Lime plaster

For those who want some mainly historic details on Lime ... are we sitting comfortably, then I'll begin.

Ordinary Portland Cement (OPC) invented in 1824 is what is used in almost all new buildings, but lime has it's uses especially in the renovation of old properties.

Lime mortar consists of a mixture of sand and lime.

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.' Chemically it is Calcium Oxide. There was a whole industry associated with this, and the guys involved were called Lime Burners.

Traditionally, lime was taken in this form to a site, a pit dug, the Quicklime put in it and then a quantity of water added. This results in a VERY vigorous exothermic chemical reaction which gives off a vast amount of heat and expands considerably. If the water quantity was accurate the lime expands, it disintegrates and falls into a powder. However normally excess water is added to produce a paste known as 'Lime Putty'.

The above action is chemically termed hydration and historically the process is 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.

It has advantages :

• It is consistent
• It is fully burnt and thoroughly hydrated
• It can be stored longer without deterioration
• 'usually' better results than lump lime

However its has a MAJOR disadvantage that it suffers from blowing ... this is the expansion of small lumps of lime causing fractures in finished work.

There are different classes of LIME

• 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, a joy 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' contains a greater proportion of impurities, making it more difficult to work. Often referred to as 'Learn' lime.

Impurities in Lime is not always a bad thing, and in fact for brickwork is often desirable as it increases strength of the finished mortar. There are many known impurities, such as magnesium, such 'Magnesium Limes' are used because they have greatly increased hardening properties compared to High Calcium limes.

Settling of Lime

Settling is the action that occurs after slaking and depends on the slaked lime absorbing CO2 from the atmosphere. This changes the soft particles of slaked lime 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 is 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.

Hydraulic limes have unusual peculiarity of setting without exposure to air, and are thus very suitable for damp conditions.

SAND

Sand is very important, the properties seriously affect handling characteristics and finished strength.

Builders sand is a 'soft sand' and totally unsuitable. What is needed is a well graded clean free from impurities '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.

Sharp sand is usually pit sand. Dredged sea-sand has rounded grains and is known as soft sand.

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