can we talk, calcium sulphate screeds

[Diamond Pool Finishers]

as a substrate, please any top blokes can write about it for us to read someone like ajax/DAVE???? i dont know much about them myself only they have been around a long time, and they are pump and almost liquid thanks for replies guys

 

[david campbell]

since i know the bare minimum about them you can read away here until your little hearts content-http://www.gyvlon.co.uk/

 

[The Legend; Phil Hobson RIP]

I have tiled on Anhydrite ( calcium sulfate) I think if you stick to the technical advice. You should be OK. Anhydrite screeds are a mixture of screeding sand and binder.

I think the last job I did, we had a tech guy mesuring the residual moisture content in the floor. I think it should not exeed 0.5% But we were using ditra, so we got away with 2%. We primed with epoxy, but I believe you can sand depending on maufacturers advice. Hope this helps

 

[DHTiling]

THe new Gyvlon screeds are the nuts compared to others...

Some info to follow..

 

[TT Tiling]

good screeds when layed right ,been laying on them down here for years ,you dont find many that are flat ,but when you do they are spot on ,make sure you epoxy or it will be up in no time , take a very long time to dry ,i nom get the guys to put the underfloor on to dry it out prop,before tiling

 

[Dan]

Best guy for this will be Ajax mate I think.

 

[Diamond Pool Finishers]

yes i hope ajax see this when he is on!? HAS DAVE got some info to share? its just for general knowledge for all of us really !!!

 

[Ajax123]

Is there anything specific you wanted to know Gooner.

In very laymans terms......

Calcium Sulphate screeds are a mixture of clean water, grade M sand and calcium sulphate binder which can be made from anhydrous calcium sulphate or from alpha hemi hydrate calcium sulphate.

The binders should be manufactured in accordance with BS EN 13545 and are usually made by third party accreditted manufacturers such as Knauf and of course my own company Lafarge (When I say my own - I wish - I mean the company I work for)

The binder is a blend of milled raw calcium sulphate in our case harvested as a waste product from the acid production industry. approx 150,000 tonnes of this used to go to landfill. We take some to make into screed binder and some goes to agricultural applications. Because it is a waste material it is classed as recycled when talking about things like the code for sustainable homes and BREEAM etc but in reality as it has not previously been used it is not truly recycled but as I say is a waste material that would previously have gone to landfill. When you land fill calcium sulphate and it begins to degrade anaerobically it produces a number of noxious gasses such as hydrogen sulphide amongst others so taking it out of landfill is great for the environment and the ozone layer. The raw material is mixed with very small amounts of additives which help to stabilize things like the pH, setting time and shrinkage etc. This material is then delivered in bulk to readymix suppliers. We supply quite a few readymix producers nowadays. For every 1 tonne of our binder used 980kg of landfill is saved.

Alpha Hemihydrate is made from the waste material resulting from the operation of coal fired power stations. Much of this waste goes into plasterboard manufacture as well. The issue is that when it comes out of the powerstation it is a composite material comprising calcium sulphate in several forms. To use this as a binder would lead to a great deal of instability and unpredictability in the final screed performance so it is processed to make wither Thermal anhydrite of alpha hemihydrate. Thermal anhydrite is the same chemical as the acid production stuff but it has been through a thermal process. The alpha hemi is made by a process called calcination and occasionally by autoclaving, ie heating up under pressure with steam. This process has to be controlled in such a way as to partially convert the calcium sulphate to gypsum. This again is supplied to the readymix manufacturers. I do not know the ladfill savings for this material but as it is temperature processed it could be argued as being not quite as environmentally freindly.

If you think of the binder as a special type of cement that is in calcium sulphate screeds you are not too far off the mark. We just refer to it as binder cos it binds the sand together to make it hard.

Next installment to follow......

