Lime bloom is the most common form of efflorescence ( formation of a salt crystal crust or covering through a chemical change) on cement based renders. It is caused by the migration of lime water molecules (Ca(OH)2) to the surface of the damp external render because of diffusion i.e. the surface of the damp external render contains a high concentration of water molecules than within the render where the lime water molecules are concentrated so this causes a concentration gradient to form resulting in the lime water molecules moving from a region of low water concentration to a region of high water concentration which is the surface of external render. As the moisture evaporates from the surface of the render, this leaves the aqueous lime water molecules on the surface where it reacts with carbon dioxide from the atmosphere to produce a thin surface layer of calcium carbonate (CaCO3) crystals. When this surface coating dries out, the crystals appear white, giving the appearance of colour fading or being washed out.
Lime bloom forms most readily when the render remains damp for several days shortly after application and this is reflected in the fact that it occurs most frequently during the Winter months. In particular, extended periods of rain, retained snow and damp foggy days are conditions most likely to brink on a severe outbreak. Although drying winds are often suggested as a likely cause, they are probably not a major factor. Lime bloom is not visible on damp render and so only becomes apparent with the onset of drying weather. Lime bloom forms most readily when the render becomes wet and damp for several days.
The steps that can be taken to reduce the risk of lime bloom range from including a water-reducing or waterproofing admixture which will reduce the permeability of the mix, to controlling the curing conditions of the render by covering with polythene sheeting, firmly fixed so that it is not in direct contact with the surface of the render, for a few days to enable the mix to develop strength. Also application of K-rend render material prevents the formation of limebloom because the hydrophobic (water hating) silicone compounds inside the render repel the water, preventing excess water building up on the surface of the render.