Found this.....
Low Laitance or Skin Free Gyvlon Screed is a new generation Gyvlon screed available from does not generally produce a friable skin. This means that it does not need the same level of preparation as the old fashioned more traditionally known material which required that the skin be sanded away using a rotary floor scarifier and a 60 grit paper. However just because it does not produce a skin does not change the fact that the screed surface still needs to be prepared. Many adhesive manufacturers recommend that all types of substrate are sanded to remove any contaminants likely to affect adhesion. Low Laitance Gyvlon Screed will benefit significantly from running a sander over it in order to remove such contaminants and to provide a simple key for the adhesives.
Scabbling screeds is far too aggressive and can damage the screed so I do not recommend this as a preparation method. There is no reason to take off the "top 5 mm" of the screed. Again this can be detrimental to the screed which should be designed to relatively thin depths.
65m of Gyvlon screed will dry in approximately 90 days given that the screed is in a drying environment which is 20oC and 60% relative humidity with good ventilation. This time of year these drying times need to be extended. Moisture testing can be carried out using a hair hygrometer and we would always encourage this practice with all screeds. Obviously take notice of the moisture requirements applicable to the different types of adhesives as some need the screed to be drier than others. Additionally, you will note that some of the materials say that they do not require a primer. We are at present uncomfortable with this and would prefer to see a primer used in order to reduce suction into the screed and thus preventing premature drying of the adhesives.
In all cases where underfloor heating is present this should be commissioned prior to the application of primers and subsequent floor coverings.
In terms of adhesives we do not manufacture these materials and so it is not appropriate for us to commenting on specific materials for specific applications. However in our experience we would generally recommend the use of either 1 of the following systems.
1. Water dispersible Epoxy Primer and Flexible Cement Based adhesive
2. Acrylic primer and Gypsum Based adhesive
3. Uncoupling membrane used in accordance with the manufacturers requirements.
We would not normally recommend the use of acrylic primers with cement based adhesives although we acknowledge that provided all criteria are met perfectly that these primers will be perfectly acceptable. We feel however that the more robust solution is the use of water dispersible epoxy primers.
The products below are some examples of the types of materials we are talking about.. You should not take these as specific recommendations and you should seek the advice of the relevant manufacturers in order to satisfy yourself that they are suitable for the specific application in question. To place some context to the use of gypsum based materials over gypsum based substrates we are unaware of any failures with these systems where they have been applied.
I hope that this is of assistance and should you or your client have any further questions or queries please feel free to contact me.
Alan Jackson
Technical and Specification Manager
Lafarge Gyvlon
07545 932723
Gyvlon Environmental Flowing Floor Screed Solutions
"Lafarge Gyvlon - Helping Achieve Excellence"
Small problem now is the developer is wanting to get tiling job started, but the reading on the tramex states its its 0.7% and Tramex have confirmed today that the reading on the middle scale of the CME 4 machine must be no higher than 0.3% with underfloor heating over anhydrite screed. If there is no UFH the reading should be no higher than 0.5%.
