St Astier NHL 2, has natural clay content which allows it to set under water – hence the name “Hydraulic Lime”. The grading system reflects the active clay content ratios within each of the three materials, NHL 2 being the weakest followed by NHL 3.5 and then NHL 5 being the hardest product. The 3 grades of NHL are described as Feebly, Moderately & Eminently Hydraulic Limes.
These gradings reflect the diminished breath-ability 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.