Anhydrite Primer

[floorlayer]

I'm about to lay Karndean tiles on an anhydrite screed which has hot water underfloor heating. The screed is thoroughly dry and has had the laitance removed. The tile supplier advised priming with Uzin 360 and then smoothing with Uzin NC110.

My questions are:- the surface is already smooth therefore why smooth it again? and because NC110 is also gypsum based, would I need to prime again with Uzin 360 before applying the Karndean HT adhesive?

If I was to go the Mapei route and use Eco-Prim T primer, what would be the next stages in preparation and tiling with their products?

 

[Ajax123]

If the surface is very smooth and blemish free it is fesible to lay karnden direct to it. I have seen it done a few times. However bear in mind that any blemishes in the screed will show through the tiles. I totally agree with PE360 and NC110 combo. The screed is heated and so the heating must be comissioned and run before you put anything at all down.

How do you know it to be thoroughly dry?

If you were to use a cement based leveller there is a real risk of chemical reaction between the screed and the leveller so you would need ideally to change the primer to a water dispersible epoxy. This has implications on the prep.

When you say the laitance has been removed....is this a low laitance screed or has it been sanded to remove skin. I have come across numerous instances where people have said that the low laitance screeds do not need to be sanded. This is not always correct as it may not need sanding to remove laitance but may need sanding to promote adhesion and primer penetration. Also if you were to use a WDE primer the screed would almost certainly need to be scarfified to provide a rougher open textured surface again to encourage penetration. The imnportant thing is that the primer has to be penetrative. PE360 is so I would have no real worries with it. If it has been sanded to remove the skin priming it will help to stabilise the dusty surface and is likely to leave a surface that needs to be levelled to remove minor blemishes which would otherwise show thorugh the karndean tiles.

You are always better not to mix cement with gypsum if you can avoid it and it does not seem like there are any technical reasons for using cement leveller in this instance so stick with a gypsum base and you run fewer risks.

There are quite a few other gypsum based levellers available.
Laybond Screedmaster Gyspum, Creative Impressions GBLC01, PCI CSP43 and Forbo europlan Alphy 920 are a few.

If you want contact details for these so you can price check then PM me.

As for priming the top of the leveller this would depend on the adhesive. I am not familiar with Karndean HT.

 

[Aston]

hi mate

the surface of anhydrate screeds are quite weak when they've dried so for tiling you would mechanically key using a scarifier etc because the surface is so smooth. then apply 2 coats of acryllic sealer prior to tiling and i suppose laying karndean/amtico etc would be the same to ensure a stable bond..

 

[Ajax123]

hi mate

the surface of anhydrate screeds are quite weak when they've dried so for tiling you would mechanically key using a scarifier etc because the surface is so smooth. then apply 2 coats of acryllic sealer prior to tiling and i suppose laying karndean/amtico etc would be the same to ensure a stable bond..

They should be pretty hard and strong if laid properly. They usually only go soft if they are over watered. The old fashioned skinned screeds do have an issue with dust once they have been sanded which is another good reason for using WDE primers as they significantly improve the pull off strength of the screed. The newgeneration screeds are much more surface stable and durable so requirements for prep much reduced. Some prep still needed same as any other screed though.

 

[floorlayer]

Thanks for the advice guys, particularly Ajax.

From reading other posts here, there is smooth and there is SMOOTH and apparently glasslike is the aim.

Regarding the dryness of the screed, since the heating has been turned on and slowly ramped up I have been checking it by placing a cold sheet of glass just above the surface. Over time, repeating the test has shown that the condensation has reduced to zero, however, prior to priming I will have a proper moisture test carried out.

There was actually very little laitance and the surface seems quite hard, harder in fact than I had expected, given the nature of the material.

Karndean HT is their high temperature adhesive for use with under floor heating and in other high temperature situations.

 

[Ajax123]

Thanks for the advice guys, particularly Ajax.

From reading other posts here, there is smooth and there is SMOOTH and apparently glasslike is the aim.

Regarding the dryness of the screed, since the heating has been turned on and slowly ramped up I have been checking it by placing a cold sheet of glass just above the surface. Over time, repeating the test has shown that the condensation has reduced to zero, however, prior to priming I will have a proper moisture test carried out.

There was actually very little laitance and the surface seems quite hard, harder in fact than I had expected, given the nature of the material.

Karndean HT is their high temperature adhesive for use with under floor heating and in other high temperature situations.

The moisture test kit you "technically" should use is the Hair Hygrometer. If you want to go down this route they are available from F Ball and Co and from Tramex (and probably other places as well) for about £80 ish. Try and buy the analogue kit if you can as these tend to be more accurate. The digital ones also work but are less resonsive and in my experience less accurate. The test method is to place the hygrometer on the screed surface once it has cooled for 48 hours and leave it in place havin sealed it to the floor with the water free tape provided for 48 hours. ead it and then leave it for a further 24 hours and then read it again. If the two results are below 75% RH and within a few percent of each other then the screed can be considered dry. If the results are significantly different and under 75% RH then the test should be repeated. If ither result is above 75% the screed is still drying.

However if you use Gypsum Based leveller you will get away with a slightly higher moisture than you would with cement. You can do the old Poly Bag test - or repeat your glass test. Place the glass directly on the surface of the screed - put a house brick on it and then look at the screed underneath after about 48 hours. If the screed does not turn darker and there is no condensation or moisture then your screed is probably dry. I have to recomend a proper moisture test as that is the British Standard and I would be a hypocrite to ignore it. Having said that I reliesd personally on the poly bage test for my own conservatory which is also anhydrite (Gyvlon in this case).

As for smooth you need to bear in mind that smoth and flat are 2 different things. Yes you want the floor flat so that no blemishes appear and the floor looks flat and good in oblique lighting etc. However smoothness is not alwaye desirable. The smoother the surface the less chance there is for the primer and the adhesive to form a mechanical bond with it as there is nothing to grab on to. This sounds to me like the new generation low laitance screed which is indeed much harder and smoother and more predictable than the old fashioned stuff. It is good practice to run a floor sander over any screed - this is no exception IMO - using something like a 60 grit paper. A bit like sanding your wood work before painting it. It roughs the surface to provide a key for the primer. It also removes any other contamination on the surface of the screed at the same time. A good going over with the dyson afterwards removes most of the dust. You want the surface such that when you run your hand over it it comes off reasonably clean. You will never acheive a completely dust free surface so don't worry if you get some dust on your hand. The reasion you need a penetrative primer is because surface primers do not penetrate the surface (PVA for example) but sit on top. If the top is covered with dust yhe primer just sticks to the dust. A penetrative primer will act as a stabilising solution so will help to add cohesiveness to the screed surface. PE360 is particularly good for this but does not prevent ettirngite reaction so you should not use cement on top of it. Stick with gypsum and you will not go far wrong.

Will have to look up the addy to give any opinion on that but the description sounds ok. However if you are using Uzin for the primer and leveller why not talk to them about the adhesive as well. Again if you need the right contact PM me and I will let you have the name and number. If you use them for the complete system there is less chance of arguments later on if things do go wrong - not that I have any reason to expect them to do so - just being pragmatic....

That's it for now so hope this helps

Alan

 

[Ajax123]

Oh by the way - forgive my typing - I having a beer or two - if you need more clarification just ask

 

[White Room]

Just come up for fresh air:lol:

 

[Ajax123]

Just come up for fresh air:lol:

It's been a while whitebeam