Which mortar for basement wall repair

[Andy82]

My basement consists of walls that are:
Upper part (50%) lightweight concrete blocks
Lower part (50%) lecablocks

In the upper part, i.e., the lightweight concrete blocks, the mortar has disintegrated and turned into powder. Anywhere from 5 to 50mm needs to be filled. This is on the inside of the basement. The lower part with lecablocks does not have these problems, although in some places it is not completely filled 100% with mortar.

I have been considering plastering the interior basement walls as well.

1) What mortar should I use to fill the gaps? I've heard low-alkaline is recommended. Fina has something they call "Laga vägg wall filler," up to 100mm. Is this good? I became a bit skeptical since it was called filler. I've found plaster mortar C low-alkaline (plaster mortar vs. masonry mortar = same same?). Should I use two different mortars considering the different blocks?

2) What type of plaster should I use that works on both Leca and lightweight concrete? Highly moisture open? The basement hasn't been drained for many years and is also not insulated. I plan to start with that this summer. The walls themselves don't feel or look wet or damp, but I am an amateur in making that assessment. Is it okay to plaster before I've drained and insulated the walls on the outside?

 

[Anonymiserad 405730]

My basement consists of walls that are:
Upper part (50%) lightweight concrete blocks
Lower part (50%) leca blocks

The upper part, i.e., the lightweight concrete blocks, the mortar has dissolved and pulverized. Everything from 5 to 50mm needs to be filled up. This is on the inside of the basement. The lower part with leca blocks does not have these problems, even though in some places it's not completely filled 100% with mortar.

I've been considering plastering the basement walls from the inside as well.

1) Which mortar should I use to fill the gaps? I've heard low-alkaline. Fina has something they call Laga vägg wall filler, up to 100mm. Is this good? I got a bit skeptical since it was called filler. I found plaster mortar C low-alkaline (plaster mortar vs. masonry mortar = same same?). Should I use two different mortars considering the different blocks?

It's probably been built with regular masonry mortar B which is not low-alkaline.
You should scrape out everything that's loose and then vacuum out the joints and pre-water and then repoint with a low-alkaline masonry mortar C.

2) What type of plaster should I use that works on both Leca and lightweight concrete? Much diffusion open? Today, the basement hasn't been drained for maaaaany years and not insulated either. I plan to start with that this summer. The walls themselves don't feel or look wet or damp, but I'm an amateur with that assessment. Is it okay to plaster before I've drained and insulated the walls on the outside?

If the walls feel dry, there are no issues with plastering them again; you can plaster with a low-alkaline plaster mortar over the entire surface.
Laga vägg also works but is used mostly in wet spaces due to the short drying time compared to regular plaster mortar and the ability to build thick layers with it.
But it's an expensive product that will not give you any advantages in your case.

 

[Andy82]

It has probably been built with regular mortar B that is not low-alkaline.
You should rake out everything that is loose, then vacuum the joints and pre-water, and then fill them again with a low-alkaline mortar C.

If the walls feel dry, there is no problem plastering them again; you can plaster with a low-alkaline plaster over the entire surface.
Lagavägg also works but is used in wet rooms often because of its short drying time compared to regular plaster and it allows building up thick layers with it.
But it's an expensive product that won't give you any advantages in your case.

Thank you for your answer.
Does it need to be primed before I start repairing the wall with the mortar?

Regarding plastering. Do I use a coarse plaster as the first layer, or is it the same plaster? Use mesh or not?

When you say pre-watering, is it just a little, like with a hand-pumped sprayer?

While I'm at it... one more question:
Will I need to break up the drain for replacement, including in the bathroom. What mortar should I fill it with? If you cast a slab, it should sit for 3 months before laying tiles. How about a refilling in this case? As I understand it, self-leveling can usually be done after 4 weeks, but if it's just new concrete for laying the new pipe. Is it also 4 weeks in that case?

 

[Anonymiserad 405730]

Thanks for your answer. Do I need to prime before I start repairing the wall with the mortar?

No primer with traditional mortar..

Regarding plastering. Do I use some rough plaster as the first layer or is it the same plaster? Use mesh or not?

It depends on the surface you want to achieve. Regular plaster mortar is normally 0-3mm in the ballast, and you can get a decent finish with scouring and floating. If you want a finer finish, there is plaster mortar with 0-1mm ballast that you can use to fine plaster with.

Since you have two different materials, using mesh is a very good complement that won't cost you much if you do it yourself, so I highly recommend it.

When you say wetting, is it just lightly, like with a hand-pumped pressure sprayer?

Low-pressure sprayer on that, the water shouldn't run. If there's too much, just let it dry for a while..

While I'm at it...one more question. Will need to break up the drain for replacement, including in the bathroom. Which mortar should I fill with? If you're casting a slab, it should stand for 3 months before tiling. How about filling in like in this case? I've understood that self-leveling can usually be done after 4 weeks, but if it's just new concrete to lay the new pipe, is it also 4 weeks in that case?

Yes, regular concrete requires a long drying time, but there are faster variants that can be leveled on days after casting, e.g., Ardex A 38 Mix. If you search for rotbetong, you might find a cheaper variant with 😊

https://ardex.se/produkt/ardex-a-38-mix/

 

[Andy82]

No primer needed for traditional use...

It depends on the surface you want to achieve; standard rendering mortar usually has 0-3mm aggregate, and you can get a fairly smooth surface by skimming and floating. If you want a finer finish, there are rendering mortars with 0-1mm aggregate for a finer layer.

Since you have two different materials, using netting is a very good complement and won't cost much if you do it yourself, so I highly recommend it...

Use a low-pressure sprayer; the water shouldn't run. If there's too much, just let it dry for a bit...

Yes, regular concrete requires long drying time, there are quicker variants that can be self-leveled on a few days after casting. Ardex A 38 Mix. If you search for rotbetong, you might find a cheaper version😊

[link]

Thank you so much