Masons Mortar Ltd
77 Salamander Street
Edinburgh EH6 7JZ

Tel: 0131 555 0503
Fax: 0131 553 7158

Masons Mortar Ltd
Unit I, Woodville Court
Woodville Street
Glasgow G51 2RL

Tel: 0141 445 4812
Fax: 0141 445 8898

Masons Mortar Ltd
Block 2
Woodend Industrial Estate
Cowdenbeath, Fife

Tel: 01383 514 460

Masons Mortar Ltd
Unit 11
Brough Park Trading Estate
Fossway, Newcastle upon Tyne

Tel: 0191 908 9234

Choosing the Appropriate Mortar
This section is intended for use by experienced professionals and craftsmen in order to assist them in deciding which mortar will be most suitable for the purpose. The Chart on the opposite page is there for guidance, please take time to read this section and consider the information provided here and opposite when choosing an appropriate mortar.

One important rule of thumb is that the mortar should not be stronger than the host masonry and be at least as vapour permeable. With this basic rule in mind applied to the case of historic traditional solid wall construction, two further criteria stand out.
First is the background. In a new build situation a designer has a choice of matching materials to suit the environment, detailing etc, this is not always the case with existing buildings.
Secondly exposure, the location, environment and exposure to wind driven rain. Water, particularly in the form of wind driven rain is possibly the greatest threat to buildings, minor failures, slight cracks, inadequate joints and small detailing defects are cruelly exposed in strong wind driven rain. Internal timbers, imbedded in walls pick up moisture from wet masonry and mould and rot proliferate at an alarming rate, often unseen until the damage is beyond repair. In addition, damp walls require higher energy in the form of heating to maintain comfortable living conditions.
The primary dilemma comes when say for instance the background condition is such that only a relatively weak mortar would be appropriate, but the environment is such that a weak mortar would be unlikely to cure properly before the onset of winter, thereby leaving the walls exposed to even greater risk of saturation. Knowing the correct balance between background, mortar choice and mortar performance is therefore all-important. This chart is provided to address some of the issues related to mortar choice, curing and performance. The assessment of the background remains with the user.
1 Our Pre Mixed Dry Bagged Range will match or exceed the general performance figures shown on the table opposite. 2 Although the exposure classification would suggest that the higher the number the better the mortar, this is based on current design criteria. A mortar class 1-4 can be successful in high exposure ratings provide it has been fully and properly cured, the difficulty comes in relation to the time it can take to achieve full curing and how proper detailing acts to protect areas of a building, both during initial curing and thereafter. Pre Mixed Dry Bagged Mortars will generally exceed these performance figures especially in relation to early freeze thaw resistance.
3 Much is made of Freeze Thaw resistance, however if something is not wet, it can't freeze, and ultimate freeze thaw resistance is closely linked to pore structure and vapour permeability. 4 The use of Capillary rise figures in isolation does not mean that a mortar is vapour permeable. 5 Vapour permeability, comes in most mortars at full carbonation, where the pore structure is such that ice crystals that form within the matrix of the wet mortar are large enough and well enough linked to allow the ice to expand harmlessly across the matrix of the mortar. 6 Full carbonation; can take many months, usually 10-12 in good curing conditions for a 20mm thick render. Partial carbonation in a hydraulic lime mortar is often adequate to resist freeze thaw depending on the strength achieved through the hydraulic set (Normally 1.8-2Nmm2) but this will depend on location and exposure.
In the majority of cases mortar joints are far less susceptible to initial or early freeze thaw action, with the exception of joints in dense impermeable masonry units, (examples would be Granite, Basalt, Slate, some sandstones, lime stones and some types of brick) This is due to a cascade effect where water that is not absorbed into the face of the masonry, flows freely down the wall face, any wind pressure then concentrates the water, exposing weak or defective joint finishing, leading to early saturation and a higher risk of freeze thaw, particularly relevant to parapets, copes, sills and chimneys.
Saturated masonry has the potential to trigger 7 Sulfate reactions, either from air borne pollutants or from soluble Sulfates in the host masonry or sands or through capillary action from ground water.
Road salts, de-icing, water thrown up by vehicle spray all affect the performance of walls.
Apart from design requirements for early initial and final 8compressive strength, such as thin wall construction, lime mortars, specified and used properly, have performance characteristic that are superior to Portland Cement mortars for every aspect of traditional building repair.