4 Year Old Gyvlon (anhydrite) preparation advice

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.

 

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.

 

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

 

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 ?

 

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.

 

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!

 

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

 

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.

 

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) by Algy posted Jul 10, 2018 at 10:24 AM

Lafarge Gyvlon before sanding - (no flash) by Algy posted Jul 10, 2018 at 10:22 AM

 

Oh and the primer I was recommended by the original flow screed layer is Ardex P51 - any thoughts on that?

 

If you removed 1mm of laitance, you would be down to aggregate
There is a lot of conflicting advice as to how much laitance to remove but personally I take off enough to expose the aggregate then prime with a good quality primer sticking a anti crack mat down with a gypsum based adhesive then cement based above mat to stick tiles with.

 

And how do you measure this mythical 1mm. Sorry but it's rubbish. I've heard everything from 1mm through 5mm to one adhesive rep who said you have to sand until it's too hard to sand further and if you get through to the pipes the screeder has to replace the screed.

You are simply sanding off any loose friable Laitence with loose and friable being the key phrase. You don't scratch off with a screwdriver, you don't scrape off with the corner of a paint scraper, you don't bang it with the edge of an old chisel. Once the surface is sound stop sanding. This could be as much as 1mm but as little 100 microns. It's not measurable in depth terms. The requirement to expose aggregate is a myth. The screed is porous enough to accept a suitable primer e.g p51, p121, prime plus, apd, ar and many others.

The most important elements are make sure it's dry enough and where possible use gypsum based adhesive

 

Typically 1:1 with clean water although it depends on whose primer it is. Some are 1:3 and some are already dilute enough. If in doubt test it in a small area first. If you are using gypsum adhesive you are in my trying to reduce suction so much less primer required. The water soaking into the surface via the life structure effectively pills the polymer in as well. If using cement you need to form a chemical barrier so more primer but then it can skin over so is higher risk. Sometimes of cement is used it's better to use an epoxy primer.

 

Why the need for such an arrogant response? I know you are a very well respected member from the screed industry and I wasn't trying to pick a fight! I was saying that this was my personal method of laitance removal from liquid screeds. I have invested a lot of money in sanding equipment, hoovers etc and have done a fair bit of laitance removal and this is what Ive found to be the better method.
From copper discs to sanding pads to diamond cups, Ive tried them all numerous times and honestly a 60grit sanding pad will do absolutely nothing to a screed. A copper disc will scratch it but not remove all of it and a diamond cup setup is by far the best .