Cement based adhesive on calcium sulphate (gypsum) screed

[ecopaddy]

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.

[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.

[ecopaddy]

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.

[sWe]

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.

[whitebeam]

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.

[Cliff Anger]

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