Hi everyone 

Our floor fitter suggests that we avoid anhydrous screed ahead of laying a herringbone floor as it apparently takes much longer to reach the required low moisture level to lay the floor. 
 

I’m struggling to find non-anhydrous cement to make up a sand & cement mix for the screed. 
 

We were looking at Lafarge General purpose + cement but that has clinker in which I believe is anhydrous. 
 

any tips please? (I also thought all cement was anhydrous so it’s a learning curve!)

 

thank you 

1 minute ago, Reginald said:

(I also thought all cement was anhydrous so it’s a learning curve!)

this was my understanding too. are you sure he's not thinking of liquid screeds?

7 minutes ago, Thorfun said:

this was my understanding too. are you sure he's not thinking of liquid screeds?

Hm I hope not as we’ll be charged more for that but that gives me something I can clarify with him. Thanks for replying

 

are liquid screeds not anhydrous?@Thorfun

Edited April 17, 20242 yr by Reginald

I have to start most of my responses by saying "I'm not an expert in this area..." but my understanding was the opposite, i.e. that the anhydrous element (calcium sulphate) *replaced* the cement as a binder.

 

Here you go - this should be useful; https://www.homebuilding.co.uk/advice/a-screed-masterclass

Edited April 17, 20242 yr by garrymartin

2 minutes ago, Reginald said:

Hm I hope not as we’ll be charged more for that but that gives me something I can clarify with him. Thanks for replying

 

are liquid screeds not anhydrous?@Thorfun

afaik from my research you can get anhydrous and cement based liquid screeds. i didn't want anhydrous due to having a laitence removal requirement and being gypsum based. the cement based liquid screed was brilliant and ticked all the boxes for me.

 

@nod is a screed expert (i most definitely am not!). hopefully he'll be along soon to comment. 😉 

47 minutes ago, garrymartin said:

I have to start most of my responses by saying "I'm not an expert in this area..." but my understanding was the opposite, i.e. that the anhydrous element (calcium sulphate) *replaced* the cement as a binder.

 

Here you go - this should be useful; https://www.homebuilding.co.uk/advice/a-screed-masterclass

This is a helpful read, thank you

 

44 minutes ago, Thorfun said:

afaik from my research you can get anhydrous and cement based liquid screeds. i didn't want anhydrous due to having a laitence removal requirement and being gypsum based. the cement based liquid screed was brilliant and ticked all the boxes for me.

 

@nod is a screed expert (i most definitely am not!). hopefully he'll be along soon to comment. 😉 

Ok it’s getting clearer!! Thank you all for helping me navigate this

 

I wonder if anyone can advise if the image is anhydrous or not? My suspicion is that it is. Tried calling the manufacturer but keep getting passed around! 
 

 

starting again with the "i'm no expert" caveat......

 

it is my understanding that a sand and cement screed will take a long time to dry out regardless of the cement you use (1mm a day iirc). i'm convinced that your floorer is talking about liquid screeds as anhydrous liquid screeds take a lot longer to dry out than cement based. if you use a sand and cement based screed you'll still be waiting a long time for the floor to dry out before you can fit the finished floor.

 

i would double check with your floor fitter as to what he's referring to when he's talking about non-anhydrous

 

 

As anhydrous mean not containing water, what is used to make the cement set?

Cement powder is anhydrous.

Add water and it hydrates.but we don't call it hydrous.

I think people throw in the word anhydrous to make it sound more interesting.

 

If minimal water is added to cement, then a large proportion reacts chemically,  and becomes fixed and isn't wet. Then there isn't much left to evaporate.

 

Floor fitters make lots of money adding  a waterproof seal on concrete floors, mostly unncessarily.

I've even had them make me sign a paper that it's my responsibility if the floor lifts.

Some are chancers,  some don't understand.

29 minutes ago, saveasteading said:

I think people throw in the word anhydrous to make it sound more interesting

There was a radio program about PR people and journalists.

Very interesting.

 

https://www.bbc.co.uk/programmes/m001y9pt

 

 

Thanks for the input everyone. It took me about 20 minutes on a call to get the floor chap to share that he meant a non gypsum based screed. 
 

Regardless, the screed needs to be <80% moisture for him to lay the wood floor and as you’ve shared, I think even a non gypsum based sand and cement dries at about 1mm per day up to 50mm then 0.5mm. Our screed will be 75mm. 
 

does anyone know if you can add stuff to traditional sand and cement screeds to make them dry faster or are we looking at a poured screed for that? Some of the poured screeds promise dry in 28days or so… is that fully dry like <80% moisture? 
 

thanks in advance 

 

tagging @nod too - I hope you don’t mind - Thorfun mentioned you were a screed expert and I’m desperately trying to avoid more delays on our build! 

Screeds tend to be

a) traditional - sand:cement either site mixed or dumped in a heap, laid damp to about 75mm

b) liquid gypsum based, mixed in a machine and pumped in normally about 50mm.  Not great if it get wet and can form a laitance coat that needs removing before tiling.

c) liquid cement based, again mixed in a machine or at a batching plant and pumped in.  Normally 40-60mm.

 

I don't think the drying times vary all that much between them.

 

I have seen some stuff that is a leveling screed and claims no time needed to dry as the water is locked in.

34 minutes ago, Reginald said:

does anyone know if you can add stuff to traditional sand and cement screeds to make them dry faster

You can use a superplasticiser, which can significantly cut the water in the mix. If there's less water, it will dry somewhat faster. However they also tend to cut the amount of time that the screed remains workable (from memory, to about 45 minutes), so you have to be set up with a good crew.

1 hour ago, Reginald said:

Thanks for the input everyone. It took me about 20 minutes on a call to get the floor chap to share that he meant a non gypsum based screed. 
 

Regardless, the screed needs to be <80% moisture for him to lay the wood floor and as you’ve shared, I think even a non gypsum based sand and cement dries at about 1mm per day up to 50mm then 0.5mm. Our screed will be 75mm. 
 

does anyone know if you can add stuff to traditional sand and cement screeds to make them dry faster or are we looking at a poured screed for that? Some of the poured screeds promise dry in 28days or so… is that fully dry like <80% moisture? 
 

thanks in advance 

 

tagging @nod too - I hope you don’t mind - Thorfun mentioned you were a screed expert and I’m desperately trying to avoid more delays on our build! 

As others have probably stated Flo screeds dry in incredibly fast About a week drying time for tiles 

Traditional is a mil per day 

There’s no additive that will quicken this up though we often tile screeds that have this additive in We make it clear that there is no guarantee if it lifts 

Two options 

Flo screed or wait 

1 hour ago, Mike said:

You can use a superplasticiser, which can significantly cut the water in the mix. If there's less water, it will dry somewhat faster.

Cement mixes don't dry as such, the H₂O reacts and changes the chemistry (trying to get back to limestone), only excess water, which should be minimal, needs to evaporate.

 

Edited April 21, 20242 yr by SteamyTea