Lime is obtained by burning limestone (which makes up 20% of the earth's crust). Depending on the type of limestone used, cooking produces different types of lime (see appendix "Technologie de fabrication"):

• non-hydraulic lime, which comes from pure limestone
• dolomitic lime, which comes from a mixture of limestone and magnesium carbonate
• hydraulic lime, which comes from limestone containing clay.
  

non-hydraulic limes

From the moment it is extracted in a quarry, to the moment that it is used on a work site, the material undergoes several transformations.

Quicklime (calcium oxide) and carbon dioxide gas is obtained by burning limestone (calcium carbonate).

This reaction is produced by burning limestone at a temperature of approximately 900° C. This process entails a weight loss of about 45% corresponding to the release of carbon dioxide. This produces quicklime, the most dangerous form of the material, which consumes water in all organic matter. Quicklime can be slaked by adding water.

There are many different ways of slaking quicklime. These are described in the appendix "Technologie de fabrication". Once slaked, the consistency of the lime depends on the amount of water which has been used:
- a limited or controlled quantity: slaking produces a lime in powdered form,
- large amounts of water:slaking produces a lime putty.

Slaked lime is the material used in construction. When it is combined with water and aggregates, it can be used to make mortar.

Once the mortar is applied, non-hydraulic lime sets by a process of carbonation. This is a very long process, which takes several months, and which requires a wet environment. Water vapor reacts with carbon dioxide to create carbonic acid. The lime combines with the carbon dioxide contained in this acid and transforms itself into calcium carbonate.

In certain types of limestone, referred to as dolomitic limestone, the calcium carbonate is combined with magnesium carbonate. This type of limestone produces dolomitic lime when it is burned at a temperature less than 900° in the same conditions as for non-hydraulic lime.

Pure limestone is very rare. More often than not, it contains traces of marl and clays which are rich in chemical elements such as iron, aluminum and silicate. When heated to temperatures of between 800 and 1 500 ° C, the calcium from the limestone combines with the other elements: calcium silicates, aluminates, calciom alumino ferrites.

When these elements come into contact with water they form insoluble hydrates which give the binder its hydraulic character. The proportions of alumina and iron oxides are very low (in white binders, the quantity of iron is less than 0,1 or 0,2%). When hydraulic lime sets, it is principally as a result of a reaction between CaO and silicates.

After this, when the lime and the hydrates come into contact with humidity, they carbonate (with the carbon dioxide in the air) and turn back into the original calcium carbonate and silicate.

In 1820, VICAT classified the different types of lime according to their ability to set and to the quantity of clay which they contained. He created a hydraulicity index which is based on the relationship between the different elements in the clay and the quantity of lime.

The hydraulic character of lime can also be defined by the length of its setting time in water.

The most common natural hydraulic limes (NHL) used in construction (Boehm, Chaux blanche Calcia, Chaux blanche Lafarge, Rabot, St-Asteir, Socli, Wasselonne...) are generally totally, or largely hydraulic.

We have already seen that when quicklime is slaked it either becomes a powder (small quantity of water), or a putty (large amounts of water). Even though the chemical composition of these two materials is identical, technically speaking, they are very different:

• Lime in powdered form is very well adapted to current practices in the construction industry where it is more common to use powdered binders, which can be easily mixed in a cement mixer.
• It is very difficult mixing lime putty with sand without the proper equipment, and it is very difficult to use the correct quantities, which always depends on the quality of the putty.
• Lime putty is easy to store and can be kept for a long time.

The differences in the results produced by powder as opposed to putty would merit detailed research. It has been noted that:

• Lime putty carbonates much more quickly than non-hydraulic lime powder (which would allow buildings to be protected from bad weather earlier) and is more resistant. It is likely that the improved carbonation potential of lime putty is a result of the absence of carbonation during slaking. Also, the quality of the lime and the presence of colloidal gels are other factors which merit further study.
• When lime putty is mixed with water it does not create sediment as is the case for other lime washes made with powdered lime.
• Lime putty produces mortar with greater consistency, which has greater elasticity and which is good for smooth trowelled finishes or stuccos.

In recent years, the rediscovery of the merits of this product has led a number of companies to distribute non-hydraulic lime putty. This is good news for those who want to make lime washes, or achieve smooth finishes. Here, the difficulties involved with mixing and dosing are largely compensated for by the quality of the final results.

However, lime putty which has been obtained by wetting powdered lime does not provide the same quality as lime putty which is obtained by slaking with large quantities of water.

For many centuries, lime was made within easy reach of the places where it was to be used, in order to limit transport. It was made using local limestone and, therefore, came from many different sources and provided many different varieties. Previous generations had a large choice of lime, depending on the type of limestoone which was employed. However, nowadays, norms have defined two sorts of lime:

Non-hydraulic lime, which is used in construction under the name CL for Calcic Lime, previously known as Air Slaked Lime, which is governed by the NFP 15311 standard. It is a very pure, non-hydraulic lime, referred to by Vicat as fat lime.

Natural hydraulic lime, which comes under the name NHL, is governed by the NFP 15311 standard.
It refers to limes which have a high level of hydraulicity.

The principal limes which are produced today are non-hydraulic limes and largely hydraulic limes. Neither is better than the other. Modern day transportation means that it is possible to choose either one. It is important to make your choice according to the quality of the working surface, the exterior conditions and the final results which you are hoping to obtain.

The difference between these two types of lime has been defined by modern day specifications which have been established as a result of research concerning their composition and their characteristics.
Depending on the type of limestone employed, our ancestors used three terms when referring to the different types of lime: fat, average and lean.

VICAT defined three types of lime according to the amount of water which had been absorbed by quicklime:

It is possible to see from this table that in order to obtain lime putty from a quicklime made from pure limestone (it is possible to store non-hydraulic lime putty for a longer period of time) 3,6 liters of water per kg of quicklime are required.

The traditional and the modern approaches to the subject agree that the fattest limes are non-hydraulic limes and a great deal of water is required for slaking them. The leanest limes are natural hydraulic limes and only one or two times their weight in water is required for slaking.