Why Lime ?
Lime mortar is softer and weaker than stone, brick or cob therefore it is able to withstand a certain amount of movement (without cracking) that comes with settlement and seasonal changes in ground conditions. Lime mortar is porous and allows moisture to freely evaporate. It is this permeability that is sometimes referred to as ‘walls being allowed to breathe’. Lime mortar helps to keep a building dry inside.
It will last indefinitely as long as it is kept from drying out. It improves with age.
Why not cement ?
Cement mortar is usually hard, brittle, less porous than lime mortar and sometimes completely waterproof. It is damaging to traditional buildings for several reasons.
Cement mortar is often harder than old brick, some stone or cob, therefore when movement occurs the edges of the stone or brick are pushed against the hard mortar resulting in the masonry being damaged and the mortar itself cracking.
Hard cement mortar can trap moisture behind it causing damage to the structure or encourage ground water to rise from the base of solid walls. Trapped water in the wall causes decay and crumbling, in severe cases the cob, brick or stone fails. Masonry or cob is also susceptible to frost damage if water is trapped in the wall.
Over time lime mortar decays but not the cob, stone or brick. It is much simpler and cheaper to re render or repoint a wall than repair the fabric of the building.
Cement will last upto 50 – 70 years depending on the design life, environmental and service conditions and preventative maintenance practices. Concrete is thought of as being permanent, strong and very durable. In actuality it is a very complex, thermodynamically unstable and chemically reactive building material that continues to cure and become stronger for years. It is also vulnerable to many external influences!