Priming Anhydrite Screed - Tile Advice

[jesteh]

Anhydrite screed on kitchen floor with UFH. There were some hairline cracks which were bonded with Flowcrete PHS. Floor is otherwise sound and dry and has been sanded.


Plan is to use porcelain tiles on Ditra mat. May also have to apply some levelling compound but only to small areas.


Question is regarding the next step - primer and adhesive before mat.
Would it be better to prime the floor using two part epoxy based primer (e.g. Laybond or Tremco CS175) and then follow on with cement based adhesive?
Or would it be better to prime the floor with acrylic primer and then follow on with gypsum based adhesive (e.g. Nicobond or Creative Impression)?
Or third choice - epoxy primer and gypsum adhesive although reading Ajax's posts thats not recommended.



It sounds like two part epoxy primer is a better primer but is not really compatible with gypsum adhesives.
On the other hand, gypsum adhesive may be a better choice of adhesive but it may be harder to use and Schluter do not recommend it for Ditra. Schluter instead recommended Ardex X7 (cement based).


Any advice appreciated.

 

[Ajax123]

If you use cement use epoxy. You will have far more chance of success. Both the tremco and the laybond systems are very good. Also merlin coatings barrier coat is worth a look. I would recommend a sand scatter to offer a good mechanical key. You can use gypsum adhesives with epoxy but in reality it would be pointless. Gypsum is more expensive than cement and as the epoxy will prevent any migration of moisture borne sulphates it is perfectly sensible to use the cheaper cement based product.

Another option would be to use a gypsum compatible adhesive but don't use a primer. This might be even more cost effective. Benfer and kerakoll do these sorts of adhesives. Don't use these if you prime the screed though.

My opinion ... Do not use an acrylic primer and a cement based adhesive.

Schluter have no real place making recommendations ne way or the other with gypsum simply because they have not tested them. They would be more correct to say they can't recommend them due to lack of information rare than saying they don't recommend them as this implied they don't work... In fact they do.

 

[Gazzer]

Another option would be to use a gypsum compatible adhesive but don't use a primer. This might be even more cost effective. Benfer and kerakoll do these sorts of adhesives. Don't use these if you prime the screed though.

As do Nicobond Gypfix !

 

[White Room]

If you use cement use epoxy. You will have far more chance of success. Both the tremco and the laybond systems are very good. Also merlin coatings barrier coat is worth a look. I would recommend a sand scatter to offer a good mechanical key. You can use gypsum adhesives with epoxy but in reality it would be pointless. Gypsum is more expensive than cement and as the epoxy will prevent any migration of moisture borne sulphates it is perfectly sensible to use the cheaper cement based product.



Another option would be to use a gypsum compatible adhesive but don't use a primer. This might be even more cost effective. Benfer and kerakoll do these sorts of adhesives. Don't use these if you prime the screed though.

My opinion ... Do not use an acrylic primer and a cement based adhesive.

Schluter have no real place making recommendations ne way or the other with gypsum simply because they have not tested them. They would be more correct to say they can't recommend them due to lack of information rare than saying they don't recommend them as this implied they don't work... In fact they do.

The word Chance sounds ominous.....

 

[Ajax123]

When you mix cement adhesive and anhydrite screed into the same system it is always a risk. My opinion... Don't

if you must you very very significantly reduce the risk of issues by introducing a sulphate resistant barrier between the two. This is what an epoxy primer does. Acrylic just doesn't work as well.

 

[jesteh]

Thanks very much for all the answers.


It sounds like gypsum adhesive is a better and lower risk option. If thats the case why on earth would anyone ever consider using cement adhesive on anhydrite - epoxy primer or not?


Just an example of using epoxy primer - I spoke to the Tremco about priming anhydrite with their stuff. Their advice:
1. Mix and apply CS175 epoxy primer first (~£100 for a tub)
2. When that dries apply CS100 epoxy primer over that (thats another £100)
3. When that dries apply levelling compound over the whole lot (thats probably next £300). Apparently you can't just use adhesive straight, you must use levelling compound first!


So thats £500 gone just to prepare floor for tiling. I am sure thats over-complicating to an extent but that is Tremco's advice on priming anhydrite.


Laybond on the other hand offer far simpler advice:
1. Mix together Screedmaster resin and the hardener (~£70) in correct quantities. Water it down to right consistency and apply.
2. Wait a day and start tiling .


How can the two systems be so different yet achieve the same result? Either system though is not cheap, then you have to get the quantities right, mix it carefully, apply it carefully, try not to get any in your eyes on your skin and if you missed a spot your tiles will fail. Why would anyone take the hassle if there is perfectly adequate gypsum based adhesive that will do the job? It sounds like people still do it so there must be a genuine reasons for it.


Finally a question for Ajax on priming for gypsum adhesive - I have been told by Creative Impressions (and others) that with their gypsum adhesive I must still prime the floor to create moisture barrier - in one of your other posts you also recommend priming to stop adhesive drying out too quickly. Are you saying in above that priming is actually not required?

Finally, what would happen if I applied gypsum adhesive on epoxy primer? Best of both worlds, absolutely no advantage whatsoever or a complete disaster :yikes:

 

[Ajax123]

Thanks very much for all the answers.

Finally a question for Ajax on priming for gypsum adhesive - I have been told by Creative Impressions (and others) that with their gypsum adhesive I must still prime the floor to create moisture barrier - in one of your other posts you also recommend priming to stop adhesive drying out too quickly. Are you saying in above that priming is actually not required?

Finally, what would happen if I applied gypsum adhesive on epoxy primer? Best of both worlds, absolutely no advantage whatsoever or a complete disaster :yikes:

Prefer the two systems you have also to bear in mind that the respective manufacturers are more used to vanilla than tiles. The tremco species one I would expect for a vanilla installation. Neither are absolutely necessary.

In a past life I wold have suggested 1coat of merlin barrier coat epoxy primer scattered with dry silica sand. Leave to dry and vacuum up any unstuck sand. Then tile away.

However to answer your final question it depends entirely on the manufacturer of the adhesive as different formulations have different requirements e.g. Gypfix from nicobond and CSA from granfix don't need a primer but the creative impressions gear evidently does.

Other adhesives that don't require a primer are benfergyps, h40ideal from kerakoll and tile to gypsum from norcross. There are possibly others that I'm to aware of.

The priming is to control suction into the dry screed which if it sucks the water out of the adhesive too quickly it hasn't Tome to set peoperly.

 

[Ajax123]

Oh and to answer your final final question if you tiled with gypsum onto epoxy your tiles would possibly not stick. Not sure though because no one I know of has done it. Yu use epoxy as a chemical barrier. If you use gypsum adhesive on a gypsum screed you don't need a chemical barrier as the two are chemically compatible.