Making a hot lime mortar

Recently there has been a lot of hype about NHL lime.  Or to give it a name, Natural Hydraulic Lime Mortar.

As Chartered Surveyors, we will only recommend hot lime mortars these days.  

The reason for this is a little complex chemically, but it's lost in the mists of time.  This is how lime used to be made.  You took limestone, burned it, added water, and mixed sand with it to make mortar.  It fizzed, hissed and got very hot - and then slowly set to the lovely breathable, flexible lime mortars we see in surveys.  It took a fair bit of mixing, often had copious amounts of cow or goat hair in it, and was often left for months to mature.  If brick dust was added, as a pozzolan - it made the mortar harder.

Hydraulic (not hydrated) lime mortars have been developed more recently and are widely sold (Called NHL - or Natural Hydraulic Lime.  In 3 common flavours - 2.5, 3.5 and 5 - where the numbers refer to set strength in Newtons)  These are easy to use - like buying a bag of cement. They set quickly, and work very much like cement in that you get a first 'set' and then harden over weeks as true 'carbonation' takes place.  The catch is that these seem to be setting much harder over time than we thought they would.  Which makes them less flexible and less breathable. New research is underlining this fact.

So, to solve the problem, we go back to traditional hot lime mixes which are really cheap to make, but you need to take care not to splash yourself with the newly wetted powder - it gets pretty warm!  We make it from 2.5 parts sharp gritty sand, and 1 part lime - Calbux 90 or the equivalent. 

This is a brilliant book by the leader in hot lime mixes - Nigel Copsey - it's a must have: