Own production of lime plaster

[LAB]

Regarding mixing directly with quicklime, I also read some dissertation, but I can't form my own opinion since I have quite limited experience with anything else, at least on a larger scale. However, I have some bad news. One side of the greenhouse base that was plastered at the end of July has completely cracked now during the winter. Probably the lime hasn't cured enough, absorbed too much moisture, which caused the surface to crack when the cold came. It's unfortunate, but it can be fixed again in the spring.

I use a richer mortar for the foundation, approximately K1:1, meaning one part lime to one part aggregate by volume, and you need to adjust the aggregate so that it is not as well-graded as regular masonry sand to avoid drying cracks. It is otherwise common to use a hydraulic lime mortar closer to the ground.

If you haven't mixed and plastered with a rich mortar before, you'll notice that it behaves quite differently compared to a regular K1:3 or K1:4 mortar, it's not harder to apply but you need some practice. In the past (19th century), K1:1 was almost standard in my area, the samples I have taken on older plaster have all been roughly in these proportions. It is still not completely safe to use for the foundation in exposed locations but significantly better than a leaner mortar.

Another thing that is often done is to rough-cast the foundation, which also reduces the risk of the plaster freezing and cracking.

 

[apotekarnes]

you then have to change the ballast so that it is not as well-graded as regular mortar sand to avoid drying cracks..

Since I am a novice when it comes to masonry terminology, I have to ask: What do you mean by "not as well-graded"? I sense a lot of knowledge here that I don't want to miss out on.

 

[LAB]

Since I am a novice when it comes to masonry terminology, I have to ask: What do you mean by "not as well-graded"? I sense a lot of knowledge here that I don't want to miss out on.

Well-graded means that the gravel contains all size fractions in such proportions that there is minimal void space. Normally, the mortar becomes strongest then, especially if it contains cement, but if you use that type of gravel and increase the amount of lime, the gravel grains don't contact each other, and when the lime dries, it shrinks and cracks form.

If the gravel is more single-graded (and preferably coarser), there is ample space between the grains that can be filled with lime before the gravel grains lose contact with each other. This allows for more lime to be added while still achieving a mortar that does not crack when it dries since the gravel grains support each other.

The problem is that the masonry gravel/sand sold is suited for C- or KC-mortar, and you have to screen and wash it yourself to get suitable gravel if you want to use richer lime mortar. As far as I know, there is also no one who sells pre-mixed K1:1 mortar, so it must be mixed on-site or specially ordered, used principally for restoration of churches and the like.

Previously, there were a couple of smaller gravel pits not far from here with really good sand/gravel, naturally washed, but their permits expired, and I missed the chance to order a good stock

I can add that the recommendation is usually to use a well-graded aggregate even with lime mortar, but I couldn't get order with richer K1:1 or K2:3 mortar until I switched to the same type of aggregate used in the 19th-century render I sampled. With leaner lime mortar (K1:3 or K1:4), it's different; well-graded sand/gravel is surely best.