 

[Ajax123]

chapter 2

Once the calcium sulphate binder is manufactured it is delivered to the readymix producer where it is stored in bulk untill time for use. The readymix producer will have a variety of mix designs available depending on the specific requirements of the screed. e.g. standard domestic screeds will generally be C20F3 or C25 F4 in strength whereas screeds over timber floors (used for accoustics and thermal mass systems) will generally need to be stronger. The screed mortar should be manufactured in accordance with the requirements of BS EN 13813 which makes incumbent upon the manufacturer certain testing regimes to ensure that the screed meets things like flexural and compresive strength criteria in a similar way to concrete manufacture. Compliance to a third party accreditted batching system is essentially optional but mos suppliers in the UK do comply to these systems. Some also carry CE marking and in a very few cases specific NHBC approval. The screeds are not usually BBA Certified these days because the thrid party accreditted QA systems make this un-necessary. The batching will be one of a variety of methods but essentially all of them aim to place binder, water and sand into the barrell of the readymix truck ready for delivery to site. The screed installer, who will usually have been on a training session to learn the technical side of the material, takes delivery of the material. The screed should be tested upon delivery for flow (like a mini slump test) and presuming it meets the required flow specification it will be discharged into a screed pump which should ideally be of the worm and stater or continuous flight auger type. This enables greater controll of the mateiral at the placement end of the pump. It can be pumped considerable distances. One of the advantages anhydrite has over hemihydrate is that the rate of reaction (setting) is slower and more predictable so allowing the installer more time to work with it on site.

The installer should foolow the guidelines within BS8204 part 7 but as we all live and work in the real world this does not always happen.

next bit to follow later cos me fingers are now aching.....

 

[doug boardley]

have we got an epilogue??:lol:

 

[Ajax123]

have we got an epilogue??:lol:

Give me a chance Doug - can't give you it all in one go cos I will have nothing left for tomorrow.....anyway just been making a cupp and a banana sandwich....

 

[Ajax123]

Now then......... what should we look at next......specification I think..

the specification of the screed is essentially in the hands of whoever is taking design responsibility for it. Sometimes this is the architect, sometimes the screeder, sometimes the builder etc etc. I am able to help write specifications but I generally do not accept design liability as there are too many things outside of my direct control so I offer what are called Model specifications. Ideal designs if you like. The general rule of thumb is that the key to success is to play to the strengths of the material and design out the weaknesses. The one thing which crops up time after time after time is screed depth. In the uK we seem to be stuck fast into the mentality that a screed should be 65mm or 75mm deep. Take the average house ground floor where you will often see the screed and insulation zomne specified at 150mm in total i.e. 75mm insulation and 75mm screed. This zone has some merit in traditional masonry build as it fits very nicely with brick coursing i.e. 2 courses of bricks and mortar is 150mm. To acheive the necessary U value a designer will often specify a polyurethane type insulation in this scenario. Placing anhydrite screed deeper than it needs to be has a number of undesirable effects, most notably the drying time. This screed dreis naturally at a rate of 1mm per day up to 40mm depth and then add on 2 days per mm thereover so a 75mm screed will take 110days to dry. Not conducive to anyones build programme. It can be very effectively force dried but if you design it at a much thinner depth it would have a much reduced natural drying time and so you can see zoning is something which is very important. In domestic situations it can be laid to a depth of just 35mm and in commercial applications 40mm when laid over insulation. Even thinner when not on insulation but more on that in a later chapter. The more shrewd amongst you will notice that we now have 75mm of polyurethane insulation and 35mm of screed which does not fit conveniently into the brick coursing. What I tend to do here is simply increase the depth of the insulation to 110mm and the screed to 40mm. When you increase the depth of insulation to this depth it is quite possible to acheive or exceed the U value by changing the insulation type to polystyrene which is generally quite a bit cheaper. Typically an EPS or XPS 100 insulation will offer an improvement to the U Value and allow the screed depth and hence the drying time to be reduced significantly.

How am I doin so far.........

 

[DHTiling]

LIke i said Info to follow and his name is Ajax...:lol:.. i just tile them.. he makes the stuff..

