Anhydrite screed question - Tile Advice

[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.