An introduction to lime - The Hempcrete Topic: Designing and Building with Hemp-lime - page 56

A moderately hydraulic lime mortar is used for pointing in this wall, since the
stone is very hard. For softer masonry, an air lime mortar would be appropri-
ate.
A range of limes with differing degrees of hydraulicity has always been available, due
in part to the differing amounts of impurities present in the original stone, and in part to
the temperature at which they are fired. Hydraulic limes are usually classified as 'feebly',
'moderately' or 'eminently' hydraulic, depending on their strength and other characterist-
ics. In feebly hydraulic limes - the type most commonly found in the UK - there is still
a lot of 'free' lime in the material, so much of the set happens due to carbonation, with a
relatively weak accompanying hydraulic set. In moderately or eminently hydraulic limes,
there is far less free lime present, and a stronger hydraulic set is responsible for the major-
ity of the setting process.
Suitable limestone for producing moderately and eminently hydraulic limes occurs less of-
ten in the UK geology, and where such seams do exist they have been found to lack the
impurities at consistent levels to produce limes with reliable characteristics. The best mod-
erately and eminently hydraulic limes to be found close to the UK are produced in France.