Ingredients to use for repointing

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.