Yep NHL = Non hydrolic lime ?
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NHL = Natural Hydraulic Lime
it's a type of hydraulic lime
cement is also a type of hydraulic lime
hydraulic lime = lime + pozzolana
the pozzolana transform the properties of the lime
pozzolana can be brick dust, fired clays, trass, volcanic ash, flywheel ash, etc
Back in the day, craftsmen mostly just used raw earth and lime. Sometimes they would add a certain amount of pozzolana if the situation called for it. Foundations are best built with hydraulic lime because it can set under water.
But everything above ground, everywhere the wind can reach, they traditionally used pure/fat lime. Because that type of mortar is much better at sucking the moisture out of the stones so that the wind can dry it (convective drying).
Hydraulic lime doesn't do this and that's why the stones (and wall) stay wet.