Recently a job was completed for me and I'd like some advice if possible.

Floor comprises of a poured anhydrite calcium sulphate (gypsum) screed. The tile adhesive use was BAL Rapidset Flexible adhesive and the adhesive was applied directly to the floor screed. Recommended primer such as BAL Prime APD was NOT applied to the surface. Before you say, why wasn’t the primer applied, the job is now completed and I am looking for the best way to deal with the situation.

I have been informed that there may be a reaction between the calcium sulphate screed and the Bal adhesive as the Bal adhesive contains Portland cement and ettringite failure may occur.

The tiler that completed the job has informed me that he has never see any issues with BAL Rapidset Flexible but I do not think that the gypsum screed is something that he has applied tiles to before.

My question for you experts is, what are the chances of a reaction occurring and the tiles lifting off the floor. Would you take all of the tiles up now, prime the floor and refit or take the wait-and-see approach as perhaps there is a very small chance of failure occurring and the refit of everything now may be and unnecessary step not to mention very expensive.

Thanks,
Paddy.

Depends on what the surface will be exposed to. If it is exposed to water/damp/heat, then you could have some serious problems. Never had to deal with such a problem myself, so I'm not an expert on this particular matter.

Originally Posted by sWe

Depends on what the surface will be exposed to. If it is exposed to water/damp/heat, then you could have some serious problems. Never had to deal with such a problem myself, so I'm not an expert on this particular matter.

Hi sWe.

The screed has underfloor heating pipes embedded in it so there is a heat factor. Some of the tiles are located in bathrooms also.

Thanks,
Paddy.

Originally Posted by ecopaddy

Hi sWe.

The screed has underfloor heating pipes embedded in it so there is a heat factor. Some of the tiles are located in bathrooms also.

Thanks,
Paddy.

As far as I know, water is chemically bound to both the gypsum and the cement based adhesive as they cure. Heat might facilitate a reaction between the gypsum and the adhesive by freeing the water up, little by little, and any damp might have an accelerating effect on such a reaction. It can also work the other way around, with water triggering a reaction and heat adding to the effect.

As I stated previously, I'm not an expert on the subject, but if I were you, I'd contact BAL and ask them for advice. They made the adhesive, so they should know more.

Hi eccopaddy, Bal recommend's that apd primer is used and in wet area's tanking is used with Bal wp1 I have never laid tile's on this screed but have heard of problem's but think this may have been because a lack of primer and probably because tiler's where'nt informed about the screed. Also heard that ditra has been laid over as well, Personaly I would wait to see if you get a problem rather than rip up you could be lucky and not get a complication, And if you do I would be ringing your tiler.

Hi Paddy

Calcium sulphate and portland cement are not compatable - a whilst a failure is not guaranteed it is a real possibility. Over time ettringite crystals may appear and as they expand will cause the tiles to lift and de -bond. It may well take several months to determine how bad the reaction might be - certaily worth a call to Bal for their advice - or even the TTA

really annoys me when people give advice even though they havent got a clue.
however did the tiling is an idiot, the adhesive he used is cement based the floor is gypsum based, if your using a cement based adhesive you should NOT take Bals advice, but instead use a two part primer which will seperate the two products, or alternately use a gypsum based adhesive with a basic primer which will stop the dry floor sucking water out of the adhesive and making it dry to rapidly.
even if he had been tiling directly to a cement screed he should have used primer for the reason ive just mention at the end of last statement.
unfortunatley there are lots of cowboys out there thinking they know what there doing, (MellonS)

Paul

Originally Posted by paul00692

really annoys me when people give advice even though they havent got a clue.
however did the tiling is an idiot, the adhesive he used is cement based the floor is gypsum based, if your using a cement based adhesive you should NOT take Bals advice, but instead use a two part primer which will seperate the two products, or alternately use a gypsum based adhesive with a basic primer which will stop the dry floor sucking water out of the adhesive and making it dry to rapidly.
even if he had been tiling directly to a cement screed he should have used primer for the reason ive just mention at the end of last statement.
unfortunatley there are lots of cowboys out there thinking they know what there doing, (MellonS)

Paul

Welcome Paul..

Use the forum search feature in the top right of the forum and you see this product discussed in greater depth than it was 3 years ago..

how do you no wot type of screed it is before tiling it ? not being rude , just would like to no . is there a test or something you can do ? just alot of floors look the same to me give or take a few different shades etc... ?

Ecopaddy. Sadly I believe that there is a very high indeed almost certain risk of the tiles failing - it is just a matter of time. If no primer was used between the adhesive and the screed the damage in terms of chemical reaction is likely to have already been done. The action of heating the screed with the underfloor heating will only exacerbate the problem. I hope it is ok for years to come but bitter experience tells me that this is a very optimistic prognosis. Cement Based adhesive simply MUST be divorced from the screed by means of a suitable primer. Better still use a gypsum based adhesive. My suggestion is that once the tiles begin to delaminate (leave it as long as you can) then you will need to lift them and if possible save them (this is unlikely to be possible) You will then need to sand the surface of the sscreed to remove any residual ettringite and adhesive and then prime the screed with a suitable primer. I would then suggest using a Gypsum based adhesive to re stick the tiles.

In terms of saving the tiles the only hope I have is that as the screed was not primed it will have sucked the moisture from the adhesive very quickly and it may not have stuck to the tiles very well.

Sorry to be so pessimistic

Originally Posted by terry the tiler

how do you no wot type of screed it is before tiling it ? not being rude , just would like to no . is there a test or something you can do ? just alot of floors look the same to me give or take a few different shades etc... ?

Often it is a case of back to basics and simply ask the question. If it is a gypsum screed it is likely to have been pumped and will have a surface texture somewhat similar to that of a smoothing compound (although this is not clear cut) It is also likely to be lighter in colour and much thinner than sand cement. My over riding advice generally is that if you cannot find out what the screed is by asking the questions then treat it like anhydrite anyway as anything you do with anhydrite will work with cement. Alternatively you can always give me a shout at work or PM me through here for advice.

FAO: Paul 00692:

The first time I had to lay slate onto a 30 sq/m anhydrite screed (3+ months old to tile onto), I phoned up BAL, and they informed that if I used BAL APD as a primer, applying it neat in one coat, and then another at 90 degrees to it, allowed to dry, and then fixed the slate using a BAL product I would get a guarantee for 25 years. I'm not from BAL, but I haven't had a phone call from the customer yet. They had piped under floor heating which I informed them the low-down on heating times/degrees with regards to adhesive. BAL grout is crap to work with, I used Ardex which is very reliant and very quick. Looks good too.

I've sort of replied already, but not really answered your question Paddy...

My question is: Who knew it was an anhydrite screed and when? It sounds very strange that you are now realising possible problems of the substrate screed after tiling has commenced. Why would this be investigated unless a failure in the installation had occurred? However, regardless of all this, I believe what the experts in the adhesive industry say - your tiling installation will fail before it's time, so rip it up now if you can salvage anything. Sorry.