How do I recognize if a plaster also contains gypsum?

Hello,

in my soon-to-be-rented apartment (building year 1972, windows with double glazing since 2010, insulated ceiling, not insulated facade) I want to install calcium silicate boards on two interior walls in the dining room because mold has appeared in the corner in the past. The problem is that according to the manufacturer’s specifications, calcium silicate boards must not be applied on gypsum or lime-gypsum plasters, so this gypsum-containing plaster would have to be completely removed beforehand.

My question is: How can I tell whether the plaster also contains gypsum?

As mentioned, the building year is 1972 and my uncle was already on the construction site at that time. He said that it is probably a lime-cement plaster; however, he is not sure.
Unfortunately, quite thick layers of paint have been applied several times, so you cannot really feel with your fingers what type of plaster it might be. But I am not a construction expert and could only guess anyway.

Could you also break out a piece and have it examined? If yes, who does that?

Best regards
Robert

I recommend you - this also applies to other simple questions - as a layperson to also try ChatGPT. It provides quite good tips, which I would have given myself.

1. Color and Surface:

[*]Gypsum plaster has a very light, almost white color, which is often lighter than cement or lime plaster.
[*]The surface of gypsum plaster is usually very smooth and feels soft and fine-grained. It is easy to recognize when you gently run your hand over it.

2. Fingernail Test:

[*]Gypsum plaster is softer than many other types of plaster. You can easily press a notch into the plaster with your fingernail, which is more difficult with harder plasters like cement or lime plaster.

3. Moisture Sensitivity:

[*]Gypsum plaster reacts sensitively to moisture. If you slightly dampen the plaster in a moist spot, it often darkens. With prolonged moisture, it can crumble or become soft because gypsum is not a waterproof material.

4. Tapping Test:

[*]When tapping on a gypsum plaster surface, you often hear a dull, muffled sound compared to harder types of plaster, which can have a firmer, sharper sound.

5. Reaction to Sanding or Drilling:

[*]When you sand or drill the plaster, fine, white dust is released, which is another sign of gypsum plaster.
[*]Gypsum plaster is relatively easy to work with, while cement-based plasters offer more resistance.

6. Structure and Layer Thickness:

[*]Gypsum plaster is often applied in thin layers (approx. 5-10 mm), and its structure can often be very even if you expose it somewhere.

7. Identification by Small Scratch Tests:

[*]A small scratch test in an inconspicuous spot can be helpful. Gypsum plaster is typically fine and powdery when scraped off.

8. pH Test:

[*]A somewhat more technical test is the pH test: gypsum has a neutral pH around 7. Lime plasters, on the other hand, are rather alkaline (higher pH). With a special pH test strip or solution, you can test the pH value at a moistened spot.

Especially the tips on moisture, hardness, and dust when drilling are good indicators.

Thank you very much for the many tips.

I would definitely recognize a pure gypsum plaster from this. Based on your tips and the texture, I can probably rule out a pure gypsum plaster. It seems more difficult to me if the plaster consists of lime and gypsum. However, in this case, at least some white dust should be released when drilling, right? The test with moisture and hardness would be more difficult there.

What would actually be the consequence if the wall plaster behind the calcium silicate boards contains a proportion of gypsum?
The boards (2.5 cm thick) themselves are highly vapor permeable and diffusion open, so the moisture absorbed from the air is conducted through and would then be in contact with the gypsum plaster. The instructions for the boards and the anti-mold plaster mortar state that an existing gypsum plaster should be removed, especially if condensation is expected in the gypsum or lime-gypsum plaster layer.

I would scrape off some plaster and put the crumbs in water. Then you can already see whether there is gypsum in it or not. Then rub it between your fingers. If it becomes slippery and fine, then it is gypsum. Lime cement plaster is much coarser and does not dissolve in water.