Lime comes basically in two forms: Hydrated and Hydraulic.
Hydrated Lime (or calcium lime, air lime, high calcium lime):
This is the most commonly used and known lime. It comes in powder form, like Type N, NA, S or SA, or in a paste form, known as Lime Putty. Although it has wonderful physical and aesthetic qualities, hydrated lime has very strong limitations in the construction industry. Hydrated Lime sets only by carbonation (the re-absorption of CO2) and thus, thicknesses are very limited. It is not suitable or practical for scratch or brown coats in a plaster, but can do wonders as a finish coat but requires blending most of the time. The application requires very highly skilled labor and that special care must be taken to avoid physical harm, such as burns when in the form of quicklime. Consequently, it becomes a specialty with associated costs.
Although basically of the same chemical composition as hydrated lime, hydraulic lime has its initial set with water, much like cement, and a second set by absorption of CO2. This allows for simplicity of application although basic care is required, such as minimizing the amount of water, good sand and tarping. The cost of labor is comparable to normal stucco installation. Hydraulicity for St. Astier lime is achieved by the nature of the raw material.
Hydraulicity of Hydrated lime may be obtained by the addition of cement, pozzolanes, etc. In such a case, the introduction of foreign chemical elements may have immediate or long range inauspicious consequences and the sensitive user should stay away from these hybrid products.