| The Building Lime |
Building lime is one of the oldest and most used binder in the human history.
The existence of 'special building lime', obtained burning impure limestone, known as 'strong' lime, 'wild' lime and so on is well documented in the main architecture treatises.
But it is only in the eighteenth century that was understood that the reaction mechanism of hydraulic lime was linked to the presence of clay impurities.
In 1793, J. Smeaton discovered that the limestone burning, containing clay impurities, produced a type of lime (hydraulic lime) with characteristics similar to those of mixed air lime and pozzolanas.
The term 'hydraulic' (refered to a binder) has been given by the engineer Louis Vicat (1786-1861), who first settled precisely the proportions of limestone and clay needed to produce materials that can harden even in the absence of air, but in presence of water.
Hydraulic lime refered to natural products derived from the calcination of limestone marl or marly limestone (natural mixtures which have a certain level, 6 to 22% of clays or other aluminosilicates hydrates) burned at temperatures that generally are between 950 and 1100°C.
In these conditions, calcium oxide (CaO) is formed which subsequently combines with the silica and alumina clay forming silicates and hydraulic calcium aluminate; reacting chemically with water they form stable and insoluble hydrates that allow the material to harden and remain stable even under water (hydraulic action).
Natural Hydraulic Lime according to EN 459-1:2015
In recent years, the meanings of words used to describe hydraulic binders have been changed. This has produced considerable confusion and disorientation on part of final users.
Currently, according to European Standard EN 459-1:2015, the products obtained by burning natural marls or homogeneous mixtures of limestone and clay materials are referred to Natural Hydraulic Lime.
The EN 459-1:2015 provides for a distinction based on the mechanical strength at 28 days on three classes: NHL 2.0 - NHL 3.5 – NHL 5.0.
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