Plaster A, B or C

[Pettttter]

I am trying to understand the difference between various plaster mortars but can't seem to find the answer by googling. If you have a surface in the basement that is clean down to the concrete (i.e., not old plaster), what disadvantages are there to just using A, which if I understand correctly, is the strongest?

In most instructions you read, B is used on clean surfaces and C when improving old plaster according to the rule of always applying weaker than what you are plastering on. But if you've chipped/sanded down to the foundation, I'm trying to understand why you wouldn't want to apply the strongest possible, there must be differences in how pleasant they are to work with or something like that?

 

[Stefan N]

If you have any slight movement in the wall, A cracks more easily which is a disadvantage, and it is also less enjoyable to work with.

 

[Pettttter]

If you have any slight movement in the wall, A cracks more easily, which is a disadvantage, and it's also less pleasant to work with.

Ok. On the other end of the spectrum, what is the disadvantage of applying C directly on the concrete? Is it so soft/weak that it's a problem?

Which one has the best "breathability" properties to minimize the risk of new plaster detachment?

 

[Stefan1972]

Then you also have to think about what you're polishing and in what environment? concrete yes sure but what kind of house is it overall? What do you want to achieve? How should it look?

 

[Pettttter]

Then you also have to consider what you're plastering and in what environment? Concrete, sure, but what about the rest of the house? What do you want to achieve? How should it look?

It's an exterior basement wall, maybe 10m2. The old plaster has come off and looks terrible, so it needs to be redone. I don't really care about achieving a fantastic and perfectly even result, just mostly that it won't need to be redone again in 3 years. And that it's as easy as possible to apply since I'm a novice at this.

I should add that I'm not going to redo the drainage, etc., it's not worth it.

 

[Stefan1972]

Sure, but how old is the house? Can one kind of assume that they might have plastered with lime plaster or is it from the 50s-60s? Not that I work with plastering and masonry, but thin plaster on basement walls is always a bit risky considering moisture penetration.

 

[Pettttter]

Yeah, but how old is the house? Can we assume they might have used lime plaster or is it something from the 50s-60s?
Not that I work with plaster and masonry, but thin plaster on basement walls is always a bit risky considering moisture penetration.

1946. It doesn't look like lime plaster, it's probably been redone at some point since it was built. But it probably doesn't matter much since I plan to remove everything down to the concrete?

 

[kimtommy]

I'm neither a mason nor do I have formal education in the subject, but I have done some masonry, plastering, and casting at home. So, evaluate my information as you wish.

If the walls are fully cast in concrete, you can use Type A mortar. However, indoors there might not be much reason to do so. If they are brick walls, which is the most common, I believe in using Type B mortar. The general recommendation is B or C indoors as I understand it.

The difference between the mortar types is mostly the cement content, with Type A having the most cement and being the strongest. However, this comes at the expense of the ability to absorb movements in the construction. I tend to use Type A mortar for fully cast structures outdoors. It's usually called cement mortar type A. It's easy to work with, almost waterproof, and handles moisture and frost incredibly well. In my experience, Type B mortar does not withstand frost, especially not on stone hedges. Type C mortar might work in your basement, but I think it gets damaged more easily if you bump into it with furniture, etc.

 

[Pettttter]

I am neither a bricklayer nor formally trained in the subject, but I have done some bricklaying, plastering, and casting at home. So evaluate my information as you wish.

If the walls are cast in concrete, you can use A mortar. But indoors, there is probably not much reason. If they are bricklayed, which is the most common, I believe in using B mortar. The general recommendation is B or C indoors as I understand it.

The difference between the mortars is mostly the cement content. Where A has the most cement and becomes the strongest. However, at the expense of the ability to absorb movements in the structure. I think that for fully cast things outdoors, I always use A mortar. It's usually called cement mortar A. It's convenient to work with. It becomes almost waterproof and withstands moisture and frost extremely well. In my experience, B mortar does not withstand frost, especially not on free-standing walls. C mortar would probably work in your basement, but I think it would take damage more easily if you hit it with furniture, etc.

Thanks for the reply!

Do you know if there is any difference in how well B and C allow moisture through and thereby do not detach from the concrete? It actually doesn't matter how well it holds up to physical impacts on that wall...no one can directly reach it since there are benches in the way...so maybe C is best?

 

[kimtommy]

Thanks for the reply!

Do you know if there is any difference in how well B and C allow moisture and therefore do not detach from the concrete? It doesn't really matter how well it withstands physical violence on that wall... no one directly reaches it, there are benches in the way... so maybe C is best?

I'm probably just guessing now. But if you have moisture presence, C mortar will detach more easily due to the lower cement content, cement is the glue, less glue - poorer adhesion. It's a bit like building sandcastles. The sand adheres poorly and the castle collapses if water gets on it. C might be more diffusion-open, but it doesn't matter if it detaches, B will stay in place and get dark from the moisture. There is also KC mortar B. Then there's a bit of lime in it. I've used both and honestly don't notice much of a difference.

 

[kimtommy]

Finja has good and educational brochures about masonry and plastering.

 

[Pettttter]

Ok. I have another wall so maybe I'll try one of each and know in 5 years which one is smartest

 

[andreascarlsson]

In a year we'll know... 👻

(or maybe you already have some lessons to share?)

 

[kinevenow]

Ok. I have another wall, so maybe I'll try one of each and know in 5 years which is smartest

How did it go?

 

[aweste00]

Ok. Have another wall so maybe I'll test one of each and in 5 years I'll know which is the smartest

Found this thread and now 5 years have passed! How does it look?