Sand & priming anhydrite screed

[YorkieSelfBuild]

Hi

 

I'm having anhydrite screed over wet UFH heating on GF. I'm getting conflicting info on applying primer after it's sanded to remove laitance. 

 

My architect's "national tiling contractor" says sand then "need to get some sort of primer down straight after to keep the surface free of contamination and reduce the dust". Then when it's dry, add another 2 coats before tiling.

 

I've spoken to a Kerakoll sales manager who said sand about 7 days after pouring but apply 2 diluted coats of primer only when it's dry. Doing before it's dry means it takes longer to dry.

 

Who's right?

[nod]

I tile loads of these screeds 

Scrape it before tiling Then a coat of 3-1 SBR I’ve tiled 1000s of meters without any issues 

The beauty of this kind of screed is that you hardly need any drying out time and there’s very little shrinkage

No need for crack matting 

[Beau]

We had an anhydrite screed. The advice at the time was leave to dry, sand/grind the surface to remove the surface fines and then prime. I used a diamond grinder on small angle grinder which made one heluva mess but worked. The primer was something I think called Ozin 260 but can check tomorrow if helpful as I think the old tub is still sitting about. Think it was only one coat. Anyway that was 18 years ago and tiles have never budged

Edited March 13 by Beau

[Barnboy]

I had an anhydrite screed poured at the start of October. All the info I have found has lead me to give it great care. What I have been told is, after 24hrs start giving it good ventilation, after 7 days give it a sand with a proper floor sander/ grinder to remove the laitance, you'll see it on the surface. I hired a grinder as the floor contractor went over it with a sander but I think he was being chased by Usain Bolt as he hardly touched the surface.

You can turn your ufh on after 14 days and use that to help dry it out. I left ours 5 months before turning the heating on and couldn't believe the amount of moisture still coming out of it. I followed the info for tehnal shocking the floor by running my ufh later on, starting at 25⁰c and increasing by 5⁰c per day until at operating temp, keeping it att his temp for 7 days before slowly decreasing it by 5⁰c per day.

I've now got a hygrometer box on the floor and am testing the Relative Humidity of the screed, I need to be 75% or lower, I'm at 74% at the moment. 

If the screed tests ok in a few places then as we're using Mapei adhesive I'll be priming it with their Primer G diluted, once that's absorbed and dry I'll give it another coat of neat Primer G at 90⁰ to the 1st coat before laying the antifracture membrane. 

I can't get hold of any gypsum based tile adhesive locally, otherwise I'd use that to lay the antifrac membrane leaving the cement based adhesive isolated from the anhydrite by way of the membrane.

I'll also be putting in expansion joints at each doorway.

3 days after turning my heating off I noticed 1 crack in the screed, so they do have movement in them.

[Mr Punter]

I have used Cemfloor flowing screed and there does not seem to be such an issue with laitance as it is cement based.

[YorkieSelfBuild]

On 13/03/2025 at 16:35, nod said:

I tile loads of these screeds 

Scrape it before tiling Then a coat of 3-1 SBR I’ve tiled 1000s of meters without any issues 

The beauty of this kind of screed is that you hardly need any drying out time and there’s very little shrinkage

No need for crack matting 

So you wait until it's dry enough to tile before sanding? I've read it's better to do it after 7-10 days as it gets much harder later and needs diamond discs

Edited March 14 by YorkieSelfBuild

[YorkieSelfBuild]

8 hours ago, Barnboy said:

I had an anhydrite screed poured at the start of October. All the info I have found has lead me to give it great care. What I have been told is, after 24hrs start giving it good ventilation, after 7 days give it a sand with a proper floor sander/ grinder to remove the laitance, you'll see it on the surface. I hired a grinder as the floor contractor went over it with a sander but I think he was being chased by Usain Bolt as he hardly touched the surface.

You can turn your ufh on after 14 days and use that to help dry it out. I left ours 5 months before turning the heating on and couldn't believe the amount of moisture still coming out of it. I followed the info for tehnal shocking the floor by running my ufh later on, starting at 25⁰c and increasing by 5⁰c per day until at operating temp, keeping it att his temp for 7 days before slowly decreasing it by 5⁰c per day.

