Now in looking at the surface it is still sound, surface clean, BUT very, very hard. Sanding it will be a challenge.

However, I want to do exactly what is right to have a lasting result. Nearly all forum advice seems to be to sand, but it is assuming that the screed is fairly freshly laid. What about 4 year old hard screed?

One professional advice was it had to be done and to be aware it would make an awful mess (we've now had a kitchen already fitted). A second professional says not needed now. So I am confused.

I've already decided to have a decoupling mat which every way we go, I really want to have a good result.

Desperate for advice please.

 

[Ajax123]

Lightly sand to remove anything loose and friable and to produce a surface key. Anyone who tells you that you have to sand through to aggregate (it does happen) is frankly talking bull. Vacuum it well. Make sure you run your underfloor heating if present for a couple of weeks before tiling as a precaution against any damp as given it's summer it will not have been on for a while and you may havecspillages etc. Prime it with a good quality acrylic or SBR primer and then tile using a gypsum based tile adhesive. If you have underfloor heating and are choosing a natural stone tile use an uncoupling mat stuck down with gypsum based adhesive.

 

[One Day]

Anyone who tells you that you have to sand through to aggregate (it does happen) is frankly talking bull. ...

Thanks Alan. Confirming what I've been saying for years now!

 

[Flintstone]

I’ve sanded a couple of these sorts of screed that have been down for 2 years with underfloor heating in use, and they become very hard to sand with even a diamond grinding cups. You need to remove enough that it will take on some primer in my opinion. Any photos of it ?

 

[One Day]

I've seen them ground down to aggregate and all that does is expose the VERY thirsty body of the screed.
If the laitence is hard-baked on, then it's not coming loose naturally. You may as well fix to it, following Alan's advice above.

 

[On one]

In this instance what dilution ratio (if any) would you use if you were using say, SBR
Just seems be an important part of the job which can really go t*ts up,if not done correctly.

 

[One Day]

In this instance what dilution ratio (if any) would you use if you were using say, SBR
Just seems be an important part of the job which can really go t*ts up,if not done correctly.

Whatever is needed to adjust suction. The main cause of failure is failing to remove loose laitance, or laitance which will or could develop or become loose with the residual moisture/heat cycles in young screeds.
After 4 years, if that laitance is stuck fast, it's staying stuck fast.

Of course, if you are using cement based adhesive then a separation layer is required - so prime according to manufacturer's recommendations. (they vary so much)

This is my understanding - if I'm wrong... well I learn!

 

[Flintstone]

What I am saying impish is if it’s got baked on laitance, it will be like a silky crust that won’t take on any moisture, id always remove as much as possible

 

[One Day]

What I am saying impish is if it’s got baked on laitance, it will be like a silly crust that won’t take on any moisture, id always remove as much as possible

Ok, I wouldn't. I would sand off anything loose with 60grit s/c pads and only prime if I felt it needed it, or I had to use cement.
I've never fixed direct to anhydrite. Always used ditra or rapid mat stuck with gypsum based.

 

[Algy]

Thank you for your help. I've attached photos (one with and one without flash - since the flash does make it look whiter).

Based on your thoughts, I will hire a floor scarifier. I'm thinking to mainly take a small amount off to provide a decent key, rather than dig down deep (unless as I do it the floor looks like it has a lot of flaky surface). I'm hoping just doing enough to key the surface will not be as drastic with dust in our now fitted kitchen.

The amount to take off is the thing that is difficult to know. I'm supposing it is a millimetre or so?

Lafarge Gyvlon before sanding (with flash)

• Algy
• Jul 10, 2018

Lafarge Gyvlon before sanding - (no flash)

• Algy
• Jul 10, 2018