A lime binder must be cured once it has been placed because it requires time to fully hydrate before it acquires strength and hardness. Curing is the process of keeping the mortar/render under a specific environmental condition until hydration is relatively complete.
Good curing is typically considered to use a moist environment which promotes hydration. Increased hydration lowers permeability and increases strength, resulting in a higher quality material. Allowing the mortar/render/concrete surface to dry out excessively can result in tensile stresses. The still-hydrating interior cannot withstand these stresses, causing the mortar/render/concrete to crack.
Protecting Lime Renders
Standard practice for protecting lime renders would be in the form of Hessian sheeting draped over the scaffold in relatively close proximity to the render. This should be left in place for at least a week. This is a standard requirement for any kind of rendering and one that is regrettably seldom practiced. Once again it’s very important for the hydration/hardening phase of the binder.
Cornish Lime stock four grades of Hessian; 229, 273, 320 & 360 GSM (Grammes per Square) Metre where the weight is relevant to the weave and amount of fabric used per square metre. The most commonly used for curing is the 229 & 273 GSM, the heavier fabric is more generally used for frost protection.
When appropriate we would advise the addition of a proprietary polypropylene or fiberglass reinforcing fibres added to the mix as an aid to control shrinkage cracking in the base coats.
1, Drying Too Quickly
Where a render is allowed to dry out too quickly hydration and carbonation of the binder is inhibited, resulting in drying shrinkage. There are two principle types of drying problems both of which will be manifested as cracks. The first, plastic shrinkage is the consequence of the rapid evaporation of mixing water from the mix (while in its plastic state). This leads to increased tensile stresses at a time when it has not gained sufficient strength. Plastic shrinkage cracks will be manifested in the first 48 hours.
The second, drying shrinkage is from the effects of climatic conditions such as wind and high temperatures or exposure to strong sunlight (compounded during times of low humidity). Cracking from drying shrinkage tend to take that much longer to manifest but the outcome is much the same. Another consequence of rapid drying is that the mortar may become friable.
2, Excessive Water
The consequences of too much water in a mix can compound the plastic shrinkage, which as previously mentioned is likely to be manifested in the first few days following application. Water in a mix takes up volume and is given up during the hydration process.
3, Moving On Too Quickly
The consequences of applying subsequent coats of render coats to soon onto the previous coat may result in stress cracking as a result of unequal contraction (differential drying) between the two layers. We would advise that the backing coat should be allowed to achieve a sufficient set prior to applying additional coats.
4, Thick Top Coats
The application of excessively thick top coats can result in stress fracturing in the coat as a result of unequal compaction when finishing the render coat. The purpose of floating (rubbing up) is primarily decorative, however it performs a technical function in that by closing the surface will help reduce the Ingress of water. Also where a top coat is too thick it will be extremely difficult, often impossible, to compress the whole thickness to an adequate level.