Hi chaps, I've recently been doing a great deal of research about these type of screeds as it's clear a lot of fitters are cautious about the stuff. Same here Anyway, as always, the internet throws up a great deal of fluff and contradicting information, so I contacted Alan Jackson who has had a great deal of experience with this type of screed. He has worked for Lafarge Gypsum as both a specialist screeds manager and a technical and specifications manager spanning over 10 years. Prior to that he was an industrial chemist. Below is the main body of the email I sent him with his replies highlighted. I asked the questions that were most relevant to me, so I hope I haven't missed much out that you lads might want to know. <Start> I've not come across an anhydrite screed that I know of yet, but recently was made aware of problems associated with it, in relation to my field. I say problems, but should perhaps say considerations.. **Yes you should say considerations. There are generally very few problems with anhydrite screeds provided they are treated like anhydrite screeds and not like sand cement. The too are quite different**. I have several questions regards anhydrite. The first being the recognition of an anhydrite screed. My research has lead me to believe that a standard latex SLC would react with this type of screed ensuing in de-bonding, which for a floor fitter can be extremely expensive as I'm sure you appreciate. Is there any pointers you can suggest in distinguishing, on-site, between a commonly used sand/cement screed from anhydrite? I'd like to be able to assess this myself as being in contact with the installer is not always possible, with the added problem of Chinese whispers.. ** generally anhydrite screeds are lighter in colour and tend to be much smoother than sand cement screeds. If you can view a cross section they are often thinner and not honeycombed as they are fully self compacting. By far the best way though is to work from the specification which should tell you the screed type. I know this sounds like a but of utopia but it is nevertheless true. With a little experience it becomes relatively easy to recognise. If you really cannot find the answer to what screed type it is then I usually advise treating it as if it were anhydrite anyway.** The second question I have is in relation to measuring the relative moisture content of an anhydrite screed. My research has lead me to understand that these type of screeds dry out through crystallisation. Hence, a humidity box is not accurate to gain the readings. Is there an accurate evasive and none invasive method of assessing this type of screed? ** your research is a little flawed in this respect and probably driven by some of the rubbish I occasionally see on the internet. Whilst it is correct that the setting process for anhydrite involves crystallisation of the anhydrite powder to form gypsum which uses some of the water in the fresh material All screeds including anhydrite dry by evaporation of residual moisture from the surface of the screed into the surrounding atmosphere. The dryer the atmosphere the faster they dry. This is detrimental to sand cement but is the aim for anhydrite. It can also be force dried which is something else that should not be done with sand cement. This can be achieved by applying heat either from an external source or by underfloor heating. At the same time as heating the moisture coming out of the screed needs to be removed from the air above it. The accepted and correct methods of testing for moisture in all screeds is to use a hair hygrometer or surface digital hygrometer. Other methods are the carbide bomb test and the oven dried sample test both of which require samples of the screed being removed. ** With a sand/cement screed, should the RH read high, a liquid dpm can be installed but as my research has taught me, the use of a liquid dpm is not possible with anhydrite, therefore accurately assessing the moisture content is extremely important to me. ** surface damp proof membranes can and are used on anhydrite screeds subject to their being below 1.5% moisTure (approx 92% rh) and subject to them being at least 28days old. In all cases whether sand cement, concrete or anhydrite these membranes should NOT be used on screeds with underfloor heating. This makes anhydrite very advantageous as you can use the heating to force dry the screed thus removing the need to DPM's. It is usually cheaper to force dry than to to DPM. anyway.** Linked to the last question. Is a dpm required? Is the surface evaporation of moisture completely none existent due to these screeds drying out through crystallisation? If a floor covering was applied (impervious or pervious), would the Anhydrite screed dry out naturally without air? **see my previous answer** <End> Well, I found that pretty useful and hope it's given a little insight. I think we may be seeing a good deal more of Anydrite screeds in the future. Any thoughts?
So they can be forced dry via ufh coupled with a dehumidifier then, good to know, id come to the conclusion that they had to dry naturally and fully to >75rh before you do anything with them.
Same here pf...Plus the testing method seemed sketchy (or the reading from a humidity box anyway). Good to know that's possible. I also found the use of a surface dpm, really good information to know. I got in touch with Colin Ward from ardex, regards this question and asked his suggestion on suitable products.. Here is his reply.. "As a company we have been using our products When installing anything on an Anhydrite screed the screed should have been laid, prepared and tested in accordance with the manufacturer’s instructions. This generally means that the screed has to be sanded and be less than 0.5% mc or 75%RH in accordance with BS 8204 and laid on a floor that has an effective dpm before any floorcovering is laid. Cement based products cannot be laid direct to Anhydrite screeds which are gypsum based as there is a reaction so a primer is always used which in our case is usually ARDEX P51. ARDEX DPM 1 C can be used on a sound, Anhydrite screed so long as it is not heated and is less than 85%RH, we state this on our literature. The quickest way to test the moisture content is to use the Carbide Method but these are expensive to buy. Hope this helps Colin" Proper learning curve for me all this
Fair enough Spacey lol...Might be worth saving the post in case you ever need the info, but hey, that's up to you
calcium sulphate/anhydrate are totally fine to work with so long as u realise that's what it is and not mistaken for the std s/c gear. grind, prime, screed. tho try telling mrs smith in her new build that your gonna charge her x amount more than the shop down round to grind the floor. 'but the shop down the road arnt charging me for all that!?' ok missus, I'll hear about the failure soon enough lol
All this should be given as literature to consumer would make our job easier to just say check your info
I thought that the people who lay the anhydride were meant to come back and remove the skin a few days after it's put down?
I'm glad to hear it Cool...I got a bit panicky after reading a thread on here and hearing about popping SLC etc, then going to price a recent job with a newly laid slab which had a RH of 76% within 6 weeks...Put two and two together and poo'd my pants lol
prob I'm coming across of late is pricing up against other company's who obviously arnt realising it's an anhydrate floor! pricing for a std setup and I lose out coz I'm charging more. but I always highlight to the customer to make sure the other quote says ANHYDRATE/CALCIUM SUPLHATE preparation etc lol
Everyone else in Europe lets there subfloors dry out naturally they don't use dpms. It's only us daft &@£?! Who can't wait and are rushed into doing so.
