Application of Lime

 

Lime should only be applied externally when there is an ambient temperature of at least 10ยบ C. Therefore it is generally not advisable to apply lime outside between November and early March. Traditional mortars should be treated as the sacrificial element within the construction process. They should be weaker than their surrounding structural elements of stone or brick simply because it is cheaper and more beneficial to the structure to replace the mortar occasionally than the necessity to replace the structural elements. Water, either by precipitation or capillary action will enter into an old building! The use of traditional mortars will allow moisture to evaporate. Cement/ gypsum mortars will trap moisture allowing a gradual degradation to occur which is then aggravated by the freezing process as water expands when frozen. When applying pointing or plastering mortars do not over work them as to do so will allow the smaller particles of sand as lime to rise to the surface giving both a more brittle finish and giving a whiter appearance. Lime is white with the sand giving the colour to the mortar from a grey tint to a muddy colour. Natural pigments can be applied but the colour can be difficult to replicate over a successive batches of mortar.

Hydraulic limes can withstand more extreme ranges of temperature because of their pozzolannic properties that incorporate calcites and silicates which gives their constituent parts their ability to harden naturally under damp conditions. The critical point beyond which a mortar will break down under both low and high temperatures can also be reduced by covering the mortar with hessian or other insulating materials that still allow the drying process to continue whilst maximising protection. This harder hydraulic mortar is therefore more suitable to more exposed areas of a building and specifically those areas that are likely to be affected by a greater degree of weathering ie chimneys or wall cappings.

The initial set can take a few days depending on the environmental conditions at the time so it is important to keep the mortar covered. This period is critical to the eventual long term strength of the mortar. Once this has passed the mortar will continue to carbonate for a year or more to reach its full strength.