Lime mortar ‘hot mix’ for Cob house, in october / Mortier de chaux mélangé ‘à chaud’ pour la maison en bauge (cob), en octobre

In October, we learnt hot mix of lime mortar, and started using it to build the stone stemwall of our cob house. In this post, we explain how we prepare it at our farm, and how others on internet make it.

What is lime mortar and why to use it ?

Lime mortar (and plaster) is made from lime, sand and water. Lime is an ecological natural building material, as it can be easily prepared in small scale from calcareous stones (see the video below), and does not requires chemicals and huge amount of energy that Portland cement fabrication requires. Additionally, while setting, lime mortar fixes CO2 from the air, and when fully set, lime mortar revert to the calcareous stone that lime was made from while keeping an amazing self-healing ability from cracks.

Lime is made from calcareous stones (limestone, marble, etc) by cooking it at 800°C. Chemically, calcareous stones are calcium carbonates (CaCO3). When cooked, they loose loose Carbon in CO2 form, to become quick lime stones (i.e. CaCO3 + heat → CaO + CO2, cf. lime on wikipedia). When in contact with water, quick lime (CaO) react releasing heat, to become slaked (or hydrated) lime (i.e. CaO + H2O → Ca(OH)2). Then, when exposed to air CO2, slaked lime slowly sets and becomes the calcareous stone (CaCO3) it was in the beginning, a process known as carbonation (i.e. Ca(OH)2 + CO2 → CaCO3 + H2O, cf. slaked lime on wikipedia).

Why make lime mortar hot mix (or dry slaking) rather than using lime putty?

Hot mix (or dry slaking) preparation of lime mortar and plaster from unslaked lime, sand and water, is not a very known method that has the benefit to not require maturation time. In this method, lumps of quicklime are lightly dampened to initiate slaking, and layered or covered with damp sand, and the mortar is ready to use after only few hours (See next section for the details of preparation). But most people only know lime mortar preparation from a lime putty, prepared by slaking lime in water, and left in water to mature for at least three months.

Lime putty preparation method is often considered as the historic way to prepare lime mortar or plaster. In Antic Rome, lime putty had to be matured five years by law, and until now, historic building restoration rules requires using lime putty matured for years. But recently, historians began to consider that lime mortar was probably made on site with hot mix or dry slaking from unslaked lime, for practical reasons, as lime putty was heavier to transport and required a long maturation period (cf. p76 of Mortar, Renders & Plaster by English Heritage). Today, few advocates of hot mix even consider that hot mixed lime mortar is superior to lime mortar from putty, because the slaking happen with lime and sand altogether, which improves binding between them, and also because the hot mix uses less water and leave a small portion of unslaked lime which may improve the self-healing properties of lime mortar after setting.

Matured lime putty, hydraulic lime ready to use and even ready lime mortar, are sold by material shops in certain countries (e.g. Conserv. in UK). If they are available to you, or if you are ready to make putty and wait at least three months, you may not need to make hot mixed lime mortar. But if you have no time and only find quick lime (that you can make yourself, see the video below), like we do, it is the way to go. In our case, in Tunisia, it is impossible to find other lime products than traditional quick lime, we dont even find unslaked natural hydraulic lime (the only factory in the country, Interchaux at Thala, is not selling hydraulic lime any more). It is how we got interested in making hot mix mortar.

Quick lime preparation in DIY kiln

How we make lime mortar hot mix, to build our cob house stem wall ?

In the video above, we show how we use hot mix (or dry slaking) method to make lime mortar in our Permaground RasJebal Farm, to build the stone stemwall of our cob house, following these steps :

1 / We place 1 volume of non-hydraulic crafty quicklime stones within a circle of 3 volumes of damp coarse sand.


Add lime to sand

2 / We add 1 volume of water in the center.


Add water

3 / We draw the sand rapidly over the quicklime. The heat produced by slaking drives off some of the moisture from the sand in the form of steam.


Draw sand over lime

4 / We leave it slake for at least 45 minutes. As the quicklime slakes, it expands, forming fissures in the sand.


Let it slake for at least 45 min

5 / The result is a dry powdered lime and dried sand. We mix the lime with the sand.


Mix lime with sand

6 / To have a texture of mortar (and to store it), we add 3/4 volume of water, and we mix it again.


Add water

7 / We finally crush (or beat) the mix to expel air from the mix, and we use it right away or we pile it and cover it for storage.


