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Natural hydraulic lime (NHL) comes from limestone containing natural impurities of clay and other minerals. The amount of impurities within it determines how hard it will set.
There are three European classifications of natural hydraulic lime: NHL 2, NHL 3.5 and NHL 5. These grades are based on the compressive strength of laboratory mortars after 28 days. These are often somewhat misleadingly termed feebly hydraulic, moderately hydraulic and eminently hydraulic.
The strength of a hydraulic lime mortar varies depending on the manufacturer of the hydraulic lime. Some natural hydraulic lime will test higher in the qualifying band, and some test lower. Strength will also vary depending on the mix ratio and the type of aggregate used.
Natural hydraulic lime works by setting in the presence of water, hence the term ‘hydraulic’.
We generally recommend the use of hydraulic lime where the need for breathability and lower strength is outweighed by the desire for an earlier and harder set.
For lots of detailed information on lime and its uses visit www.naturalhydrauliclime.net