Recommend a primer for tiling plastered / painted wall?

[Ajax123]

Quote; whe portland cement and gypsum are placed in direct contact with each other in the presence of small amounts of moisture (bare in mind your cement add is mixed with water) they react chemically and the calcium alluminates hydrate to become ettringite and thaumesite salts which are very soft and friable. The bond between the cement and gypsum are destroyed.

Walls were plastered for 40+ years used Sirapite gypsum plaster on moisture damp sand/cement without any problems ???

Hmmm interesting. I haver to admit I thought Sirapite was a Lime based plaster but it appears from research it was indeed Gypsum based (similar to plaster of paris. I do know it contained cellulose and petroleum additives so I shall do some research ans see if I can come up with an answer.

It may be to do with the direction of the sulphate rich moisture. with the gypsum being at the outside surface the sulphate laden moisture would be towars the outside so these sulphates have less contact with the cement. I shall come back to you on this. To date I can find little information on Sirapite Plaster.

 

[White Room]

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Hmmm interesting. I haver to admit I thought Sirapite was a Lime based plaster but it appears from research it was indeed Gypsum based (similar to plaster of paris. I do know it contained cellulose and petroleum additives so I shall do some research ans see if I can come up with an answer.

It may be to do with the direction of the sulphate rich moisture. with the gypsum being at the outside surface the sulphate laden moisture would be towars the outside so these sulphates have less contact with the cement. I shall come back to you on this. To date I can find little information on Sirapite Plaster. [/COLOR]

Last time I used it was 23 years ago, it's actually pronounced siraphite but obviously spelt sirapite ???