 

[Ajax123]

The next thing I always look for in the specification is the divorcement of the screed from any residual construction moisture from concrete sub slabs etc. A typical ground borne concrete slab will have a depth of 100mm and can take in excess of 12 months to dry. There will often be a DPM under the concrete so the only direction the residual moisture can go is upwards. If this moisture is allowed to rise through the screed it can extend the drying time of the "floor" - notice I do not say of just the screed - significantly. This can in turn have a number of undesirable effects - e.g. delaying fixing of final floor coverings to in extreme cases poly sulphide attack ( although the latter is very very rare). Concrete being cement based does not mind moisture being trapped within it, in fact it is desirable. By the simple expedient of placing a 1200 gauge polythene membrane over the top of the concrete prior to placing the insulation and screed can save loads of time in the build programme. I like double membrane systems where the DPM is under the insulation and a slip membrane is ont top altough I accept the commercial aspects associated with using the DPM as a slip membrane as well. this does not however suit heated screeds where a double membrane system should always be used IMO.

 

[Ajax123]

LIke i said Info to follow and his name is Ajax...:lol:.. i just tile them.. he makes the stuff..

Write de theme tune....sing de theme tune......:lol::lol:

 

[Ajax123]

In heavier duty environment consideration has to be given to the deflection created when the insulation compresses under loadings so it is often wise to stick with the PU type insulations where heavy traffic is to be used e.g. hospital beds, school library shelves etc.

Blimey that one was dissapointingly short........now where di I hear that before....oh yes the Mrs is sitting on the sofa as well.....😳

 

[Ajax123]

tune in folks tomorrow for the next exciting installment of "Calcium sulphate screeds - a thesis"

 

[The Legend; Phil Hobson RIP]

Thanks Ajax, Thats what I meant. :lol::lol::lol:

 

[Ajax123]

Sorry I missed an installment tonight - went off to play Snooker instead.......Lost:incazzato:.....so as it happened would have been better coming here. Will be back with the next installment tomorrow. We'll probably talk more about design and perhaps about underfloor heating next Any other special requests considered unless they invlove contortion....:yikes:

 

[DHTiling]

Tut tut snooker...:smilewinkgrin:

 

[Ajax123]

Calcium sulphate screedforms a perfect compliment for an underfloor heating system being suitable for use with both wet systems and electric systems. The nominal septh of traditional screed over underfloor heating would be 75mm (yes I know sometimes it is laid thinner) with anhydrite you are able to reduce the depth of the sacreed significantly to just 25mm to meet BS8204 part 7 and 30mm to meet the requirements outlined by most manufacturers. This offers a number of advantages with heated screeds but before we look at them let us look at the standard design requirements and some of the possible pitfalls.

1. It is very important that the screed is divorced from the substrate by means of a Damp Proof Membrane, usually a 1200 gauge polythene over the substrate. It is common to see this folded up the walls behind the edge strip but in my opinion this is un-necessary as there should be a damp proof membrane under the concrete which serves to protect the srtucture. provided the screed does not bridge this then the DPM over the concrete can just be cut almost flush to the walls as it is only there to prevent the residual substrate moisture from rising through the floor and extending the screed drying time. I also see a lot of installations where the DPM has been moved upward on the zone ot on top of the insulation. This is a bad idea if you are using underfloor heating pipe clips to secure the pipes as these will clearly then breach the DPM so it becomes a seive instead.

2. The screed depth should be designed to meet as close to the minimum as possible. I see a lot of designs where the insulation is designed to meet the u-value required and then the screed placed to make up the zone. What would be better would be to design the insulation depth based on the minimum depth of screed (normally 50mm nominal). placing the screed too deeply results in significantly extended drying times.

3. The screed should not be placed at belos the nominal inimum design depth as this can lead to overheating at the surface of the screed and the posibility of pipe mapping and plastic settlement cracking over the pipes.

The screed should be designed with consideration for expansion and contraction. Whilst it is much more dimensionally stable than traditional materials it should still be joined across all door thresholds and where there are abrupt or significant changes to aspect ratio or direction. Joints should also be placed at the interface between independently controlled heating zones, remembering that generally there will be up to 8 curcuits within the same zone. All of these joints should be formed using a full depth isolating former ideall of a compressible material and should be between 5mm an 12mm wide. All joints should be fully reflected through the tiled surface. Remember uncoupling membranes are not designed to replace joints.

