Removal of laitance from anhydrite screed before tiling

[hellopaul2]

Hello,
I have about 38sq.m of floor to tile. It's currently an anhydrite screed with wet underfloor heating in it. It was poured 5.5 weeks ago. A week ago I started gradually warming up the underfloor heating, and switched it off this morning. I'm hoping to tile it tomorrow - the floor should be cold again by then.

The screed is about 60mm thick. I'm hoping the often-quoted "1mm per day drying time" has been reduced by the underfloor heating being switched on. There are no cracks in the screed.

My question is regarding laitance, which I didn't know anything about until this morning, when researching primers. I have read that the top layer (insert random thickness here) needs to be removed to get down to the screed with a decent amount of strength. The floor seems hard, not crumbly at all. It's a bit dusty, but it's not like the screed 'comes away' as dust when swept - if you know what I mean. So is this removal of the laitance really necessary? Or will I just end up with a load of cracked/unstuck tiles in a few weeks if I skip the laitance removal step?

I've read about using shot/grit blasting and all sorts of expensive-sounding methods of laitance removal, but I've also seen a video on YouTube (always a reliable source of information, I know!!) of someone apparently removing laitance using a floor sander with a coarse grit paper.

So, I'm obviously hoping that laitance removal is overkill - the room is a sitting room/kitchen, so won't get a huge amount of moisture/traffic, but obviously the underfloor heating will add some movement to the whole equation.

Any thoughts or experience of this, anyone, please?

Many thanks in advance!

- Paul

 

[AdamCH]

Paul

I'm not sure you're going to get the guidance you need on here. Anhydrite screeds can be tricky and if you don't complete the preparation correctly and use the correct materials you will have all your tiles popping up.

You must make sure its sufficiently dry, remove the laitance use the appropriate tile adhesives maybe use a decoupling mat etc. Experienced tilers get caught out with these screeds so you need to make sure you get the correct advice.

 

[hellopaul2]

Thanks Adam.

My plan is to attack the floor with a stiff wire broom to remove anything remotely loose, sweep, vacuum then prime it before tiling. And to go with the "it'll probably be fine!" attitude!!

- Paul

 

[AdamCH]

I think the recommended approach is floor sander, vacuuming and moisture readings - they can take a long time to dry.

Ditra decoupling mat may also help.

Make sure you use the proper adhesive - normal cement based adhesives won't work (for long).

 

[tpt]

It really depends on the adhesive you are using, but whichever adhesive you really need a moisture test on the screed before proceeding. Going by the book the screed must contain <0.5% residual moisture before any tiling can take place.

Usually abrading and hoovering the surface will remove the laitance, shot blasting sounds very extreme, never seen it done, sanding with medium grit paper or 60grit on a floor polisher to remove any loose and friable material and damn good vacuum to get rid of the dust is usually enough. but your best to speak to the adhesive manufacturer to get their method, if your using a brand like BAL, Granfix or Keracol they will guarantee the installation if you follow their prescribed method so its worth contacting them and asking them for a specification.

If your using a cement based adhesive you must prime the surface with 2 neat costs of acrylic bond and prime, this is because the 2 surfaces dont really like each other, one is gypsum based and one is cement based. You can get specialist anhydrite adhesives which are gypsum based and need no priming such as Keracol H 40 Eco ideal, but sometimes they can't be used if underfloor heating is present.

Either way, make sure the slab is completely cold before you prime\tile it, leave the adhesive 48hrs and then slowy (over several days) bring the heating up to temperature.

Just a word of warning - if your laying natural stone, you'll need a decoupling membrane, which changes the above advice!

Sorry for the long post, if you need more just ask!

 

[hellopaul2]

Thanks for all the advice and suggestions!

Last night I swept an area and put a rubber mat down; this morning the area under the mat was very, very slightly (almost imperceptibly) darker, so I'm postponing the tiling. I've put the mat back to see if it gets any darker and will get the underfloor heating up and running again for as long as it takes. As I said, the screed is around 60-65mm thick, so theoretically it should take a couple of months to dry. I'm hoping the heating and constant ventilation will help (the windows and doors have only been in a couple of weeks). I'm keeping the windows open. It's been down for 39 days so far. I'm (possibly too optimistically) hoping another 6 days of heating will do the trick.

I've spent most of today attacking the surface with a wire broom (the type normally used to clean moss off a patio), and occasionally going at it with a sharp chunk of steel. My theory is that if the wire brush or the chunk of steel don't take off the surface laitance, then the tiles will stick like the proverbial to a blanket. Some areas of laitance come off with the wire brush, others are very firm.

The floor's been down for just over 5 weeks now, and there's been plenty of building work going on, so a lot of the looser laitance has been worn away by the builders' foot traffic.

I realise in my original post I said something about one of the rooms being a kitchen so not getting much traffic - silly thing to say - surely it's the most trafficky of all rooms in the house! So I've been attacking the floor here with added vigour.

I've got some Bal Prime APD. I'm planning to apply two coats to isolate the gypsum from the cement-based adhesive, after everything's had a chance to dry.

It would have been nice if the company who poured the floor, or the idiot builder, had told us about the time it would take to dry, and to remove the laitance after a few days. It's just really lucky that I researched all this online - else I'd have merrily slapped the tiles straight onto the screed, and got rather angry when they all started falling apart in a few weeks/months.

Normally I do all my own building work/project managing, but this time I didn't have the time. Stupid, stupid decision, letting a "professional" (ha!) get involved. Pretty much every aspect of the build has been a disaster - over budget, waaaaay over time, and generally a bit ****. Oh well, we live and learn!

This has become more like therapy than advice-seeking for me!! Thanks for listening...maybe I'll be back here "on the psychologist's couch" next week....!

- Paul

 

[AdamCH]

Paul

I'd suggest that you do a lot of research on this - not youtube. You may find that adhesive that sticks well to the screed doesn't stick well to the tiles and vice versa. And don't slap on the primer until you're sure on the adhesive you'll be using.

You may also need to take a moisture measurement properly.

I don't know how big the area is or how expensive the tiles are but get it wrong and you could end up with an unwanted second go at it.

 

[tpt]

i'd second adams post. get a proper moisture test, and dont prime it until just before tiling, foot traffic will affect it if you do.

Choose your adhesive before choosing the primer.