Ingredients to use for repointing

Ingredients

Sand - A variety of sands are available throughout the country and should be chosen in terms of colour and texture to match original mortar and complement the stone. It should be hard, durable and free from contaminates such as soluble salts.  Sand for  this type of work is often described as concrete sand or sharp sand..

BS 1199 - Water - Should be clean and used for mixing and rinsing of the joints where applicable.

BS 3148 - Lime - Lime putty should ideally be used, although bagged hydrated lime is more commonly used, and can be slaked prior to use to increase qualities and improve workability.

BS 890 - Cement – Used to ‘gauge’ a lime based mortar, particularly in exposed positions, but current best practice is to follow traditional mixes and to avoid its use completely.

Dyes - Rarely used in mortars, natural colours being obtained by using appropriate sands/aggregates any being considered should be oxide based.

Aggregates - As with sands, local varieties are available, small rounded grains less than 3 mm are often used to give a good bond. Larger particles however may be desirable to match the appearance of a historic mortar. If larger particles are to be included these should be well graded and include a mixture of sharp and rounded grains. Sharp sand gives the best bond but rounded grains are often found in older mortars. All aggregates should be washed and free from impurities. 

Moisture Inhibitors. These chemical additives designed for use with cement mixes should never be used with historic buildings. Their purpose is to prevent moisture entering a wall but they also prevent it leaving. While this may be appropriate for a modern cavity wall where moisture can drain out of the cavity, for historic buildings this can contribute to the build up of damp within a wall.

Mortar Types

Mortars used on historic buildings fall into a number of distinct types:

Masonry cement

These mortars are typically 1:4 or 1:5 portland cement with sand and should never be used on old buildings. Their use in the recent past is the source of many of the damp problems associated with old buildings today. It is usually best to remove it.  Any decision should be based on an inspection of the condition of the mortar (are hairline cracks allowing water to penetrate?), the damage this may be causing by trapping water and the likely damage which may be caused by trying to remove it.

Non Hydraulic Mortar based on lime putty

These mortars are easily worked, they provide a good bond to masonry when cured and are flexible and permeable. This type of lime mortar (‘fat lime’ free from impurities) does not have a chemical set and it cures slowly by reaction to air (carbonates). It must therefore be carefully supervised to avoid rapid drying out and can be vulnerable to frost and salt damage during this period. Because of this it is normally only recommended in very sheltered locations. Appropriate mixes depend on conditions  and use but often fall within the proportion of 1:3.

Artificially Hydraulic Mortars

These have similar properties to fat lime mortars but there is also a slight chemical or ‘hydraulic’ set introduced. This acts to strengthen a mortar while the main component carbonates and leaves it less susceptible to early damage. The chemical reaction is introduced by the addition of a reactive aggregate to the mix known as a ‘pozzolan’. Brick powder and ash are two common types. Many historic mortars are of this type because clays in the limestone used to make the mortar introduced some pozzolanic elements. The mortars are stronger than pure lime mixes and therefore have better weathering properties though they may not be appropriate in very exposed conditions. They are often used in the proportion 1:3 to 1:4 where the pozzolan is part of the aggregate.  Due to the chemical set, care needs to be taken to follow the manufacturer’s instructions.

Hydraulic Mortar based on hydraulic lime

Hydraulic lime is created from limestones which contain clay impurities. Because of this they have a chemical reaction as well as carbonation and set in differing degrees of hardness.  The majority of this material is imported from France or Germany. Their classification is based on strength: NHL2 is also known as ‘feely hydraulic’, NHL3.5 ‘moderately hydraulic’ and NHL5 ‘eminently hydraulic’ They all have good compressive strength and are flexible and have resistance to frost within 28 days. They are often mixed with sand in the proportion 1:2 or 1:3.  NHL2 for is used for pointing.  NHL5 for severe exposure and chimneys.

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