mortar for limestone blocks

[Nick Durnan <durnan@zzzzzzzzzzzzzzz>]

Norman
Gypsum plaster or Plaster of Paris is made from heating the mineral
gypsum (calcium sulphate) also known in its pure form as alabaster. It
is fast setting and expands very slightly as it sets. It has the
disadvantage of being slightly water soluble. Putty lime is made from
heating limestone, chalk, marble, coral, shells (all calcium carbonate)
and then slaking the burnt material with water to make the putty. Lime
putty (calcium hydroxide) will only set when exposed to carbon dioxide
in the air. Hydraulic lime is made from burning limestone which has a
clay content, the more clay in the limestone the more hydraulic the
lime. Hydraulic lime sets partially by reaction with water and
partially by reaction with carbon dioxide. There are three strengths:
feebly hydraulic (NHL 2) moderately (NHL 3.5) and eminently (NHL 5).
The stronger the hydraulicity the less plastic, less permeable the lime
becomes. Hydraulic limes harden quicker than putty limes but putty
limes are more workable. So you can select lime you need depending on
the strength and durability of the mortar required. Current wisdom is
that the mortar should always be more permeable and softer than the
stone it is being used with. Putty limes can be made hydraulic by
mixing them with pozzolans (naturally or artificially fired clay
minerals like trass, pumice, brickdust,)

Nick