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Introduction and Properties
Lime has been used in mortars, plasters, renders and paints for centuries. It was continuously used in building work from Roman times up until the 20th Century. It is only in the last few decades, particularly since the 2nd World War, with the development of Portland cement that lime products have ceased to be used. Lime mortars, renders, plasters and limewashes have many properties that have encouraged a healthy resurgence in their everyday use:
- they are porous and absorb moisture from the surrounding bricks or stones; any salt or frost damage occurs in the lime, thus protecting the surrounding materials
- they allow walls to 'breathe'; moisture will evaporate as rapidly as it enters (whereas most modern mortars and paints hold moisture in the wall)
- they are relatively flexible and will accommodate some movement in a wall; if they crack they will 'self-heal' when exposed to air
- they are 'soft on the eye' and compliment the beauty of other natural and traditional materials
- they are considered to be more environmentally-friendly than conventional materials
- they allow building materials (eg stone, brick) to be re-used, cement is so hard that we will be unable to reclaim much of the stone used today.
Principles of Selection/Specification
It is important that the most appropriate material for the well-being of the building is chosen. Every situation needs to be carefully assessed in terms of:
- softness of stone, brick etc - this should be a major factor in selection; the mortar should always be 'softer' than the stone/brick because it is the 'sacrificial' element designed to take water away from the main fabric of the building
- the environmental conditions - where the environment is very wet (eg sea-defence walls, canal walls) a hydraulic lime is more appropriate because of it's ability to set under water
- the existing materials; repairing like-with-like will usually provide the most satisfactory solution both aesthetically and technically, but it should not be done without considering the performance of the original materials
- the time of year - lime work (like cement) should never be done when the temperature is 5oC and falling; ideally, fat lime work should be done in spring / early summer, otherwise consideration should be given to adding a pozzolan (brickdust, calcinced clay etc) to a fat lime to speed up carbonation or to using a hydraulic lime (eg in very wet conditions)
- the available time - occasionally work is programmed with insufficient time to support the use of the materials specified; rapid builds necessitate rapid sets in which case consideration should be given to adding a pozzolan to a fat lime or to using a hydraulic lime.
Frequently, the choice still seems to be based on the availability, or lack of availability, of skills, which means that cement is often introduced into buildings where it has no place and will - in the long term - almost certainly damage the very fabric of the building.
There is nothing 'magic' about using the basic lime-based materials - mortars, plasters and limewashes but there are specific rules which will ensure the right result - the materials are often blamed despite the fact they have been used pretty much continually for 1000s of years and structures and parts of structures such as the Great Wall of China have survived. |
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