Mortars used on historic buildings fall into a number of distinct types:
1. Masonry cement
These mortars are typically 1:4 or 1:5 with sand and should never be used on historic buildings. Their use in the recent past is the source of many damp problems associated with historic buildings today. Were this is encountered a decision will have to be made if it is better to remove the mortar or leave it in- situ. 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 damaged which would be caused by trying to remove it.
2. 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. Pre mixed mortars of this type are available in Northern Ireland.
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 often because impurities in the production of 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. Mortars of this type can also be bought ready made in Northern Ireland. Due to the chemical set, care needs to be taken to follow the manufacturer’s instructions.
A second group of these mortars is those gauged with some ‘hydraulic lime’ to give a chemical set rather than a pozzolan. There has been some controversy over this practice in recent years but if the hydraulic lime and the fat lime are mixed in equal proportion there will be little cause for concern. They can be used in similar exposure conditions.
4. Hydraulic Mortar based on hydraulic lime
Hydraulic lime is created from limestones which contain impurities of silica and iron. Because of this they have a chemical reaction as well as carbonation and set in differing degrees of hardness. There are no naturally occurring hydraulic limes produced in Ireland and the majority of this material is imported from France. 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- 2.5. NHL2 for pointing. NHL5 for severe exposure.
5. Hydraulic Mortar based on natural cement
Mortars such as ‘Roman Cement’ were manufactured during the nineteenth century and preceded the invention of artificial or Portland cement. Ebrington Barracks in Londonderry was originally rendered with this material for example. These mortars have similar properties to artificial cement in that they tend to be very strong and prone to hairline cracks and water retention. There are some modern equivalents but they are rarely applicable in conservation
6. Hydraulic Mortar based upon artificial cement
Artificial or Portland cement invented in the 1840’s has the advantage of quick curing due to its chemical set. Its strength is however much greater than that required for historic buildings and problems result from its Impermeable nature and poor flexibility. Mixes gauged with lime which maintain the proportion 1:3 have proven to be less damaging than others but with the reintroduction of a wide range of pure lime products in recent years there is no exposure situation were this should be considered the better option. Well executed lime mortars without cement are much better for long term results.
1:1:6 cement lime sand is suitable for severe exposures though it is prone to hairline cracks and needs to be very carefully monitored. The lime and sand are mixed to a ‘coarse stuff’, and this is gauged with the cement.
1:2:9 cement lime sand is suitable for moderate exposures The sand component can be amended to include a grit aggregate to provide a more robust mortar. Often, this will also give a closer match to an existing mortar.
1:3:12 cement lime sand is suitable for more sheltered exposures.
N.B. The use of very small amounts of cement with a lime mortar can result in failures. Research carried out by English Heritage (Smeaton Report) and Historic Scotland (TAN1) has indicated this. EHS advocates that cement content should never be less than 1:3:12.