Beat the mix

We have done this process many times now. We use sand from our land, that we screen before making the mix. The mix may not be right the first time, but we adapt the dosage for the next times. Sometimes, sand is still damp when we mix it with lime (at step 5), we then use less water next time. The crafty quick lime stones are not always well cooked, and we may find unburnt stones in the mix. Also, quicklime may not be well air tightly stored with a lot of powder in it, i.e. already slaked from air humidity,  rather than stones, and we get only small fissures when using it for the hot mix. In either cases, we then use more quicklime than 1/3 of sand volume the next time. If you try this method, use gloves and protective glasses, as lime is very caustic, don’t do like we (and others) do.

How others hot mix lime mortar and plaster?

Hot mix (or dry slaking) in four steps

Mortar, Renders & Plaster by English Heritage, in p76, explain the process of dry slaking or hot mix as following (see image on the right) :

  1. Top left : Quicklime is placed within a circle of damp sand.
  2. Top right : The sand is drawn over the quicklime. The moisture in the sand initiates slaking. The heat produced by slaking drives off some of the moisture from the sand in the form of steam.
  3. Bottom left : As the quicklime slakes it expands, forming fissures in the sand, which is by now almost dry.
  4. Bottom right : The quicklime is slaked to a dry hydrate, which is easily mixed with the dried sand.

Mortar, Renders & Plaster by English Heritage (p76) explains that the result “can be passed through a screen to remove lumps of unslaked lime”, if used as a lime plaster. “However, unscreened material, containing both unslaked lime and under-burned limestone, was often used, imparting a particular character to many historic mortars, such as those often found in wall cores”. Later this book explains that”If non-hydraulic or feebly hydraulic lime was employed, the screened mixture could be stored for a few days either in its dry form, or wetted and mixed to form mortar, known as coarse stuff. Non-hydraulic or very weakly hydraulic coarse stuff could be stored indefinitely as the water in the mix inhibited carbonation, (as for lime putty)”. According to Practical Building Conservation: Mortars Plasters and Renders by Ashgate Publishing, “the resulting mix is beaten to improve its performance, better incorporate lime and sand, and expel air.”

Below is video of hot mix lime preparation and final screening. Notice the proportion of 1 bucket of quick lime stones, 3 buckets of damp sand and 1 bucket of water. Note the size of the fissures produced as the lime expands. Note also that the final result is a dry powder, as the heat of slaking dry the sand.

Hot mix lime mortar preparation and screening

Below is another video of a lecture on hot mix lime mortar preparation. It includes at 1:30 a part of the previous video of hot lime mix, but this time more water is added to make a mortar, and the result is beaten to expel the air. Notice the consistence of this mix, and how it turns to an homogeneous gray mortar. Later in this video, at 6:20, you can see another hot mix to make plaster, that is kept dry and screened, like in the first video. You may want to watch the entire video, as it provides comments on the process and on lime mortar in general.

Lecture on the hot mix lime mortar method

Below another video example about lime mortar hot mix preparation by Dr Gerard Lynch making lime mortar at the Weald and Downland Open Air Museum. They follow the procedure we described and explain that historic building has 1/2 ratio of lime to sand rather than 1/3, because quicklime prepared with 1/3 ratio expand when slaked to reach 1/2 ratio. In another video, he show a variation of dry slaking without banking, i.e. without leaving the lime to slake inside the sand, mixing it with sand immediatly, used as he explain, when you have only few minutes to prepare the mortar. This method is referred to “hot lime mortar”, in which mortar was used hot and this may have some advantages in cold damp conditions, according to the book Practical Building Conservation: Mortars Plasters and Renders.

Hot mix lime mortar preparation (see also part2 & part3 on youtube)

We hope that you found this post informative. We are only beginners in the science of masonry and would be glad to have few comments and/or critics on this post and our method of lime mortar preparation.


4 thoughts on “Lime mortar ‘hot mix’ for Cob house, in october / Mortier de chaux mélangé ‘à chaud’ pour la maison en bauge (cob), en octobre

    • Yes it is hard, it is mortar, it holds rocks together and I can jump on the wall no problem. But it is soft and breathable, you can scratch it into powder with the nail.
      As I mentioned, this way of preparing mortar gives it a self-healing property, because of the partially hydrated lime inside. But harder, I am not sure it is as hard as 5 years aged lime putty. Probably harder than mortar prepared from hydrated lime powder. I remember reading that hardness of the lime mortar depends of the type of rock used to make lime, lime made from marble gives an extra hard mortor, when lime made from chalk is very friable.

      Do you have an actual project using lime mortar ?

  1. I have project only “in my mind” for now. I have heard once on TV , from a man that build straw ball house that at the end he use hot lime to covere it. He said it became so strong that a nail with the hamer you can’t put into it.

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