Anhydrite screed question

[Matthew77]

Hi All

I have a couple of questions regarding tiling prep for Anhydrite screeds and was hoping someone here might be able to offer some advice?

A few months back i quoted for a floor tiling job for a wet underfloor heating system. The screed the builder decided to use was a gyvlon thermio+. I was sent some info regarding drying times, priming, adhesives and removal of surface laitance by the company that laid the screed.

The screed has been down a few months now so I went to the job today and started to sand the screed to remove the laitance. What i was expecting was to be sanding a fairly loose dusty film off the surface and when i started sanding it with my floor polisher and some 60grit carbon paper the laitance was coming off quite easily revealing the courser texture of the screed underneath, but this soon started to be really hard work and after a few hours it seemed like i was getting nowhere with it. So I went and hired a big floor sander and some 40grit sanding belts for it and spent the next few hours making lots of dust, but hardly starting the surface. :mad2:

So I guess my question is do I need to remove this surface layer of laitance if its not coming off when sanding? I have about 30m2 of screed to prep and today I think I've just about done 10m2. Also after doing a bit more research into removing laitance I've found a few sources saying that the laitance should be removed between 4 and 6 days after the screed is poured, this info was not mentioned in the documents the screed company gave me just said "Ensure that the surface ofthe screed is free from laitanceor surface skins and then priorto continuing, the surfaceshould be sound and dust free. The removal of loose material suchas laitance or dust is normally a straight forward procedure"

Any advice here would be handy thanks.

I'm also going to include a photo as that might help

 

[Stef]

Hi, the longer the laitance is on the harder it is to remove as you have found out.
This must be removed before tiling, I take it the heating has been commissioned?
It has baked it onto the surface.

 

[Matthew77]

Hi Stef

Thanks for your reply. Yep your right about the heating was turned on last week, its been on for about a week now!

Any tips for removing it? The sander weighs a tonne so should be putting plenty of pressure onto the sanding belt and 40grit was the lowest grit they had?

Its literally not even touching it any more!!

 

[stusgsx]

I thought the laitance was meant to be removed within the first 2 weeks?

 

[Stef]

I've used a belt sander before to remove the laitance & it removed it easily but the screed hadn't been down long.
Boys on the job I'm on the now used a carbinunbrum machine. Tried to google it but can't find any info on it.
No mess with it as it has an industrial vacume with it to extract all dust.

 

[radim]

i used something like this before
http://www.hss.com/hire/p/htc-gl450-grinder

 

[Matthew77]

I thought the laitance was meant to be removed within the first 2 weeks?

This wasn't mentioned in the info given to me by the screeding company, just that you should remove the laitance. I also spoke to 2 different adhesive technical guys explaining it was my first job onto anhydrite asking for suitable materials and the process involved, they both failed to tell me this had to be done within 2 weeks.

 

[Matthew77]

I've used a belt sander before to remove the laitance & it removed it easily but the screed hadn't been down long.
Boys on the job I'm on the now used a carbinunbrum machine. Tried to google it but can't find any info on it.
No mess with it as it has an industrial vacume with it to extract all dust.

thanks might have to look into one of those. If I can find some info on it.

 

[Matthew77]

i used something like this before
http://www.hss.com/hire/p/htc-gl450-grinder

Thanks will look at this option too. did you not catch the laitance in the first few weeks either?

 

[Stef]

thanks might have to look into one of those. If I can find some info on it.

It's the same one as the link that Radim just put up.
What adhesive have you been advised to use on the job?

 

[Matthew77]

It's the same one as the link that Radim just put up.
What adhesive have you been advised to use on the job?

There were a few suggestions but I decided to go with Anhyfix by tilemaster as it was fast setting and I already use their level flex leveller and ultimate flex adhesive on certain jobs and have been very happy with both of those products.

So the machine Radim posted is a sander that uses silicone carbide type abrasives which is the same abrasive pads i started using originally at a 60grit these seem to be doing nothing now!

 

[judge]

Ive not had one job where the latience has been removed.Only the jobs that ive been involved with from the start and removed it myself after 7 days.Nobody seems to advise on latience removal.I still get "i was told a mm a day to dry !" with most the jobs i look at.:smilewinkgrin:Oh,ask your local hire company if they can get you an"STR" with a copper plate.

 

[Matthew77]

Would anyone be able to please explain to me why if the laitance is stuck so well to the screed that the sanding pads are having real trouble removing it why it can't be tiled onto....then I can relay this information back to every other trade that asked me every 10 mins why i can't just tile straight onto it as "its obviously fine to tile onto"?

