Houtje Uytdenbos

Arlie Grunseth wrote:Hydrated lime is lime that started as "hot lime" that has been slaked, then dried. It is not a setting material when water is added because the chemical reaction has already occurred from the kiln, then the slaking and drying process. This lime is good for binding other materials together and making them more plastic but not really a strong material by itself. Hot lime is slaked by sifting lime powder into water carefully (with eye protection!) and letting the chemical reaction commence until is is fully worked and excess water still stands on the surface.The lime is allowed to settle into a putty and completely hydrate before it is used. Hot lime is a setting material, it will take 50 years to reach it full strength but is sufficiently strong for construction even  after the initial set. Hydrated lime putty can be mixed 50/50 with gauging plaster for smooth plaster work, gauging plaster is a gypsum based plaster. I have worked as a plasterer for 25 years and as a hod-carrier for another 10. Most of that work was commercial work so most all materials were processed and bagged. I am now retired and really enjoy reading, on this site, about old/new methods of earthen plasters and would like to work with them. I don't see where gypsum is discussed or used much by permies which makes me wonder if it might not seal off walls from water vapor on the inside just as cement stucco seals from the outside, or maybe there is another reason.

No it's when the quicklime pebbles are slaked with a pozzollan or when using hydraulic lime or portland cement that the compressive strength pretty much increases indefinitely. Maybe a pure hot lime created from pebbles or rock does increase in strength over time, but that's only because there are some unslaked bits in there that might only slake years after fact after there is some water ingress. But hydraulic lime does indeed keep settings. It does reach it's maximum'ish potential in a year or so, but after that, year by year, it gradually gets stronger over time, but only so little that it is almost insignificant. In theory, scientists do not know the true compressive strength of hydraulic limes because no one has measured it over the span of say 50+ years.

Jeremy VanGelder wrote:I've got a pile of red bricks, a bag of hydraulic lime

use either stone or concrete block for the foundation. Brick doesn't do well when perpetually wet.
Just use regular portland cement for the foundation because it's much cheaper. Hydraulic lime and portland cement are essentially the same thing. Unironically, portland cement is probably significantly "greener" than hydraulic lime. Portland cement production is very effient due to large demand. So to cut production costs, their manufacturing process is much more efficient and thus "greener".

When you marry your wooden mud sill to the stem foundation, put it in a bed of air lime mortar on top of you stem foundation. Then fasten your anchor bolts. You want the wood to be in contact with air lime mortar because it dries faster so your mud sill is less likely to rot.

Rule of thumb: everything below ground: use hydraulic lime (aka Portland cement)
everything above ground: use air lime.

for the stem wall it's better to do it in portland cement entirely, even the part that is above ground which would be about 50cm's or so. The foundation/stem wall is designed to stop and resist moisture: rising damp, rain water, splash water, ... The idea here is that your foundation and stem wall have to be able to resist the erosion effects of being perpetually damp. The portland cement mortar will also act as a plug and it will stop most of the rising damp going up into your wooden frame. So finish off your stem wall with a layer of portland cement mortar, make it flat and let it set. When it has set, put a bed of air lime mortar on top and then sandwich it with the mud sill.
But starting from the mud sill and up, everything needs to be designed to be able to dry out fast. For example: don't paint your wooden frame with latex paint or something silly like that. If you want it painted, use limewash or whatever.

@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!