Lime Protection, Curing and Aftercare
Lime Protection, Curing and Aftercare is a vital process in the successful use of lime mortars. This part of the process does not receive the attention that it deserves particularly if work is carried out in the winter, when frost may be a problem, or in the summer when the mortar dries out too quickly.
Traditionally the time to carry out work using lime products are in the mid to late spring season or in the autumn. This is not very practical in the modern age; therefore we are forced to initiate certain steps to mitigate the negative effects of the weather.
Protection starts before the work takes place. It is important to assess the state of the structure before work commences. Robust detailing, properly installed ground drainage, roof membranes, gutters etc should all be in working order to avoid water ingress and over saturation of fresh mortars.
Water entering the masonry at any level will cause localised saturation within the masonry and this in turn causes problems in the application of the lime finish. Temporary measures may need to be put in place for the disposal of rainwater.
Freeze Thaw Action:
Freeze thaw action can only occur if the mortar pore structure is saturated, such as in fresh mortar. If there is a danger of freezing of fresh or uncured mortar, close covering with hessian or tarpaulins should be used. Polythene should not come in contact with fresh mortar.
If the mortar dries out too quickly, hydration and carbonation will be inhibited, drying shrinkage may occur and the mortar may become friable. Rapid drying is best avoided by screening against direct sunlight and provision of physical barriers to reduce wind action.
Rapid drying whether by wind or sun can lead to shrinkage (cracking), separation from background, crumbly and powdery mortar. Proper curing conditions involve the process of gradual drying and should ideally result in no shrinkage.
It is important is to provide suitable scaffold for carrying out the safe and effective operation of harling, pointing or smooth rendering. Drape green mesh or in very exposed situations monoflex on the outside of the scaffolding. This cuts down the harmful effects of wind drying the material too quickly.
On the inside of the scaffold, hessian sacking combined with a plastic sheet provides an excellent barrier against the negative effects of the weather. The use of hessian and polythene close to the wall creates a microclimate that can raise the temperature a few degrees.
This needs to be left in place for at least ten days or longer in adverse weather conditions in the case of lime. In extreme conditions it may be necessary to use bubble wrap or to install heaters to protect the work against frost attack. Work should never be carried out if the temperature is below +5oC.
Fat limes or non-hydraulic limes will need protection for longer than hydraulic limes to ensure they are not damaged, particularly by winter frosts. Therefore timing and material selection are very important ingredients when choosing the correct time of year to commence work.
When lime mortar dries too quickly it results in whitening on the surface. This is the lime being drawn to the surface. As mortar carbonates it needs to dry out slowly. It needs to be kept moist (not wet) for at least 10 days. This aids the process of carbonation. Keeping the work moist can be achieved by the use of a garden pump sprayer.
During and after application, protection should be in place for as long as necessary for the mortar to cure properly and dry sufficiently.
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