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What is Natural Hydraulic Lime?

Materials used in the repair and restoration works of historical buildings have changed from past to present. These changes, together with observations and experiments conducted depending on factors such as the characteristics of various materials used, their impact on structure, and their environmental impact, aim to be able to choose a more suitable material over time. 

Although lime has been used as a building material in various ways since the construction of the pyramids in the 4000s BC, the production & use of lime has significantly decreased with the invention of Portland Cement (PC), since the first half of the 20th century, cement has been used as the dominant binder in the repair and restoration works of old buildings / structures.

Lime is divided into two categories as hydraulic and non-hydraulic lime. Non-hydraulic lime becomes a building material by burning limestone (calcium carbonate) in a kiln to obtain quick lime (calcium oxide), then it is slaked with water to get lime putty and thus acts as a binder. Lime putty is mixed with aggregates and carbonated with the help of air, resulting in a hardened form similar to limestone.

Natural hydraulic lime (NHL) is similarly derived from limestone, but it is not pure limestone, there are pozzolanic amorphous, reactive silicas from different minerals naturally present in it. Therefore, after burning, belites are formed along with quicklime. After slaking with water vapor, it takes the form of natural hydraulic lime which can be packed and ready for sale. Natural hydraulic lime is hydrated when water is added, creating calcium silicate hydrates similar to cement and it forms a harder, stronger and less permeable structure than non-hydraulic lime by setting. 

Natural hydraulic lime has a porous structure allowing vapor permeability, that is, breathable, which prevents the formation of dampness and decay. It is a sustainable material compared to Portland Cement, it does not contain salts which reacts with mortars used in historical buildings and damages them like cement does, and it allows small deformations. These can be listed as reasons why natural hydraulic lime is preferred instead of cement as a binder in historical buildings. 

Natural hydraulic lime is commercially sold in packages in the form of NHL2, NHL3,5 and NHL5.
They get their name from their 28-days compressive strength as 2MPa, 3,5MPa, and 5MPa respectively. These values are much lower than 42,5MPa which is the 28-days compressive strength of commonly used Portland Cement.