What types of mortar?

[Henningelvis]

Hello

This summer, it's time to install flex conduit in the old chimney and seal up what I opened last year. It's quite a complex operation, as I'm installing four conduits in the same chimney for four different fireplaces, two iron stoves, and two tiled stoves.

There will be quite a bit of masonry work needed. I partially have to reconstruct what the iron stoves are standing on, which is a brick structure. I need to seal the chimney where the conduits are being installed, and I also need to seal a flue for a tiled stove that won't be used. I plan to do all this with bricks and some form of lime mortar, as the house dates back to the late 1800s/early 1900s. So the first question is: which lime mortar should I use?

Then, I need to secure the conduits at the top of the chimney. It's an old brick chimney that has received a new cap of some sort of coarse concrete. Should I choose a different mortar here, or can I continue with the lime mortar? It's outdoors, as mentioned, but there's a metal cap over it, and since the rest of the chimney is built with some sort of lime mortar, I thought I could continue using that.

Lastly, I need to reattach the caps on the tiled stoves. I've had to remove them to access the flues. Does anyone have a good tip on a clay mortar I can use here?

 

[Claes Sörmland]

Lime Mortar

I have worked with classic lime mortar, i.e., slaked lime and sand. Unfortunately, wet slaked lime is hard to come by here (you have it in Skåne, right?), so I've used bagged lime mortar E, which I mixed into a lime paste with a whisk. This lime paste is best made in advance in larger quantities and then placed in a large mason's tub covered tightly with plastic (like a piece of construction plastic). A little water floating on top is good. It doesn't cure, but it must not dry out. It gets better and stickier after a few days to weeks.

When laying bricks, take some lime paste and a suitable portion of masonry sand (natural sand) 0-8 mm that I bought from the local sand pit. It may need to be sifted first if there are larger stones in it. Mix with a whisk.

How much sand? Well, it depends on the age of the house how much sand was used. After the turn of the century, a lot of sand was often used, e.g., 1:5 lime and sand, check the existing mortar on the chimney and copy that. Lime mortar is very forgiving, so the proportions don’t seem to be crucial.

Clay Mortar

The simplest way is to buy clay powder in bags. For me, it's the local hardware store that stocks this—where the occasional stove makers shop. This is mixed with natural sand 0-8 mm (again sifted if needed) with a whisk. Typically, 1:3 clay-sand by volume is sufficient. Mix either in advance and let it sit for a few days to weeks; it takes time for the clay to fully hydrate, or mix a clay paste like the lime paste above and let it sit.

You can also dig up clay from the ground, but this requires more effort and knowledge. Natural clay contains a certain amount of sand/silt and is naturally varied in fat content.

Outdoors

Using clay and lime mortar outdoors is not a good idea. So either hydraulic lime mortar or cement-lime-mortar. None of these mortars can withstand direct wetting like on top of a chimney's crown, so sheet metal work, a roof, is required.

 

[Henningelvis]

Lime mortar

I have worked with classic lime mortar, i.e., slaked lime and sand. Unfortunately, wet slaked lime is hard to get hold of here (You have it in Skåne, right?), so I've used bagged air lime E, which I've mixed into a lime paste with a mixer. It's advantageous to prepare this lime paste in advance in a larger quantity and then place it in a large mortar bin covered tightly with plastic (like a piece of construction plastic). A little water floating on the top is good. It doesn't cure but mustn't dry out. It becomes better and more malleable after a few days to weeks.

When building, take some lime paste and a suitable proportion of mortar sand (natural sand) 0-8mm that I bought from the local sand pit. It may need sifting first if there are larger stones in it. Mix with a mixer.

Amount of sand? Well, it depends on the age of the house how much sand you have used. After the turn of the century, much sand was often used, e.g., 1:5 lime and sand, check the existing mortar of the chimney and replicate.

Clay mortar

The easiest is to buy clay powder in a bag; for me, it's the local Optimera store that has this in stock - that's where the few remaining stove builders shop. This is mixed with natural sand 0-8mm (again sifted if necessary) with a mixer. Type 1:3 clay-sand in volumes is usually suitable. Either mix in advance and let it stand for a few days to weeks, as it takes time for the clay to fully hydrate, or mix a clay paste like the lime paste above and let it stand.

One can also dig up clay from the ground, but it requires a larger effort and more knowledge. Natural clay contains a certain amount of sand/silt and is therefore naturally variably fat.

Outdoor

For outdoor building, clay and lime mortar are not a good idea. So either hydraulic lime mortar or cement-lime-mortar. Neither of these mortars withstand direct wetting that occurs, e.g., on top of a chimney crown, so sheet metalwork, a roof, is required there.

I live in Uppsala and the house to be built is outside Härnösand, so it's far from Skåne.

I can probably mix a lime paste myself and arrange for sand. The amounts of mortar I need are not unreasonable. Perhaps the chimney should be built with C-mortar? As mentioned, it has a metal hood, so what I build there is to some extent protected from rain and snow. Maybe I should take the opportunity to repair joints in the chimney as well while I'm up there and working.

 

[Norrtull]

Lime mortar

I have worked with classic lime mortar, i.e., slaked lime and sand. Unfortunately, wet-slaked lime is hard to get here (You have it in Skåne, right?), so I have been using E lime on sack, which I have mixed into a lime paste with a mixer. The lime paste is advantageously made in advance in larger quantities and then placed in a large builder's tub covered tightly with plastic (like a piece of construction plastic). A bit of water floating on top is good. It does not harden, but it must not dry out. It becomes better and stickier after a few days-weeks.

When building, a portion of lime paste and a suitable proportion of mortar sand (natural sand) 0-8 mm, which I bought from the local sand quarry, is used. It may need to be sieved first if there are larger stones in it. Mix with a mixer.

Amount of sand then? Well, it depends on the age of the house how much sand was used. After the turn of the century, a lot of sand was often used, e.g., 1:5 lime and sand, check the current mortar of the chimney and copy. Lime mortar is very forgiving, so the proportions aren't entirely critical, it seems.

Clay mortar

The simplest way is to buy clay powder in sacks. Here, it is the local Optimera store that has it in stock - that's where the few stove makers shop. This is mixed with natural sand 0-8 mm (again sieved if needed) with a mixer. Typically 1:3 clay-sand in volumes is usually suitable. Mix either in advance and let it stand for a few days-weeks; it takes time for the clay to fully hydrate or mix a clay paste as with the lime paste above and let it stand.

You can also dig up clay from the ground, but it requires more effort and knowledge. Natural clay contains a certain amount of sand/silt and is therefore naturally variably rich.

Outdoors

Using clay and lime mortar outdoors is not a good idea. So either hydraulic lime mortar or cement-lime mortar. Neither of these mortars can withstand direct wetting that occurs, for example, on top of a chimney cap, so metalwork, a roof, is required.

What an impressively clear guide, almost gives me the courage to tackle our old chimney stack.