Anhydrite screed prep

[Silane]

A simple question I'm struggling with.

Up front, it's a self build - my 4th, so done a fair bit but 1st time using anhydrite.

Screed down for 4 years+. Prepping for ditra. Started sanding with 40 grit using 6"festool orbital to remove laitance - solid - (so may have to use diff tool).

Issue I have - surface (when sanded with 40 grit) is a tough mushroomy colour - I can see whitey/ creamy colour aggregate beneath it where the air bubbles have burst.

Before sealing do I need to take off top mushroomy coloured surface to reveal agg beneath? -

Mushroomy surface is solid and we'll bonded to agg beneath - hard to even scratch it with screwdriver - my common sense brain is telling me that primer will bond well with top surface as is - so why remove and expose the more fragile agg beneath.

Appreciate your thoughts

Si

 

[Colin broom]

I've only fitted on anhydrite screed once the builders removed the brown stuff that is on the top with a industrial sander I think. I do know I was told to used a anhydritetiling adhesive and could only start fitting once the screed was below 65% Rh. Hope this helps

 

[Dave]

Hi. Is the screed heated.

 

[Silane]

I've only fitted on anhydrite screed once the builders removed the brown stuff that is on the top with a industrial sander I think. I do know I was told to used a anhydritetiling adhesive and could only start fitting once the screed was below 65% Rh. Hope this helps

Cheers Col

 

[Silane]

Hi. Is the screed heated.

 

[Silane]

Dave,

Wet ufh. Pressurised but not commissioned - I know ideally should commission b4 flooring but passive House insulation level and decided to use infared panel heaters combined with log burners and mvhr instead of gas boiler - using ditra in case do commission later on.

Floor is dry as a bone and (mushroom coloured) surface rock hard

 

[Dave]

Dave,

Wet ufh. Pressurised but not commissioned - I know ideally should commission b4 flooring but passive House insulation level and decided to use infared panel heaters combined with log burners and mvhr instead of gas boiler - using ditra in case do commission later on.

Floor is dry as a bone and (mushroom coloured) surface rock hard

It really needs it’s first heat into the screed imho.
As for the laitence , if you contact a flow screed pourer etc , most do a sanding service.

 

[Silane]

It really needs it’s first heat into the screed imho.
As for the laitence , if you contact a flow screed pourer etc , most do a sanding service.

Understand that point re first heat: taking a chance on basis that any subsequent expansion & contraction will be similar to initial if starting from similar base temp (as ufh wouldn't be needed that often - installed it as retro fitting not poss).

I'm happy to do the work - it's part of what I do - just don't want to remove what I think is a good solid bondable surface - guess my concern is that primer won't penetrate surface & therefore surface liable to debond from the screed - maybe just overthinking - wouldn't be first time.

 

[Ajax123]

To use “traditional” prep methods The surface needs to be clean sound and absorbent. As the surface is currently solid it is definitely sound. Clean … well that’s fairly straight forward… A simple test for the level of absorbance is the water drop test. Get a few drops of water (you could use perhaps a spent lateral flow test bottle or similar) to put a few drops of water on the surface of the screed. It should soak in pretty much immediately. This, when you prime it allows the water in the primer to dial in pulling the polymer in as well. If the water sits on top of the surface for more that a couple of seconds it’s not absorbent. You can get specialist primers for non absorbent surfaces but I’ve never had much success with them. So you are best to sand/grind it to open up the surface a little and make it more absorbent.

Alternatively you could use a pressure sensitive vynil flooring adhesive to stick down the sites matting.

Final alternative you could use bal flexbone and avoid any prep at all.

 

[Dave]

Understand that point re first heat: taking a chance on basis that any subsequent expansion & contraction will be similar to initial if starting from similar base temp (as ufh wouldn't be needed that often - installed it as retro fitting not poss).

I'm happy to do the work - it's part of what I do - just don't want to remove what I think is a good solid bondable surface - guess my concern is that primer won't penetrate surface & therefore surface liable to debond from the screed - maybe just overthinking - wouldn't be first time.

If you heat slowly then it should be fine. I would deffo sand it though for an acrylic primer method.