I've now got a hygrometer box on the floor and am testing the Relative Humidity of the screed, I need to be 75% or lower, I'm at 74% at the moment. 

If the screed tests ok in a few places then as we're using Mapei adhesive I'll be priming it with their Primer G diluted, once that's absorbed and dry I'll give it another coat of neat Primer G at 90⁰ to the 1st coat before laying the antifracture membrane. 

I can't get hold of any gypsum based tile adhesive locally, otherwise I'd use that to lay the antifrac membrane leaving the cement based adhesive isolated from the anhydrite by way of the membrane.

I'll also be putting in expansion joints at each doorway.

3 days after turning my heating off I noticed 1 crack in the screed, so they do have movement in them.

That's more like my spec, sand early on then long gap before priming because you have to wait for it to be dry enough to tile.

Did you put down boards or sheets to cover the screed while people walked and worked over it?

[YorkieSelfBuild]

17 hours ago, Beau said:

We had an anhydrite screed. The advice at the time was leave to dry, sand/grind the surface to remove the surface fines and then prime. I used a diamond grinder on small angle grinder which made one heluva mess but worked. The primer was something I think called Ozin 260 but can check tomorrow if helpful as I think the old tub is still sitting about. Think it was only one coat. Anyway that was 18 years ago and tiles have never budged

I've heard of Ozin. Was looking at Tilemaster but some things are discontinued. So currently it'll be Kerakoll Active Prime Fix primer, Kerakoll Biogel No Limits to lay Ditra mat and Kerakoll Standard Set Setaflex to lay tiles. Then Tilemaster Grout 3000 and TileMaster Silicon 3000 to finish.

[nod]

35 minutes ago, YorkieSelfBuild said:

So you wait until it's dry enough to tile before sanding? I've read it's better to do it after 7-10 days as it gets much harder later and needs diamond discs

Yes once dry Scrape it with a floor scraper 7 days is ideal Weve tiled many after 3 days 

[Barnboy]

5 hours ago, YorkieSelfBuild said:

That's more like my spec, sand early on then long gap before priming because you have to wait for it to be dry enough to tile.

Did you put down boards or sheets to cover the screed while people walked and worked over it?

I understand that the longer you leave it before sanding then the harder it is to sand and the laitance seals the top of the screed and stops it drying. These screeds dry from the top down so they can look dry but still be wet down below. 

I haven't put any boards down, it's just been left to air, whilst I was plasterboarding I had a stack of boards on bearers laid down, the bottom of the stack was damp and this was a good 2 months after the screed had been poured.

[Nickfromwales]

Why not use cementitious self-levelling screed and have zero of the above? @nod what say you?

[Nickfromwales]

9 hours ago, Mr Punter said:

I have used Cemfloor flowing screed and there does not seem to be such an issue with laitance as it is cement based.

☝️ 👌

[nod]

33 minutes ago, Nickfromwales said:

Why not use cementitious self-levelling screed and have zero of the above? @nod what say you?

Cons This type of screed costs more than traditional screed and requires more prep work Pros You don’t need anti crack matting and you can get it tiled within days without much prep 

Manufactures are always cautious 

75 days for a sand and cement screed to dry out before tiling Is unworkable in the commercial world 

Hell Before the advent of flexi adhesive We would screed in the morning and tile in the afternoon 

[Nickfromwales]

53 minutes ago, nod said:

traditional screed

Dry sand & cement? Or gypsum?

 

53 minutes ago, nod said:

75 days for a sand and cement screed to dry out before tiling Is unworkable in the commercial world 

Yup, and the Adrex (A35 iirc) stuff is too expensive to compete against. Undeniably good stuff though, and can take ceramic tiles 45 mins after laying at 80mm. I cheated by laying porcelain the same day, but insisted that the grout was done 7 days later......still alive to tell the tale  

[nod]

7 hours ago, Nickfromwales said:

Dry sand & cement? Or gypsum?

