What is Hot Mixed Lime?
Hot Mixed Lime is when dry powdered, or kibbled Quicklime is gauged (mixed) with an aggregate e.g. sand
Water is then added, so the mixing and slaking process are undertaken simultaneously.
The chemical reaction creates a lot of heat, hence the term.
(It can be quite a violent reaction so great care must be taken. Being strongly Alkali, Quicklime is a hazardous material which can burn skin and eyes as it can spits, bubble and steam).
The resultant mortar, plaster / render or limewash can be used whilst still hot or once it has cooled. It can remain useable for some time after mixing and will usually only require ‘knocking up’ before use.
Our own research here at Cheshire Lime Ltd has shown that many historical Lime Mortars were more Lime Rich than some of the mortars made today using pre-slaked Lime Putty.
The general modern ‘recipe’ to make a basic mortar using pre-slaked Lime Putty is:
1 Lime Putty : 3 sand
In equal volumes e.g. buckets
Very little water is added , if any, since the Lime Putty is already quite wet.
It is then mixed until a useable mortar is obtained.
A recipe for hot mix Lime mortar recipe might look very similar on paper:
1 tub kibbled quicklime : 3 tubs sand
However, because the Quicklime expands on contact with water (up to 2.1x for Powder or, because they are more dense, 2.7x for Kibbles), the resultant mortar will be closer to:
2.7 Lime : 3 Sand
Which is almost 1:1
Hot-Mixed Lime mortar or plaster is also more forgiving of aggregate with impurities.
Historically, the Lime mortar would have been slaked and mixed on-site, often either in pits or upon the bare earth.
The earth dug to create the pit might be used as aggregate in differing ratios, since not only was it the cheapest, most locally available material, but the clays and impurities contained within had many different effects on the final mortar.
Not only on the colour / appearance and the workability, but also had a Pozzolanic effect on the speed, and final strength, of the set.
It would be down to the experience of the workers to understand all the many variables and would mean many local variations in mortars due to geological conditions.
Nowadays, because much emphasis is placed on using certified pure Lime Putties with a well washed and graded sharp sand, the resulting mortars will behave and perform quite differently and could be less varied.
This ‘one size’ fits all approach, whist certainly better than using cementitious materials, may still not be ideal if attempting to match or restore the historic fabric of a structurally sensitive or listed building.
Written by: Peter Borgs