Anhydrite screed & Laitance removal - Tile Advice

[Holster]

We had a 50mm anhydrite screed down over a DPM around 100 days ago. For the first 50 days the conditions were far from ideal for drying @1mm/day (early Feb, definitely no where near 20 degrees air temp, fresh plaster throughout, room open to elements). After around 50 days we comissioned the UFH as recommended, and this has been running along with a dehumidifier for a few weeks now.

The floor wasn't sanded after 7 days, but was sanded with an STR around 70 days in.

We've done a RH floor box test and also a multiple tramex tests. Unfortunately the readings still appear to be too high. The driest part of the floor is where the floor prep machine has taken the scree down to white colour with a rough "sandpaper" like feel. This currently reads 1.0 on the tramex and 58% RH. The rest of the floor is pink/brown in colour with a very smooth-to-touch feel. Readings on this part of the floor are consistantly around 3.0 on the tramex everywhere (87%RH on the one section I measured).

So... In summary, the UFH has been on for around 3-4 weeks during good weather, with 1.5 weeks in combination with a dehumidifier.

When you remove the laitance, what sort of texture should you be left with? I'm wondering if the laintance has been baked on by the UFH, and we've only actually properly removed it in one place (where it's more white and rough to the touch).

Any advice would be gratefully received! We're getting desperate now!

 

[Chalker]

Once the latience has been on for that long, you will need a cup grinder to remove it. Or a proper surface grinding machine, if it a large area.
I'm surprised that the ufh can't dry it out though. Are you on gas or ground/air source?

 

[Flintstone]

It will struggle to dry out properly until that Laitence is removed, the super smooth surface is Laitence I think, once its sanded off it is lighter in colour and feels More like 2000 grit sandpaper imo. Smooth but not glossy

 

[Flintstone]

That's sanded with diamond cup vs none sanded on a floor that was down for 18 months and never sanded initially.

 

[Holster]

Thanks localtiler- that picture is exactly the two types I'm seeing. I think we have about 1-2mm of Laitance to removed

 

[culrossblue]

hi. just wondering what tramex machine you have and if you have the scale for anhydrite readings.

 

[Holster]

It is the Tramex CEMexpert II

 

[Ajax123]

Are you tiling the floor?

 

[Time's Ran Out]

Are you tiling the floor?

Good question !

 

[Holster]

Yes, but not me personally. I know it needs to be primed & ideally use a gypsum-based adhesive. The other half of the floor will be engineered wood.

 

[Ajax123]

Yes, but not me personally. I know it needs to be primed & ideally use a gypsum-based adhesive. The other half of the floor will be engineered wood.

It depends on timescales. There is a new system about to be launched which will allow tiling to commenced on Gypsol Anhydrite screeds after just 7 days and with a moisture content up to 95%rh. Should be available after 1st July.

 

[Ajax123]

It is the Tramex CEMexpert II

Not suitable for accurate moisture measurements on Anhydrite.

 

[Holster]

So what would you recommend Ajax? Also been doing hygrometer box RH tests

 

[Ajax123]

So what would you recommend Ajax? Also been doing hygrometer box RH tests

Hygrometer is ok. Carbide bomb Is most accurate.

What sort of temperatures have you run the underfloor heating.

Independent tests tell us that if force drying with underfloor heating the presence of laitance does not interfere with drying times.

 

[One Day]

I wish MY customers took an interest in moisture readings and sanding screed! It's always left to me to raise the issue, convince the client and take the readings and usually sand the screed!