Natural Hydraulic Lime or NHL is a powdered material.
Generally, NHL is harder and less kind to stone or brickwork, with a lower porosity and less breathability than Non-Hydraulic Lime. However, it has properties that make it the ideal choice for certain applications.
NHL has natural clay content which allows it to set underwater – hence the name “Hydraulic Lime”. The grading system reflects the active clay content ratios within each of the three materials, NHL2 being the weakest and NHL5 being the hardest product. The 3 grades of NHL are described as Feebly, Moderately & Eminently Hydraulic Limes.
These gradings reflect the diminished breathability of the finished product.
Strength and frost resistance can also be varied by altering the mix ratio, typically the ratios will range between 1 lime : 1·5 Sand to 1 lime : 3 Sand (by volume) Generally exterior hydraulic lime mortars will be 1 lime : 2 or 2·5 Sand. With cement mortars as well, mix ratios determine the strength and these are detailed in BS 5628. Type I mortar being the strongest and Type v the weakest.