Houtje Uytdenbos

@Childers

I'm not familiar with dolomitic limes but thanks for pointing it out. Good luck with your experiments!

Well I'm sure there are many differences and variables at play when you're shopping for lime commercially. However, the bread and butter of any type of mortar you make is very simply quicklime (CaO). What you're looking for is quite simply limestone that was fired in a kiln which is now CaO. And you want a product of a high purity (+90%).

When you make your mortar and add water to your mix, that is when you're slaking the lime. It then becomes hydrated lime. You can buy hydrated lime as well, but it is presumed that it is better to use quicklime and slake it yourself. The reason is because after the lime is slaked, it has a limited shelf life. Generally you want to avoid using hydrated lime that is more than a year old. Look on the bags for the dates. But still I would certainly recommend to use quicklime in stead, if you can source it. Of course, quicklime is a little bit more dangerous, so when you're adding it to your mix, take the necessary safety precautions.

When mixing mortar, you can just add the quicklime to the mix and give it the right amount of water while mixing. In terms of lime earth blocks, you probably need to slake the lime first by itself and then mix it with the earth. My guess is that you would have to slake the lime beforehand because the mixture for the blocks is rather dry since a machine is going to compress the earth into a block.

If you want to delve into the subject of lime much deeper, look into the work of Nigel Copsey.

There are mainly 2 types of lime: lime (pure +90%) and hydraulic lime (lime + pozzolanic additives)

Rule of Thumb:
generally (pure) lime is used above ground and hydraulic lime is used underground.

Pure lime sets slowly by exposure to CO2 and hydraulic lime sets when exposed to water (it even sets UNDER water)

Generally, when making any sort of pure lime based mortar, it is best practice to use quick lime.
These are often referred to as hot mixes because your mortar is essentially going to be warm when you're working with it.

In terms of hydraulic lime, best practice is to avoid NHL (natural hydraulic lime) at all costs because it is impossible to predict their quality.
If you make hydraulic lime based mortar, add your own pozzolanic material to the pure lime in order to have a consistent mix. This will require some testing of your own. If you need the mix to be stronger, add more pozzolan. However, rarely are you going to need a super strong mortar.

Pure lime and earth-lime mortars have great capillary action and this is why they're used above ground: the wind dries out the outer shell of your wall which will then "suck" the inside of your wall dry. At the same time, lime mortar offers protection from water ingress. However, water always finds its way into your wall so it is important that your wall is designed so that it can dry itself out very efficiently.

Hydraulic lime is stronger and has has much less capillary action. When exposed to water, it gets stronger over time. No one actually knows when "setting" stops. It seems that there's no limit to it. Roman concrete port piers are getting stronger still, to this day.

In practical terms though; the things you need: find a good supplier of very pure quicklime and a suitable source for pozzolans if you're going to be doing foundation work with lime. The rest is just sharp sand and/or loam or very fine silica sand or marble dust and natural fibers of varying sizes depending on what kind of mortar you're making.

masonry bedding mortar: loam + 10 - 15% quicklime
masonry pointing mortar: 1:1 quicklime/sand + small fibers
base layer plaster: loam + 10 - 15% quicklime + hay fibers
finishing plaster: 1:1 quicklime + fine sharp sand + very fine fibers

experiment with your own mixes. Depending on the type of soil that you use, some of them are gonna have more clay in them, in which case you might want to add some sand, or example. If your soil is very sandy, then you could either add some clay or just add a little more quicklime. What you're ultimately looking for is a mortar that is sticky but not too sticky because the stickier it is, the more it will shrink. Too much shrinkage causes cracks. For a plastering base layers, cracking is fine, since you'll be filling that in with a brown or finishing coat.

In general I would advise to avoid cracks as much as possible. In my opinion a good mortar is a fine balance: it should be workable and sticky enough for a skilled hand to do efficient work. But that doesn't mean that it sticks like glue. You need just enough stickiness so that it doesn't run off your trowels straight away, but not too sticky because that suggests that there is not enough large aggregate in your mix which -again- causes cracking and also makes your wall less strong and more unstable in terms of expanding and contracting due to temperature/humidity variations.

Unfortunately this is something you have to learn to feel for yourself. And the only way to do that is to work. Build some flats in your yard, get some bricks, whatever and mix up some mortar. Test different soils in your area. Add sharp sand if there is too much clay in them. Lay some bricks with your different mixes, do some plastering and take notes. If you keep at this for a month or so, you will quickly understand what a good mortar feels and SOUNDS like. The keyword here is BALANCE. A good mortar has a good balance of different aggregate sizes and performs as you want to perform: when it sets it becomes a stable mass with minimal to no cracking at all.

My final suggestion is: don't overthink it. Get your hands dirty. A book is not going to teach you what a good mortar is. Your own hands and your ears will. Good luck!