Now to look at some of the benefits. The screed has a very high thermal conductivity at around 2.2W/mK. The fact that the screed is fully self compacting means that there are no air pockets trapped within the screed which would lead to a less efficient heat transfer. Additionally the screed flows freely and completely around the heating pipes to offer a full contact surface for the heat transfer. The material is laid thinner so that there is less material to physically heat up. All of leads to greater efficiencies in the heating system meaning effectively that to get the same heat output compared to traditonal materials you need put less energy in. This makes anhydrite perfect for use with geo thermal heating systems as well as other more traditonal systems. It allows trhe screed to act more like a radiator than a storeage heater.

The fact that it can be laid thinner than traditional materials means that the design of the insulation can be considered in a more flexible way. For example if there is no desire or no possibility to alter the depth of the overall floor zone we can look to change the depth of the insulation. This allows the use of potentially more environmentally freindly insulations to meet or exceed the required U Value. When the U-Value is calculated using for example a poly urethane insulation having a r value of 0.023 at say 75mm I can look at changing the insulation type to an EPS100 polystyrene at 100mm with an R aValue of 0.030 and generally exceed the u-value we achieved using the PU. this would offer a cost saving in most instances. Alternatively and possibly more desirably in many smaller installations we can simply increase the depth of the insulation to enhance the u-value significantly and make the floor more environmentally freindly and in the long term cheaper to run.

The alternative of course it to physically reduce the depth of the floor zone although this can generally only be acheived at design stage because once the build gets past ground floor level it is too late. In this instance though reducing the screed depth by 25mm meansd that less material is used in the overall construction. This would often not suit traditional masonry construction where floors are usually designed to fit in with brick coursing. Where it can be done though the reduction in material use leads to lower levels of emboddied energy in the whole construction. I have seen instances particularly where the timber frame systems have been used where an entire course of bricks per floor has been removed with very significant material cost savings to the builder as well. The possible reduction in floor zone is up to 35mm.

Where electrical underfloor heating systems are used it important that the cables are controlled by means of floor stats instead or as well as room stats as there can be a risk of the floor overheating.

Turning the general floor temperature down not only leads to energy savings but also reduces the risks associated with thermal expansion and contraction of the screed and the inpact this has on the finished floor coverings.

The temperature of the underfloor heating system should not exceed 55oC for anhydrite during the comissioning phase and it is generally recomended that this is limited to 45oC during the operation of the floor. This would normally lead to a surface temeprature which is comfortable.

The surface of the screed in operation should feel at best slightly warm to the touch based ont he temerature differential of this (maximum 27oC) and the temperature of the average human hand. If the floor feels hot it is generally too hot.

 

[Ajax123]

Another major benefit of calcium sulphate is its ability to be force dried. The underfloor heating can be comissioned as soon as the screed is more than 7 days old. This means that the underfloor heating can be very effectively used as a means of force drying it. If you were to force dry traditional screed it will cause cracking, crazing and curling. None of these are likley to happen with calcium sulphate provided it has been correctly installed. Of course it is not impossible for cracks to appear and the fact that the underfloor heating can be comissioned very early means that any cracks which do appear can be attended to very quickly. It is imperative that an underfloor heating system is comissioned properly before tiles are laid. The recomendation in the relevent standards is that the underfloor heating in traditional screeds should not be comissioned for at least 28 days following installation of the screed. This means that if you want to use the heating to dry the screed if you use calcium sulphate it will probably be dry before you can even switch on the heating if you use a traditional screed.

Unfortunately it is not recomended that surface damp proof membranes be used on heated screeds but the fact that anhydrite can be force dried means that it is largely unecessary anyway.

The comissioning procedure takes approximately 10 days from start to finish although it can be extended in order to use it to force dry the screed effectively. Bear in mind that just heating the screed will not coause it to dry. It will of course force the moisture into the atmosphere above it. If this moisture is not removed either by extraction, ventilation or dehumidification (Not my personal favorite method) the relative humidity in the atmosphere will quickly equilibrate with the screed and drying will stop. This is true of all substrates.

