Alfa Heavy Anhydrate.

[CJ]

Anyone heard of it?

looks like an ordinary screed. No laitence.

 

[Stef]

Alan's (Ajax) your man for this!!

 

[Ajax123]

Had to laugh .... Sorry.... No offence.

It's alpha hemi hydrate. And it is a calcium sulphate screed. It will have Laitance but it will not be a soft friable Laitance. In terms of prep it should be treated the same as any other calcium sulphate screed. I.e. Sand it, vac it, dry it, heat it and cool it (if heating is present), test it for residual moisture and if dry prime it as appropriate and tile it

 

[Stef]

Alan, sorry to hijack this but how soon can I remove laitance from a Gyvlon, poured on Monday 55mm.
Does this cold weather affect the drying times of calcium screeds?
Heating hopefully going on end of next week.

 

[Ajax123]

Alan, sorry to hijack this but how soon can I remove laitance from a Gyvlon, poured on Monday 55mm.
Does this cold weather affect the drying times of calcium screeds?
Heating hopefully going on end of next week.

Typically removal would be about 5 to 7 days.

yes cold weather will affect the drying time. 1mm per day up to 40mm and 2 days per mm there over in hood drying conditions at 20oC and 60%rh. We have neither at present so drying rates will be slower and drying times longer. Force drying with the heating will help enormously.

 

[CJ]

Had to laugh .... Sorry.... No offence.

It's alpha hemi hydrate. And it is a calcium sulphate screed. It will have Laitance but it will not be a soft friable Laitance. In terms of prep it should be treated the same as any other calcium sulphate screed. I.e. Sand it, vac it, dry it, heat it and cool it (if heating is present), test it for residual moisture and if dry prime it as appropriate and tile it

I was close.

Job I looked at today......has this stuff, (although it does look like an ordinary screed, no laitence) ...with piped UFH

I explained the procedure to the customer, (mech sanding, Hoover, prime) so he phoned the company there and then......and this company told him it "just needs priming"

This floor has been down 3 months (so he told me)

 

[Ajax123]

I was close.

Job I looked at today......has this stuff, (although it does look like an ordinary screed, no laitence) ...with piped UFH

I explained the procedure to the customer, (mech sanding, Hoover, prime) so he phoned the company there and then......and this company told him it "just needs priming"

This floor has been down 3 months (so he told me)

i have heard this a number of times recently and I think there is a knowledge gap with the reps who supply the binder in terms of floor coverings. It is fair to say that it probably won't have a loose friable Laitance but Laitance is a function of migration of bleed water. Even sand cement screeds produce a Laitance. The advice of the contract flooring association, the tiling association, the adhesive and primer manufacturers and all of the industry experts is that it should be sanded. If you don't and it fails, do you think the company that told him "just needs priming" will be willing to stand by your side and defend you. And how successful do you think they would be with all of the other experts agreeing it should have been sanded. For the sake of what it costs why would your client want to risk that scenArio.

 

[beanz]

Alan, if it's not a "loose friable laitance" why would it fail? Don't get me wrong, I'm not disagreeing with you, just trying to understand the process!?!

 

[Ajax123]

Alan, if it's not a "loose friable laitance" why would it fail? Don't get me wrong, I'm not disagreeing with you, just trying to understand the process!?!

Because there is often very fine dust on the surface of these screeds which can interfere with the penetration of primers. Also the screed has been in a construction site for a number of weeks or months and will have surface contamination from extraneous materials e.g. Mud, plaster, mortar, paint etc etc. Also the surface of these screeds can be very hard and very dense which further interferes with the penetration of primers. Sanding opens the surface texture and subsequently provides a mechanical key for said primers etc. it also opens the surface to promote faster drying. I can obviously only offer guidance based on experience and the national and industry best practice codes. It may be that if you tile it you would get away with it and it would not fail. But, if it does, you would stand alone in sorting it out because the first questions you would be asked would include "did you sand the screed as per the instructions of the tiling association guide to tiling to calcium sulphate screeds and as per the instructions on pretty much every bag of tile adhesive in relation to these screeds" I know that is what I would ask....


sanding is is not just to remove Laitance but is to prepare the screed to receive bonded floor coverings...

 

[Stef]

Alan is there any other level screeds on the market that dry quicker than a calcium screed?
Or can be tiled quicker I should say.

 

[Ajax123]

Alan is there any other level screeds on the market that dry quicker than a calcium screed?
Or can be tiled quicker I should say.

Yes but there are always trade offs. There are cement based screeds that dry in hours rather than days but they are expensive and don't offer benefits of depth reduction and the environmental aspects. There is not a fast drying pourable cost effective screed that can be laid very thin and offers all the advantages that anhydrite gives. That would be utopia but to be fair the Gypsol technical bowfins are working on it. We can produce an anhydrite screed that dries very fast but is not pourable so only gets us half way there. Still....small steps eh!

 

[CJ]

These newer pourable screeds do look like an ordinary sand/cement screed. Sometimes unless it's obvious, we could end up in the cack.

 

[Ajax123]

These newer pourable screeds do look like an ordinary sand/cement screed. Sometimes unless it's obvious, we could end up in the cack.

Hemi I hydrates will often look like cement screeds in colour because there is often a small amount of cement in them. The colour is also affected by the sands used. I have seen a few in the south west that use a very dark sand which makes them look dark grey.

 

[Ajax123]

These newer pourable screeds do look like an ordinary sand/cement screed. Sometimes unless it's obvious, we could end up in the cack.

Thats why you need to ask the questions...

 

[CJ]

Thats why you need to ask the questions...

I always do......but quite often, they don't have a clue.
I'm in the South/West, and this particular floor DOES look like a standard cement screeded floor. Surprisingly, they told ME!