Removing laitance - when is enough, enough?

[Tom]

Hi all - our anhydrite screedhas been down just over a week and I'm trying to s rape off the laitance. It's pretty hard going with a scraper, but when is enough enough? A few passes with it gets up a good load of the laitance, leaving the surface glassy smoothe but there still seems to be a bit there (see pics) , but do I need to keep going? 

[Tom]

[Kelvin]

This was why I didn’t go for this type of screed. 
 

I thought the recommended method was to use a sander to abrade the surface? When I researched it you keep going until the loose broken up surface becomes stable. The doesn’t isn’t the kind of stuff you want to be breathing in either. 

 

[Tom]

Hmmm, I was told a floor scraper would do it, and it's getting a good load up but there's still some left behind. I could keep going with the a scraper I guess, if I wanted the arms of Zeus. Looks like I might have to hire a sander. Is there a time limit for getting this stuff up before it gets too hard?

[Kelvin]

The ideal time is between 7-14 days. A sander will be a lot less effort than scraping it off. Why make the build harder for yourself. Saying that I’ve just sealed our exposed beam using a wobbly ladder that is two rungs too short really 

[saveasteading]

Why do you need or want the laitence off?

If it is properly mixed and laid , it all goes hard.

 

[nod]

We normally have an hour Once pass then tile it over Never has an issue 

[Tom]

@saveasteading I don't think the stuff on an anhydrite screed does though, unless I'm wrong?

Edited September 30, 2023 by Tom

[Tom]

8 minutes ago, nod said:

We normally have an hour Once pass then tile it over Never has an issue 

Sorry, don't follow you nod, an hour after what and a pass with what?

[Kelvin]

2 minutes ago, Tom said:

#saveasteading I don't think the stuff on an anhydrite screed does though, unless I'm wrong?

All the advice I got from the supplier, installer, and tiler was it needs to be sanded off. The only reason I was considering it was to do with screed depth but in the end we had 55mm screed depth so went with Cemfloor. 

[Nickfromwales]

22 minutes ago, saveasteading said:

Why do you need or want the laitence off?

If it is properly mixed and laid , it all goes hard.

 

Nothing will stick to the laitence, no primers / adhesives / bonding agents, nothing. If a floating floor is going down, or carpets, not a problem.

[saveasteading]

4 minutes ago, Nickfromwales said:

Nothing will stick to the laitence, n

OK I see that from a quick read.  I wasn't aware of this.  Very glad we used cemfloor.

What is the point of anhydride?

[Nickfromwales]

Just now, saveasteading said:

OK I see that from a quick read.  I wasn't aware of this.  Very glad we used cemfloor.

What is the point of anhydride?

I think it was just popular, and possibly there were not the alternatives available 'then' as we have 'now'. I've used dry cement based screeds for the last 20+ years without an issue, so I will be interested to see how one of my build clients self-levelling cement screed turns out tbh. I will continue to avoid anhydride screeds personally, just cant see the attraction myself 🤷‍♂️

[saveasteading]

I've only done concrete floors until our conversion.  Choice of cemfix or a designed concrete pumped screed. Nobody offering anhydrite that I came across, hence my ignorance of this laitence issue.

The cemfix pumped easily and flowed well.

The only issue has been a couple of shrinkage cracks where another crack inducer would have been better. 

Completely hard and ready for tiling.

 

With concrete there should be no need to take laitence off. Twice had to grind it but for super-flat industrial spec, not because of laitence. But yes, too much "fat" is a problem if not controlled.

[Kelvin]

43 minutes ago, saveasteading said:

OK I see that from a quick read.  I wasn't aware of this.  Very glad we used cemfloor.

What is the point of anhydride?

You can pour a shallower depth of screed using it. 

[saveasteading]

25 minutes ago, Kelvin said:

You can pour a shallower depth of screed using it.

Cemfloor quote 35mm minimum.  I wouldn't want to aim for that unless really struggling for the last few mm.

Allowing for real life, there will be even thinner areas. With UFH id think 50mm is the minimum sensible.

Anhydrite sources appear to quote 25mm or 30mm or 35mm variously. 50 for UFH

 

Less than 35mm is becoming a levelling screed. The bagged stuff from Merchants will become favourite at some thin-ness, as it is diy and doesn't need further treatment.

 

If you lay 35mm of anhydrite then remove laitence, what thickness remains?

 

Is a marketing side of it that it uses waste products rather than cement?

 

Sorry for these queries. It is an area of ignorance for me, having never come across it in my centuries in design and construction. This simply means that it wasn't marketed at my sector. FRom what i am reading here it woulsnt have been attractive anyway.

 

 

[Kelvin]

You need a minimum of 25mm above the UFH pipes with Cemfloor which means a minimum depth of 45mm assuming the pipes are stapled to the insulation rather than tracked. It also assumes a flat level slab so 45mm is probably still a bit tight. 
 

When I had my mild panic about how much depth I had available for screed I briefly considered anhydride but we ended up at an average depth of 55mm so it was fine. 

[MR10]

2 hours ago, Kelvin said:

I’ve just sealed our exposed beam

What are you using to seal as I'll have to do the same soon with our glulam beam but not sure what to use. 

[Kelvin]

13 minutes ago, MR10 said:

What are you using to seal as I'll have to do the same soon with our glulam beam but not sure what to use. 

Osmo oil. Are you leaving it exposed? If so don’t make the mistake I did and wait until it’s boarded out 😂 
 

Edited September 30, 2023 by Kelvin

[MR10]

On 30/09/2023 at 18:58, Kelvin said:

Osmo oil. Are you leaving it exposed? If so don’t make the mistake I did and wait until it’s boarded out 😂 
 

Yes, leaving it exposed.  Yours is looking good. Thanks for the tip, will get it done before plasterboarding

[nod]

3 hours ago, Tom said:

Sorry, don't follow you nod, an hour after what and a pass with what?

As. Business we tile lots of liquid screed 

 

3 hours ago, Tom said:

Sorry, don't follow you nod, an hour after what and a pass with what?

As part of my business we tile lots of these What doesn’t come off in one pass Doesn’t need scraping 

If it’s hard to scrape 

[Tom]

3 hours ago, nod said:

As. Business we tile lots of liquid screed 

 

As part of my business we tile lots of these What doesn’t come off in one pass Doesn’t need scraping 

If it’s hard to scrape 

 

Thanks @nod, so my attempt at scraping in the pics above should do? Going over it again with the scraper lifts more off, but less and less each time. Are you saying one pass with the scraper then leave it? 

[nod]

Definitely 

As soon as it becomes hard going It’s time to stop 

[Tom]

Thanks @nod.

I've been over the whole area now and while some areas seem easier to scrape than others, there are a few patches where the scraper digs in and the surface seems really soft, the consistency (and colour) of firm putty. Is this normal? It's been down for 10 days now.

[Tom]

Here's a pic of one of the areas