 

Yup, and the Adrex (A35 iirc) stuff is too expensive to compete against. Undeniably good stuff though, and can take ceramic tiles 45 mins after laying at 80mm. I cheated by laying porcelain the same day, but insisted that the grout was done 7 days later......still alive to tell the tale  

Yep traditional semi dry Lay ply on in the afternoon and tile We would always pour the grout on like water to catch any hollow spots and brush wood shavings to clean and polish 

Something we only do now for very heavy vehicle traffic 

 

In short Times move on liquid screed is bombproof Don’t overthink it If it won’t come off with a floor scraper It’s fine Don’t use expensive primers As with plasters renders etc The primer is there to seal and kill any suction (Slow drying )

[saveasteading]

8 hours ago, nod said:

Manufactures are always cautious 

75 days for a sand and cement screed

All very interesting.

75 days is just silly. Based on 1mm per day I think.

An industrial slab can be 200mm. I've had client's consultants want 200 days ' drying'. They didn't understand that most of the water becomes chemically incorporated and not much evaporation is necessary. That's unless it is still being rained on.

And some of the advice on screeds seems contradictory. "Scrape off laitence because it is loose. But don't wait too long or it gets too hard?"

[dpmiller]

we went hemihydrate and anything that happened to land on it seemed to stick like sh*t to a blanket. We *have* primed it which makes them stick even better now...

[Nickfromwales]

I went to one gypsum laid job where almost the entire ground floor of tiles had separated. You could lift a few m2 by putting a chisel under the tile that was centred in the doorway to the wooden floor of the living room.

 

It had been primer coated too, but it was the primer + adhesive + tile that was loose, zero had stuck to the screed so I assume they’d not scraped / mechanically prepped it at all. 

[YorkieSelfBuild]

Architect agreed to not prime until it's dry, said cost of using Protecta boards is same as primer.

[Nickfromwales]

3 minutes ago, YorkieSelfBuild said:

Architect agreed to not prime until it's dry, said cost of using Protecta boards is same as primer.

Just remember to apply a dilute mix of water / primer before you prime undiluted.

 

I use a proper janitors mop and really go to town, with the dilute primer solution being left to saturate the screed / slab and soak right in. Then back mop to get rid of any that’s pooled anywhere and leave to dry for a few hours before final

priming.

 

You should be priming immediately prior to laying, and never leaving this down first days to become contaminated.

 

Specialist primers will come with instructions, follow those in that instance. 

[nod]

2 hours ago, saveasteading said:

All very interesting.

75 days is just silly. Based on 1mm per day I think.

An industrial slab can be 200mm. I've had client's consultants want 200 days ' drying'. They didn't understand that most of the water becomes chemically incorporated and not much evaporation is necessary. That's unless it is still being rained on.

And some of the advice on screeds seems contradictory. "Scrape off laitence because it is loose. But don't wait too long or it gets too hard?"

Sand and cement screed has alway been a mil per day 

LA and Housing associations will hold you to that 

[saveasteading]

27 minutes ago, nod said:

hold you to that 

Noted. It doesn't need that though unless it's a wet mix or not able to dry properly  because of conditions.

On s and c screed do you cover in polythene to cure first?

[YorkieSelfBuild]

On 15/03/2025 at 10:11, Nickfromwales said:

Just remember to apply a dilute mix of water / primer before you prime undiluted.

 

I use a proper janitors mop and really go to town, with the dilute primer solution being left to saturate the screed / slab and soak right in. Then back mop to get rid of any that’s pooled anywhere and leave to dry for a few hours before final priming.

 

You should be priming immediately prior to laying, and never leaving this down for x days to become contaminated.

 

Specialist primers will come with instructions, follow those in that instance. 

Yep, thanks. Kerakoll Active Prime Fix is applied diluted then neat, 2nd at right angles. 

 

Would you dry lay before or after it's prime? I think it'd be ok afterwards as tiles are clean

[Nickfromwales]

Dry lay over the dilute prime is fine, prime neat as you go, per m2, is what I always do.

If the MIs tell you to prime 2 layers and leave to dry before tiling then follow those instructions.

Then have a sponge with a 50/50 primer water mix on a damp sponge to spot clean each m2 as you lay the adhesive.