Lime is obtained through the calcination of limestone (wich represents 20% of the earth layers). Depending on the nature of limestone, the calcination allows the production of several types of limes :

•  aerial lime, from pure limestone
•  magnesial lime, from a blend of limestone and magnesium carbonate
•  hydraulic lime, from a limestone that contains clay.

Aerials limes

From the quarry to the construction site, the materials go through several transformations, from its fabrication to its use.

The calcination of limestone lead to the formation of quicklime and carbon dioxide.

This reaction occurs at a temperature close to 900° C and comes with a lost in weight of 45%, corresponding to the loss in carbon dioxide. The quicklime is the most dangerous form of this material, grasping for water, it burns all organics elements by absorbing the water. Quicklime can be put out, by the process of extinction, or hydratation, by adding water.

The extinction can be realised through various process. The form obtained after the extinction depends on the quantity of water :

• in controled and limited quantity : the exctinction produces powder lime,
• with an excess of water : the extinction produces lime paste.

The exctincted lime is used in construction. Mixed with water, agregates, it is used for the realisation of mortars.

When the mortar is used, the aerial lime is going to set through carbonatation. This slow reaction (several months), occurs in a damp environment : steam has an affinity witch carbon dioxide and forms carbonic acid; the lime settles down the carbon dioxide contained in this acide and transforms into limestone.

In some limestone, known as dolomitic, calcium carbonate is associated to magnesium carbonate. This variation of the limestone forms magnesial lime, during the calcination at a temperature under 900° C, in the habituals conditions for the obtention of aerial lime.

Pure limestone are rare. They are usually mixed with marl and clay rich in chemical elements such as iron, aluminium, and mainly silica. Between 800 and 1 500° C, the limestone's calcium combine with those elements and forms calcium silicates, but also aluminates and calcium's iron-aluminates.

Through contact with water, these bodies are going to form insoluble hydrates that give to the binder an hydraulic nature. The proportions of iron alumine are very low (in white binders, iron grade are lower than 0,1 or 0,2%). The phenomenon of hydraulic setting is a result of the reaction between calcium oxyde and silicates.

Next, by contact with moist air, lime and hydrates thus formed are going to carbonate (with the carbon dioxide contained in the air) to give the calcium carbonate and the silica back as original. This reaction takes several months : it is the aerial part of the setting.

In 1820, Louis Vicat is trying to class lime according to their setting type and their clay grade. He defines the hydraulicity rating. It is the ratio between differents composant associated to clay and the quantity of lime.

The hydraulic nature of limestones generally conveys by their setting and their hardenning speed under water.

The natural hydraulic lime of building trade are generally hydraulic or eminently hydraulic.

We saw previously that after the extinction of quick lime, the slaked lime obtained can become powder (controlled water quantity) or paste (lot of water). On the chemical level these materials are identicals, they are totally differents on the technical plan :

• Powder lime is responding well to actuel building usage, where we favors the resort to binders "in powder" : cement mixer can be used.
  
• Paste lime is more difficult to mix with sand, and the dosages are more difficults to master.

On the performances level, many differences would deserve specific studies. Indeed, we can notice that :

• Paste lime is carbonating faster than an aerial powder lime and products a better resistance. We can assume that this better carbonatation of paste lime can have as origins the absence of carbonatation in the operation of extinction. Under water, away from air, the reaction doesn't anticipate the setting. Moreover, the thinness of the obtained lime, the presence of colloid gels, are elements that deserve a study.
• Paste lime mixed with water doesn't sediment, in contrary of lime milk made with powder lime.
• Paste lime products the slimier mortar, smoother and allows the use of fine finitions more easily.

Since a few years, the re-discovery of these differences lead some industrials to diffuse some aerial lime in powder, and we can only be glad, in particular for the realisation of lime milk, layer coating, where the inconveniant of mixing, dosage, is widely compensated by the quality of finished works.

For centuries, lime has been made close to its utilisation place, in the main objective of limiting the transportation. Lime is made with the local limestone. It represents a lot of deposit and therefore a lot of varieties : "We have the lime that we can" we might say. Our predecessors disposed of a wide range of lime, related to all of the limestone used. Today, norms have defined two terms :

Non-hydraulic lime, that we find in these "building" application under the standar term CL, for Calcic Lime, is determined by the norm NFP 15311. It matches non-hydraulic lime, very pure, defined by Vicat as common lime.

Natural hydraulic lime, under the term NHL (Natural Hydraulic Lime) is determined by the norm NFP 15311. It is similar to lime eminently hydraulic.

Today's production only conserved non-hydraulic lime and natural hydraulic lime, wich are both good. Today, distributions ways give us the choice. It is important to use this choice, depending on the support, the condition of application and the choice of finish.

The difference between these two types of lime and their name follow moderns specifications, whose have been established through research related to composition and characteristics of these binders. Depending on the limestone they extracted, the elders used three names to designate non-hydraulic lime, medium lime or natural hydraulic lime.

Vicat defined three lime families, by determining the water volume absorbed for a given quick lime mass :

Results are, to obtain paste lime from a quick lime made with pure limestone, you need to add, at most, 3,6 litres of water for one kilogramms of quick lime.

These two approachs, issued from tradition and standard, meet since the most common lime corresponds to hydraulic lime and they needs a great quantity of water for their extinction. In contrary, the exctinction of natural hydraulic lime needs only one or two times their weight in water.