Lime mortar has been used as a building material for thousands of years and is found in most listed properties in the UK. There is plenty of published material on the topic and I don't wish to replicate it here. However, I thought a brief summary might be helpful to any listed property owners looking to learn a little more.
Mortar consists of three basic parts - a binder, sand and water. The binder in a traditional home is likely to be lime and in a modern home it's likely to be Ordinary Portland Cement (OPC). The sand can be of varying colour and grade and mixed in differing proportions to the binder to alter a mortar's strength.
Fundamentally, a mortar should never be stronger than the brick or stone it surrounds. This is an intentional design of mortar to ensure the overall stability, flexibility, and longevity. The compatibility of mortar and bricks is crucial for the structural integrity and performance of a masonry structure over time. Traditional properties are often made from bricks that are softer than modern ones. This is because they were manufactured using different firing techniques, different clays and without special additives to make them more durable. This means hard Portland cement mixes used in modern buildings today are incompatible with older bricks and soft stone.
Masonry issues, such as spalling and cracking, in traditional properties can often occur because an incompatibility between brick/stone and mortar has been introduced as part of a repair. A good example of this where original lime mortar has been repointed using a harder OPC mortar.
Not all lime mortars are equal. Some will set in damp conditions (hydraulic), others won't (non-hydraulic). You would want to use a hydraulic mortar for any repairs below ground level, for example. The more hydraulic the mortar, the greater its strength and the lower its flexibility will be. Hydraulic lime mortar comes in different strengths (denoted 'NHL' followed by a number). A very strong hydraulic lime (e.g. NHL 5) may not be an appropriate choice if the surrounding brickwork were soft and friable.
To make matters a little more complicated, it is possible to make non-hydraulic lime progressively more hydraulic! The Romans realised that by adding volcanic ash to their lime mixes, it made it stronger and set faster. The volcanic ash and a range of other similar lime additives are called pozzolans. So, if you want a stronger non-hydraulic lime mortar that will set in damp conditions, you can add a pozzolan.
Cement used in modern construction is essentially a very strong hydraulic lime - its principal constituent is made from limestone, but it also has a high proportion clays and other additives.
Choosing to what extent a lime mortar should be hydraulic (i.e. how strong and flexible it needs to be) and mixing it correctly to achieve it is where the skill and experience of a traditional builder comes in. Ultimately, mortar should never be stronger than the brick or stone it is used with but there are other considerations too, such as how exposed masonry is to the elements - a stronger mix may be warranted for a chimney, for example.
Lime, brick and stone - A wide range of articles on lime, masonry and stone in building conservation.
A Guide to the Repair of Old Brick Walls - Some practical guidance on what to look out for.
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