Lime mortar/plaster consists of a mixture of sand and lime. Ordinary Portland Cement was invented in 1824, this is what is used in almost all new buildings, but lime plaster has it’s uses especially in the renovation of old properties.
Lime is produced by the burning of limestone (chalk) in a kiln for a period of around 4 days at between 900 and 1000 degrees C. This is known as Calcining, and the resultant substance usually in lump form, or crushed to a powder is known as Quicklime. There was a large industry associated with this and the guys involved were called Lime Burners.
Traditionally, quicklime was to a site, a pit dug, the Quicklime put in it and then a quantity of water added. This results in a VERY vigorous chemical reaction which gives off a vast amount of heat and expands considerably. This action is chemically termed hydration and historically the process was known as ‘Slaking’. It is very important that full thorough slaking occurs, otherwise any unslaked lime would on the addition of water to the mortar continue slaking in the wall … causing expansion, cracking and weakness. If Quicklime is left exposed to the atmosphere it will eventually ‘Air slake’.
Nowadays lime is usually purchased in bags of scientifically prepared ‘Slaked Lime’ this is made by a fast continuous process, making it cheaper. Either by controlled burning or by a pure chemical process this results in a product that is superior to the traditional methods as all the quicklime is hydrated. However it does have a major disadvantage in that it suffers from blowing; this is the expansion of small lumps of lime causing fractures in finished work.
There are different types of Lime plaster
- High calcium lime
- Semi-Hydraulic lime
- Hydraulic or Eminently Hydraulic Lime
High calcium lime is also known as ‘White lime’ or ‘Fat lime’ and is classed as quick slaking due to being almost pure calcium. It has outstanding handling characteristics and is very plastic, making it easy to work with.
Semi-Hydraulic lime is also known as ‘Grey Lime’ has a lower Calcium content and higher level of impurities, and is less plastic.
Hydraulic or Eminently Hydraulic Lime. Also known as ‘Lias’ or Learn lime contains a greater proportion of impurities, making it more difficult to work. Hydraulic limes has a unusual peculiarity of setting without exposure to air and are thus suitable for use in damp conditions.
Impurities in Lime are not always a bad thing, and in fact for brickwork is often required as it increases strength of the finished mortar. There are many known impurities, such as magnesium, ‘Magnesian Limes’ is used because it has a greatly increased hardening property compared to High Calcium limes.
Settling Lime; Settling is the action that occurs after slaking and depends on the slaked lime absorbing cardon dioxide from the atmosphere. This changes the soft particles of slaked lime which are converted to comparatively hard crystals of Calcium Carbonate.
The sequence is thus: Calcium Carbonate…burnt…Calcium Oxide…water added (slaked)…Calcium Hydroxide…absorption of CO2 from atmosphere…Calcium Carbonate – more or less back to what it started as but all the crystals are aligned by cohesion. The addition of sand in correct proportions induces crystallisation and gives ‘adherence qualities’ It also gives it bulk, reduces shrinkage and saves on costs. Mortar MUST be kept moist during this crystallisation alignment process, and dry bricks should be dunked in water before laying to avoid sucking the moisture away before the process has completed. Excess moisture then evaporates away by exposure to the atmosphere.
SAND
Sand is very important, the properties seriously affect handling characteristics and finished strength of the lime plaster. Builders sand is a ‘soft sand’ and totally unsuitable for use in lime plaster. What is needed is a good quality ‘sharp sand’ i.e. with angular edges, with proportionate sized grains, neither too fine or too coarse, to give the necessary texture and lock the mortar together.
Line Mix Proportions
These were specified in the Ministry of Health Model Byelaws Series IV (buildings) and the typical proportions were :
High Calcium Lime (lime putty) 3 or 4 parts sand : 1 part lime
Magnesian Limes 2 or 3 parts sand : 1 part lime
Hydraulic Lime 3 or 4 parts sand : 1 part lime
Eminently Hydraulic Lime 2 or 4 parts sand : 1 part lime