 

[Ajax123]

Some stuff about moisture


The accepted drying time for sand cement is 1mm per day based on 20oC and 60% RH. so technically a 75mm screed takes 75 days to dry. Modified screeds can dry more quickly. The thing to remember is that cement based adhesive, based on the simple fact it is cement, will tolerate moisture abuse cos cement loves water. The cement simply conitnues to hydrate untill the water is all used up. It is less likely then that a cement based adhesive will delaminate if the screed is "reasonably dry" i.e. typially in my experience 6 weeks but I accept your 30 days if it has worked for you in the past. This is the case even if the adhesive (as most do) says the floor should be below 75% RH. If you do a moisture test on the material it will yeild up to 4% water to be considered dry. This is because some of the water is retained permanently within the cement pore and keeps the cement in a permanently alkaline condition
Calcium sulphate binder is added to sand and water and some additives to make a calcium sulphate screed. There are 2 distinct types but the ultimate result is the production within the screed matrix of Gypsum. Once all the source material i.e the binder is used up any remaining water - called mixing water or workability water - is surplus to chemical requirements. As the water will contain water borne sulphates and it moves around within the screed matrix predominantly in an upward direction if there is cement available for it to react with it will do so forming mainly ettringite and also some thaumesite although the latter is a very small proportion. The nominal dry calcium sulphate screed will have around 0.5% water by weight in it. to get to this level based on the same drying conditions described above will take approximately 1day per mm for the first 40mm of depth and then another 2 days per mm for each mm over the 40mm depth. i.e. a 50mm screed takes around 60days to dry in a good drying environment. Unfortunately the biggest issue we have in the UK is that people do not take this into account in the design stage and it often goes in much too deep. If you use a gypsum based adhesive there is no risk of chemical reaction and so like the cement on cement scenario it will be more tolerant to the effects of moisture. The gypsum based adhesives I know of can generally be laid when the screed is up to 85% RH which is actually closer to 1.5% moisture. i.e. almost 3 times wetter.
In terms of sanding the screed the advent of the now low laitance systems have reduced the need for this in terms of laitance removal because the laitance is bound within the screed matrix just as it is in a sand cement sceed. The down side is that the surface of the screed is likely to be much harder and smoother than the old fashioned stuff. This means that primer do not penetrate the surface as easily and as such it is often necessary to use an orbital sander to prepare it to be primed. ONe thiig I see time after time is overkill on this sanding. As for who does it .... it is generally a contractual thing. The screeders can do it and some do as a routine. Some offer it as an extra over and it is up to the main contractor to arrange it as required. Some say it needs to be sanded bu they do not offer sanding as part of the service. These latter are not my favorites but there is some merit commercially in some projects so there is place in the market. In terms of preparation for floor coverings the screed should be clean and free from surface contaminants such as plaster and mortar snots etc. The best way to remove these is by sanding. The preparation for floor coverings should in my opinion be part of the floor covering contract and as such the responsibility should be dealt with contractually between the main contractor and the flooring contractor. It is difficult to assign this part to the screeder because with the best will in the world he is unlikley to know the level of prep the flooring contractor is likely to require.
The same method of preparation should be carried out on traditional sand cement screeds, concretes and the like so in this respect anhydrite does not differ. The industry has unfortunately become fixated on the requirement for sanding. I think that because there are sometimes issues sticking the floor coverings down they get into their heads that they need to create a very rough surface in the mistaken beleif that this will help. It also provides a very convenient scapegoat for the primer and adhesive manufacturers to blame when there are issue. I have been on many problem sites where the lack of sanding and of course that devils screed anhydrite has been blamed by everyone concerned but on proper analysis and investigation in many cases the floor would have failed regardless of the level of sanding due to other problems with the build. Often the lack of a DPM or some other moisture ingress, in more than one case a leaking pipe under the screed has been a route cause.