Pointing and plastering of natural stone rubble walls (interior)

Hello

I am Roberto and recently became a homeowner. Now I have a few questions regarding "pointing/plastering" of natural/rubble stone walls, indoors. Perhaps some of you already have experience with this or even tips for me. A few details about the building fabric: The exterior walls of the building are built of natural/rubble stones, and the interior/partition walls are made of simple bricks. The house was built around 1900 and renovated in 2000 with a new exterior plaster and a new roof. Inside, nothing has been done since about 1960; the plaster had significant cracks and I have already completely removed it. The plaster was a lime/sand plaster plus some binder. The now visible natural/rubble stones were laid with lime/sand mortar but are now only loosely stacked, which presents a big problem for me. Although the walls are almost 60cm thick, there is no hold; it is absolutely impossible to install any anchors for radiator brackets or kitchen cabinets! The old mortar is just dust now; after scraping and brushing with the street/wire broom, joints and holes over 10cm deep have formed due to fallen stones/chippings. Now my question: with which material can I best fill/patch the joints? In the area of the heating pipes (to be surface-mounted later) and radiator brackets/window recesses, I used a sand/cement mixture (4/1 with washed sand). According to internet forums/guides/info pages, this is not correct because this cement mixture is too hard and supposedly destroys the wall in the long term. The cement is also said to prevent the wall from breathing and to be too hard. In forums and guides, trass cement is often mentioned for natural/rubble stone, but according to my information, this is only necessary to prevent efflorescence and weathering of free-standing natural stone walls. Some recommend again the traditional method of working with air lime + sand, but how do they get anything to hold firm in their constructions? Friends and acquaintances recommended well-known construction companies to me, which I then contacted on site for offers. The construction companies wanted to apply a bonding mortar in a net-like pattern and then plaster the walls with two layers of lime/cement plaster. However, this solution makes me suspicious. On one hand, cement should be harmful, but bonding mortar (cement-based) and lime/cement plaster are supposed to fix it??? I am also doubting that pure lime/cement plaster will provide the hold I want in the deep joints. Then there is the topic of wall breathability, which I can only imagine to a limited extent with lime/cement plaster. Wouldn't that lead to wall sweating? Because this type of plaster does not absorb and release moisture very well? Due to the deep joints, this will also be quite a thick plaster layer(s), which in my opinion (layman) will not breathe much, does not provide much hold (anchors etc.), and due to the high material effort, results in a hefty final price. According to construction companies, a long-term crack-free wall on these substrates is not possible. I also have a question about the breathability of the walls. Does the plaster breathe or directly the wall or masonry? In my profession (plant mechanic), I deal daily with reinforced concrete buildings. There, nothing breathes either; everything is regulated by the plaster (gypsum plaster). What would be the best material to fill the deep natural/rubble stone wall joints and which type of plaster would be advantageous? I am grateful for any help!!! Best regards, Roberto

Hello Roberto,

- The whole structure "breathes" – including your reinforced concrete (only very slightly); this refers to the water absorption and release of the wall and only marginally involves air.
- Since the stones were laid with lime mortar, you should also fill them with it; however, you can make the top joint (the last 3 centimeters) a bit firmer (e.g. 6/2/1 sand/trass lime/trass cement) since there is no major moisture exchange occurring there. The mortar just needs to be softer than the stone to compensate for "movements" and pressure. The trass is also important inside, otherwise your plaster will bloom. Alternatively, you only make firmer joints where you want to hang something or secure outside the joints.
- If you want to plaster the walls, I would use a pure lime plaster or a clay plaster (great indoor climate). Of course, then you can't hang anything on it either.
- So only two sensible measures remain.
1. Install a drywall partition or
2. install a masonry shell
both naturally require space, but by choosing the right stone, you can also greatly reduce your energy costs.

I hope I was able to help you at least a little.

Greetings from Berlin
Thomas