Mix lime in C mortar for plastering chimney?

[Stefan_B]

I seem to have heard that it's possible to mix more lime into regular c-mortar for plastering a chimney. In the past, it was common to use pure lime mortar, but as I understand it, it's common to use cement mortar for plastering today? Provided, of course, that all the old plaster has been removed. Mixing more lime into the c-mortar would make it more elastic. Does anyone know more about this? What kind of lime should be used in that case? How much should be mixed in?

 

[Claes Sörmland]

If you mix släckt kalk (=Murkalk E) into regular C-bruk, you get some sort of lime-cement mortar (KC-bruk). KC-bruk was a popular mortar in the past, and there are still ready-mixed KC-bruk mixtures available for purchase.

(Most mortars sold today are indeed based on cement as the binder. However, they are usually not called cement mortar since the cement is mixed with limestone powder. Portland cement + limestone powder = murcement. What is sold under the name cement mortar is typically pure Portland cement and aggregate, i.e., a variant of concrete.)

 

[Stefan_B]

Thank you for the response! Do you have any idea how much mortar lime one should mix in when plastering a chimney breast indoors?

 

[Claes Sörmland]

Thanks for the answer! Do you have any idea how much lime you should mix in when plastering a chimney indoors?

Typically, it is lime/cement/sand 0-3 mm 50/50/650 in KC mortar where the weight ratio between the ingredients is specified (approximately 1 part slaked lime, 1 part building cement, and 8 parts sand by volume). But isn't it easier to buy such mortar from the start? It is quite standard.

 

[Stefan_B]

Typically, it is lime/cement/sand 0-3 mm 50/50/650 in KC mortar where the weight ratio between the ingredients is indicated (about 1 part hydrated lime, 1 part construction cement, and 8 parts sand). But isn't it easier to buy such mortar from the start? It's quite standard.

Looked it up a bit and it doesn't seem obvious to get hold of KC mortar. You don't happen to know the mixing ratio for regular C mortar as well? Then I could calculate how much lime to mix in if I can't get hold of KC mortar.

 

[jonaserik]

The companies that sell to those involved in plastering with gypsum usually have lime in bags.

 

[torparavgrund]

Why not simply use lime mortar? Both my local Beijer and XLBygg have different variants, and Bauhaus comes up as the first option when I google. Hornbach probably has it too; as a German chain, they are likely to have good stuff for this, lime plaster is much, much more common down on the continent.
I have used both the air lime mortar Weber cal 142 and the harder hydraulic lime mortar Weber cal 148 with good results. Air lime mortar on the chimney breast indoors, beautiful white color, and sticks like a rock.