Lime Plastering

Lime is divided into two types; non-hydraulic or ‘air’ limes and hydraulic or ‘water’ limes.

Non-hydraulic limes are also referred to as ‘fat’ limes and hydraulic limes as ‘lean’ limes.  This relates to their workability or ‘butteryness’ on the trowel whilst being used by the plasterer.

The terms ‘air’ and ‘water’ are very old forms of classification that relates to lime mortars that do not set in water, i.e. non-hydraulic, and those that do, i.e. hydraulic.

The different qualities of lime as a building material have been exploited for different applications.

The use of lime as a building material has been dated to 5600BC (Ashurst, J. p.5).  the Egyptians and Greeks certainly had a thorough understanding of lime mortar, gypsum and pozzolanic additions.

In Britain lime based mortar and plaster were used from 1AD (Ashurst, J. p.5).  Oil and fat were added as waterproofers as well as different aggregates and artificial pozzolanic material, e.g. crushed earthenware, brick, tile.  Hydraulic limes were also used and developed, as a possible result of Roman influence.

Limestones, that include chalk provide the raw material for building limes.  Limestones consist mainly of calcium carbonate that was originally calcareous mud.  Other mineral contents include clays, dolomite (a calcium magnesium carbonate) and quartz.  The different relative proportions of these minerals determine characteristics such as non-hydraulic or hydraulic, and durability that in turn determines application and use.

Limestone that contains septarian nodules formed the basis of ‘natural cemments’.

High calcium non-hydraulic lime production.

Limestone (CaCO3) is broken down into lumps and heated in a kiln between 900-1200°C.  During heating calcium oxide (quicklime) is formed by driving off carbon dioxide and water.  The resulting quicklime is then ‘slaked’.

Slaking involves carefully adding quicklime to potable water.  This causes a violent exothermic reaction to occur that produces calcium hydroxide (Ca(OH)2) in the form of a soft, greasy mass of material known as lime putty.

The lime putty must be left alone to ensure thorough slaking, this can be as little as 2 weeks but preferably 3 months.  There is no upper limit of time.

Lime putty can be stored in airtight tubs.  It is important that the putty does not come into contact with air as hardening through carbonation will take place.

Carbonation

This long process occurs as a result of contact with atmospheric carbon dioxide.  The result is a completed lime cycle where calcium hydroxide converts to calcium carbonate.

An initial hardening takes place as water is lost to the porous surface of the masonry and by evaporation.  Both hardening and carbonation should not be confused with chemical ‘setting’ of hydraulic limes and cements.

Carbonation does not always occur consistently and is dependent on both moisture and to a lesser extent temperature.

Carbonation appears to be accelerated by periodic wetting of the work with a fine mist sprayer.  Rapid drying out due to sunlight or draughts retards the carbonation and results in poor ultimate strength.

Protection with hessian and spraying are vital parts of the curing process.

Hydraulic ‘water’ limes are the traditional ancient building material of masonry (Ashurst, J. p.17).

Lime was traditionally classified by its ability to set underwater, hence ‘hydraulic.  The characteristic ‘set’ is a result of silica and clay impurities.  These impurities impart greater strength and quicker setting times that make the material more versatile.

As a result of the impurities the slaking process is more complicated.  The calcium oxide must be slaked to produce calcium hydroxide, but this is a time critical process, if the material is slaked for too long a chemical set begins due to the silicates and aluminates.

Modern hydraulic limes are now classified according to their compressive strengths – N/mm²@28 days.  The 3 types of Natural Hydraulic Lime are NHL2, NHL3.5, NHL5.  The increasing strength and therefore hardness determines their uses in different settings. 

Pointing repairs

Local failures should be re-pointed using colour matched samples.  Weathering processes will rapidly tone down the colour of new mortar within 2 years.  For conservation purposes it is important to resist the temptation to re-point an entire area unless necessary.

The soft nature of an existing lime joint, demonstrated by ‘jabbing’ a point into it does not always imply a weak failing joint.

Raking out – the removal of disintegrated material followed by washing out with a water sprayer or hose pipe at domestic pressures.

Cutting out – this is usually inappropriate hard material that has been inserted, e.g. natural cements, OPC’s.  Care needs to be taken as the removal of hard material can damage the existing masonry.  This can be time consuming and must be allocated for if it is to be carried out carefully and effectively.  Joint are washed out afterwards.

Desirable depth of joint  -

• 8mm for a 3mm wide joint

• 18 mm for an 8mm wide joint

• 38mm for a 15mm wide joint

• 50mm for a 20mm wide joint

If only the joint face has been broken down the old mortar should be taken back to a square face.

Filling joints – all joints must be pre-wetted.  Mortar must be packed firmly into the joint and built up until it is full.  New mortar must be protected from drying out and rain using dampened hessian and intermittent fine spraying.

Joint finishing – a stiff bristle churn brush is used to hammer the joint face exposing aggregate and giving a weathered look.  The brush must not be dragged across or beaten when the mortar is too soft.

Summary

1.      Remove moss and small plants from the wall.

2.      Rake or cut out defective or inappropriate mortar using fine chisels or quirks that fit within the joint. Cut to clean, square edges and to depths not less than twice the thickness of the joint.  As much original mortar to be retrained as possible.

3.      Flush out the opened joints with water and allow to dry.

4.      Where appropriate, working from the top of the wall down, treat the wall with biocide.  Allow the growth to dry off and brush down.

5.      Treat the wall with biocide for a second time.  This should be carried out between one and two weeks before re-pointing.

6.      Brush down the wall and wet until the old mortar and stone faces remain damp.

7.      Working from the wall top and ensuring that working areas are kept wet, place the mortar into the damp joints, packing it in layers from the back using pointing tools of appropriate dimension.  Bring the mortar out over the joint face ironing it on hard to ensure a good bond between the stone and mortar.

8.      Within the same working day scrape, shape and spray the mortar face in order to match existing or sample.

Protect the work from sun, wind and rain under damp hessian.  Carry out intermediate fine mist spraying of the surface to control shrinkage and promote carbonation.  In hot dry conditions spraying may need to be hourly with reduced periods over as long as a week.