 

[Matthew77]

Ive not had one job where the latience has been removed.Only the jobs that ive been involved with from the start and removed it myself after 7 days.Nobody seems to advise on latience removal.I still get "i was told a mm a day to dry !" with most the jobs i look at.:smilewinkgrin:Oh,ask your local hire company if they can get you an"STR" with a copper plate.

yeah I get the classic "its defiantly dry now" after 2 weeks, which I reply "nah its defiantly not dry yet" which then comes" we've had the window open, Its defiantly dry"

Could you please explain what an STR is?

thanks

 

[Stef]

Would anyone be able to please explain to me why if the laitance is stuck so well to the screed that the sanding pads are having real trouble removing it why it can't be tiled onto....then I can relay this information back to every other trade that asked me every 10 mins why i can't just tile straight onto it as "its obviously fine to tile onto"?

The surface layer is a friable layer that must be removed.
All the contaminates that are in the screed float to the surface.
I'm not exactly clued up from the real technical side of these screeds, our resident expert Ajax is the technical guy for explaining it.
I have never tiled to an calcium screed without the laitance being removed & I never will.
AnhyFix is the adhesive to use, it's sticks like the proverbial.

 

[Matthew77]

The surface layer is a friable layer that must be removed.
All the contaminates that are in the screed float to the surface.
I'm not exactly clued up from the real technical side of these screeds, our resident expert Ajax is the technical guy for explaining it.
I have never tiled to an calcium screed without the laitance being removed & I never will.
AnhyFix is the adhesive to use, it's sticks like the proverbial.

Thanks at least by the sounds of it I have chosen the right adhesive!

I don't plan on tiling onto it without the laitance being removed either as everything I've read about these screeds the emphasis seems to be on removing the laitance and making sure the screed is dry. Its a shame theres not a bit more info around regarding the importance of the removal time of the laitance as it is hard as hell now!!! I've tried floor scrapers, a saw blade in a floor scraper, wire brush attached to an angle grinder and nothing seems to want to touch it!

 

[Stef]

The link that Radim put up is a floor grinder, it's got replaceable pads on it.
If that doesn't remove the surface laitance then nothing will.

 

[judge]

Here you go mate http://www.brandontoolhire.co.uk/en/surface-preparation/534-floor-preparation-system.html

 

[Matthew77]

The link that Radim put up is a floor grinder, it's got replaceable pads on it.
If that doesn't remove the surface laitance then nothing will.

So just to clarify, in my picture the light parts on the floor are laitance free and the darker parts are the laitance that need removing? I'm only asking this as i went over the top of the darker bits (with a new sander belt on) and let it spin over the same place for a few minutes and all it seemed to do was make it darker in colour!!

 

[Matthew77]

Here you go mate http://www.brandontoolhire.co.uk/en/surface-preparation/534-floor-preparation-system.html

Thank you.

 

[Coop5]

I feel like a dumba$$ as ive literally just posted the same question!

I had a gyvlon screed layed over my ufh, and now I realise that it needs sanding and priming before tiling. . Doh !! I was just told it takes a day per mm to dry out.

Anyhow, my system has not been on yet so may not be as tough as yours. But I now need to get sanding asap!!

On inspecting the surface, I can see a few black bits which look like rubber! I assume this is the type of thing we need to remove !?

 

[Stef]

The laitance on top of the screeds probably only 1mm.
I remember the 1st one I done, I done the exact same thing, you will end up going into the actual screed.
It's only the top friable layer that needs to come off.

 

[Matthew77]

The laitance on top of the screeds probably only 1mm.
I remember the 1st one I done, I done the exact same thing, you will end up going into the actual screed.
It's only the top friable layer that needs to come off.

So the top pink layer which I thought was the laitance which is about 1mm thick doesn't need to be removed it's just the friable layer on top of the pink stuff that needs to come off!?

None of the top layer is friable it's all really well stuck down! This is the part that confusing me the most so I've kind of decided to remove all the pink stuff just to be safe?

I've been using a diamond disk grinder this morning and have managed to remove a lot of the pink layer down to the whiter coloured screed. After lunch I will carry on and finish removing the rest.

Thanks for everyone's input on this it's been really helpful, next time I have a anhydride screed to tile I will be better prepared due to the information here and hopefully help others too. Especially the part about removing the laitance shortly after the screed has been poured!

 

[Coop5]

Hi mate,

I have the exact same issue with my new extension.

Its been down now for nearly six weeks and I've only just realised about the laitance needing to come off.

I have been on the phone to the company who make my screed, they state that the surface (which is currently very smooth) needs to be sanded down so that it appears to have the same texture as a matchbox strip!

I have ordered a str710 for this job, which comes with copper disks and varying other corses.

what sander did you end up going for ?

Cheers

 

[Coop5]

Hi mate,

I have the exact same issue with my new extension.

Its been down now for nearly six weeks and I've only just realised about the laitance needing to come off.

I have been on the phone to the company who make my screed, they state that the surface (which is currently very smooth) needs to be sanded down so that it appears to have the same texture as a matchbox strip!

I have ordered a str710 for this job, which comes with copper disks and varying other corses.

what sander did you end up going for ?

Cheers