Screed scrubbing?

[Post and beam]

I need to scrub the 'scum' off of our new wet screed. Initially this generates a lot of dust so is clearly removing material. I have no idea how far to go with this process. I am using an industrial rotary floor scrubber like you used to see at schools etc. Anyone know the process for this task? Thanks in advance as always. Keith

[Nickfromwales]

Is this gypsum anhydrite?

 

On one such screed where the clients opted to have the screed company scrub it off, they stopped when the surface looked like a pumice stone. Visible open pours in the surface, and dry ‘looking’.
 

Not sure how much material was removed as I had to be off site for the screed works after laying the UFH.

[nod]

I use a sharp long handled floor scraper Just take the fat off An hour should be enough for a 150 m2 floor 

[nod]

7 minutes ago, nod said:

I use a sharp long handled floor scraper Just take the fat off An hour should be enough for a 150 m2 floor 

https://www.facebook.com/share/r/16SpT3JiRz/?mibextid=UalRPS

[Post and beam]

Yes Anhydrite of some variant.

I basically scrubbed until the amount of dust being given of reduced to almost zero. I hope thats good enough.

[Nickfromwales]

That looks a bit like you've removed loose stuff and polished the stubborn residue? Have you tried a scraper on that polished bit vs where it appears 'matt'?

[nod]

2 hours ago, Post and beam said:

Yes Anhydrite of some variant.

I basically scrubbed until the amount of dust being given of reduced to almost zero. I hope thats good enough.

Don’t overthink 

I tile dozens of these each month 

Quick scrape Then brush up Seal with diluted SBR

[Post and beam]

1 hour ago, Nickfromwales said:

That looks a bit like you've removed loose stuff and polished the stubborn residue? Have you tried a scraper on that polished bit vs where it appears 'matt'?

No scraper yet but i will pick one up tomorrow. To be honest the rotary thing has a mind of its own anyway.

[Nickfromwales]

Just now, Post and beam said:

No scraper yet but I will pick one up tomorrow. To be honest the rotary thing has a mind of its own anyway.

Not sure how aggressive the pad you used was, but it looks like you may need to up your game a bit  Looks like a bit of the laitence is still there, but that's not easy to tell from these pics obvs.

[Barnboy]

This is my anhydrite screed after I'd taken a floor grinder to it and started hoovering after 3 rounds of sweeping. If you zoom in on the hoovered patch you can see that the aggregate has become visible and looks more open.

[Post and beam]

In case my scraper does not do the trick tomorrow would you indicate exactly what floor grinder you used please

[Nickfromwales]

 

 

 

👌

[Barnboy]

https://images.app.goo.gl/uKd67

The one I hired was the same as this link, it had a disc of cutters rather than individual stone like the Husqvarna I've used before which was a handfull and nowhere near as clean finish as this one was.

[Andehh]

On 08/05/2025 at 22:06, Nickfromwales said:

 

 

 

👌

 

 

 

I'm not ashamed to say I watched that twice, that shits cool.

Edited May 10 by Andehh

[Gus Potter]

When designing raft slabs and screeds I set a level and flatness tolerance so this should not happen.

 

It can add to the initial price but at the end of the day someone has to do it. Remedial works always cost more than getting it right first time.

 

 

[Post and beam]

On 09/05/2025 at 23:54, Barnboy said:

https://images.app.goo.gl/uKd67

The one I hired was the same as this link, it had a disc of cutters rather than individual stone like the Husqvarna I've used before which was a handfull and nowhere near as clean finish as this one was.

If i could find one to hire close to hertfordshire i would do so.

[Post and beam]

Does anyone that has experience know if the dust extraction is essential. If i need it i will hire also but it more than doubles the price

[Nickfromwales]

5 hours ago, Post and beam said:

Does anyone that has experience know if the dust extraction is essential. If i need it i will hire also but it more than doubles the price

If you’re running a CDM site then M class extraction is requisite, vs sweeping up, to control dust etc that otherwise then becomes airborne. 
 

If you’re DIY, then keep the family out and get a decent cartridge filter mask.

 

Do NOT expect contractors to breathe that in (plz), but I’m assuming you’re doing this whilst the site is vacant for practicality. 

[Post and beam]

Nick no i dont expect anyone else to endure my efforts to remove the Laitence. I intend to use my trusty Henry hoover attached to the grinder, and if i have to change the bags often then its a reasonable compromise against another £240  for an extractor. More than the grinder itself as it happens.

[Barnboy]

I had hardly any dust whilst grinding, the skirt on the machine kept it down, the only time there was dust was when I swept everything up.

[Nickfromwales]

11 minutes ago, Barnboy said:

I had hardly any dust whilst grinding, the skirt on the machine kept it down, the only time there was dust was when I swept everything up.

On sites were now supposed to use M class filtration vacuum cleaners (dust extractors) and avoid sweeping up wherever possible; with good reason tbf. If you're all alone, this rule doesn't really apply as you are not affecting others, so a good mask would suffice (not the shitty paper ones btw). 

[Benpointer]

Dumb question but why are you having to scrub (sand/scrape?) the top off your screed?

 

I was kind of hoping that once the screed company lay it my screed will be flat, smooth and (subject to drying our properly, fit to tile onto.  Have I missed something?

[Roundtuit]

1 minute ago, Benpointer said:

Dumb question but why are you having to scrub (sand/scrape?) the top off your screed?

 

I was kind of hoping that once the screed company lay it my screed will be flat, smooth and (subject to drying our properly, fit to tile onto.  Have I missed something?

To remove laitence; the dodgy weak surface layer.  

[Post and beam]

On 24/05/2025 at 22:20, Benpointer said:

I was kind of hoping that once the screed company lay it my screed will be flat, smooth and (subject to drying our properly, fit to tile onto.

That was my understanding as well, how wrong was i?

 

At the least i would expect that the screed company would do this. I paid for a one stop shop approach and said so clearly on every ocassion i had any communication with the people i contracted to do this.

 

It is done now but has cost me £500 that i would rather not spend on this. I did it myself, the cost was hiring the kit.

 

Now just have to wait until the floor is dry enough to tile.

[Post and beam]

On the subject of screed drying.......

 

Up to 105 days is bonkers & less than helpful.

 

What is a relaible way of testing when the floor is in fact dry enough to tile. I have heard of a humidity test. what exactly is involved in this please?