Natural Hydraulic Lime or NHL is a fine powdered, off-white material made by the calcination of specially selected siliceous chalks or limestone. It has natural clay content and this product is grade NHL 5.
Generally NHL is harder and less kind to stone or brickwork, with a lower porosity and less breath-ability 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 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 diminshed 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.