Lime Render

Lime render is different types of lime mortar;

• Backing coat, ultra porous. Prevents wind driven rain penetration by not having interconnected pores and draining downwards internally like a wall heart with gravity. Mesa nad macro porous with interconnected voids.

• Mid coat, normal lime porosity. Draws water, when possible, from the backing coat towards the face. Micro, mesa and macro porous.

• Top coat, dense porosity. Prevents 'casual' rain from penetrating by creating a smoother surface for rain to run over. Allows the passage of vapour as a minimum. Mesa and micro porous.

• Lime wash. Sacrificial coating that protects the other layers from erosion. Dries faster too due to the increased lime content. It's also more hygienic to apply hot lime every few years. Micro porous.

The ratio of lime to aggregates should increase towards the face and the size of them should get smaller. ie. 3:1 backing, 2:1 mid, 1:1 top and 1:0 paint. Sharp sand 4mm down for the backing, plastering sand 2mm down for the mid, silver sand 1mm down for the top coat. the size of the aggregates will increase the rate of capillary action.

There is currently talk about vapour vs liquid capillary action. Both are required, capillary at the back and vapour at the front. Because that's how capillary action works, big to small. And it is is two scales with lime, kind of. Within the layers and the entire substrate. Each layer can move moisture and each layer does it at different speeds, therefore for them to perform exceptionally this needs to be addressed. Yes you can just put a thick layer of mesa/macro followed by a thin layer of micro but it will not work as effectively as the system described above.    

There are obviously a lot of variations of this but ultimately you need two things; highly porous back coat and surface protection from erosion. So a minimal requirement would be an earth mortar, lime washed. I have seen a lot of old housing stock that would have been for poor people which is just dirt out of the ground mixed with quicklime and hair, thrown on and limewashed. Hoylake is plastered using the sand out of the ground which is a silver sand. This was applied very hot and then limewashed or roughcast. North Wales is similar in that a lot of cottages are coated in the same way. I've seen decomposed Granite and slate as well as the usual sand/clay/limestone aggregates. 

•  I suspect that contrary to popular belief that Olde Worlde Folkes would be a bit disgusted with our approach to maintenance and hygiene with regards to masonry. We never wash it anymore... nevermind maintain it correctly. 

• I am a heritage mason who works on vernacular properties therefore i fix the rainwater detailing as part of my works. Ergo the issue with run-off due to improved water resistance is almost never an issue. However this could cause problems in a few cases but very specific ones. eg. if you had render above stone with no rainwater detailing the stone beneath could suffer, however making the render more porous would just introduce the water into the stone from the inside instead as it falls by gravity, trapping it on top of the wall. I am also ruthless and will not leave stone exposed because a client wants it to be if it could be harmful. I would prefer not to undertake the contract rather than do something incorrectly. 

An observation on the use of organic additives to improve renders. 

https://link.springer.com/article/10.1617/s11527-023-02175-z

This;

The action of the water repellents is based on the

properties of: (a) hydrophobicity, which prevents

water ingress by reducing the surface tension, and

(b) water repellence, which prevents water adhesion to

the surfaces. Lime mortars contain electronegative

elements that can easily absorb liquid polar water; the

effect of hydrophobization relies on the existence of

non-polar parts in the water repellents that are able to

increase the water contact angle. Those agents did not

completely fill the capillary pores but are rather

deposited on the pore wall allowing the water vapor

diffusion

Means this;

Hydrophobicity vs. Water Repellence:Hydrophobicity: This refers to the tendency of a substance to repel water. It's achieved by reducing the surface tension between the water and the material. Think of how water beads up on a freshly waxed car – that's due to hydrophobicity.

Water Repellence: This is a broader term that encompasses any mechanism that prevents water from adhering to a surface. It can include factors like surface roughness, chemical composition, and the presence of hydrophobic coatings.

Lime Mortars and Water Absorption:The quote correctly states that lime mortars contain electronegative elements. This is true, as lime is primarily composed of calcium oxide, which contains oxygen, an electronegative element.

The electronegativity of these elements contributes to the ability of lime mortar to absorb polar water molecules.

Role of Water Repellents:Water repellents work by introducing non-polar components to the surface of the lime mortar.

These non-polar parts interact poorly with water molecules, increasing the "contact angle" between the water and the surface. This means water tends to bead up and roll off instead of being absorbed.

Capillary Action and Vapor Diffusion:The quote emphasizes that water repellents don't completely block pores. This is crucial because it allows for the diffusion of water vapor.

Water vapor can still pass through the treated mortar, which is important for preventing the build up of moisture within the wall structure.

In essence: Water repellents for lime mortars aim to balance water resistance with the need for some level of moisture exchange. They create a hydrophobic barrier that minimizes water absorption while still allowing for the necessary movement of moisture vapor to prevent damage to the underlying structure.

Key takeaway: The quote accurately describes the mechanism of water repellents in lime mortars, highlighting the interplay of hydrophobicity, water repellence, and the importance of allowing for water vapor diffusion.

The point it's making is that some organic additives will not inhibit breathability but will reduce water ingress by lining the pores with a waterproof coating. Very important for exposed applications because this will reduce wind driven rain without preventing it from drying. Not for liquid water under pressure though, so no good in a cellar or retaining wall with persistent water ingress. And potentially good for some sedimentary stone to prevent mineral redistribution from percolation. AKA the formation of the hard shit on the face of stone from using putty mortars and NHL's with them. This is also how some breathable stone 'sealants' work.  

But... I'm not entirely convinced on this one as it may inhibit the self repair function of an air lime resulting in friable mortar. They are supposed to have water run through them and pick up free lime then deposit it in tiny cracks that are created as it expands and contracts. I guess that's why there isn't one mortar to rule them all after all.

But... There is still a lot of original mortar in the wall which can do that so I guess this lime stuff is a little more complicated than it may first appear. 

But... This can prevent salts from reaching the drying face resulting in deterioration behind it when the same method is applied as a coating to stone as a surfactant. So is this going to also be an issue. Perhaps not: salt resistant red mortars and full depth application rather than at surface level. 

That's a lot of but's...