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Curing Lime Based Renders & Mortars

Curing is the process of keeping a mortar or render under a specific environmental condition until the chemical set (referred to as hydration) is sufficient to withstand the environment into which the mortar has been placed. Lime binders are generally weaker than cements taking longer to acquire their strength and hardness, leaving them potentially more vulnerable for a longer period than cement equivalents, and curing them once placed is simply regarded as best practice.

Good curing is typically considered to be that of providing a humid environment stimulating full hydration of the lime binder, providing strength development along with other quality benefits to be had from using lime. Conversely, and the most common cause of failure in our experience, is allowing a mortar/render to dry out too quickly, impeding the chemical process for hydration, stressing the mortar resulting in cracking, especially to renders.

Curing Lime Based Renders & Mortars 1 General view of Hessian sheeting attached to the inner uprights of a typical inside board scaffolding. Summer working with the Hessian dampened down to aid the curing regime.

Protecting Lime Renders

Standard practice for protecting lime renders is with Hessian sheeting draped over the subject area in relatively close proximity to the render. This should be left in place for at least a week and while this is a standard requirement for any kind of rendering it’s one that is regrettably seldom practiced.

Cornish Lime stock three grades of hessian; 229, 273 & 366 gsm (grammes per m2) 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.

Issues

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, 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.

Curing Lime Based Renders & Mortars 2 Close up view showing a simple method of wiring the sheets together (alternatively, small cable ties could have been used), with the base of the Hessian and stapled to 2”x1” batten to weight it

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 too soon onto the previous coat may result in stress cracking as a result of unequal contraction between the two layers (differential drying). We 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, closing the surface in this manner helps 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 let alone an even level.

As well as supplying Hessian and a range of ties etc. Cornish Lime also supply a Wintermix product as part of our Cornerstone range. Please contact us for further information on this product or further advice on curing lime mortars and renders.

For high suction backgrounds especially we 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.

5 thoughts on “Curing Lime Based Renders & Mortars”

  • Margaret Panter
    Margaret Panter June 12, 2018 at 8:49 am

    I’m putting lime render on internal mudbrick walls and I want the final surface to be very stable (ie no powder / dust / sand grains coming off). I’ve been told that frequent misting is the key to achieving this. How often should I mist? And for how long? And should I taper off the frequency of misting?

    The air temperature ranges from 16-22 deg C. The final coat of render is one and a half parts lime putty made with hydrate lime to 3 parts fine double-washed beach sand applied with a steel hawk.

    As well as frequent misting, is there anything else I should be doing to achieve a stable surface?

    Reply
    • Adam Brown

      Dear Margaret

      Misting can help and will certainly help aid the curing of the plaster, which in turn can help limit the dusting. There is no hard and fast rule to misting as there are several variables, such as temperature fluctuation, relative humidity, substrate condition and moisture content within the substrate. We would recommend spraying for the first 3 to 5 days, depending on temperature and background suction, you will probably find that the amount/frequency of spraying can be reduced over time. However it is worth mentioning that all lime plasters are prone to dusting.

      Kind Regards

      Adam

      Reply
  • Margaret Panter
    Margaret Panter June 19, 2018 at 7:47 am

    Thanks very much for your answer, Adam. I've been misting about every hour or 2 on the first day (but not between 10pm and 6am) and then gradually reducing misting frequency and stopping after 2 weeks. Is this overkill?

    I've also been experimenting with different sands and different ratios on different substrates and different use of tools. After about a week none of the trials has much dust coming off, but three trials have developed hairline cracks. I've put this down to the fact that the final lime render coat was too wet, there was too much beach sand which is very fine and not sharp, and that the infill coat underneath was mud (clayey with coarse washed river sand in but no lime). The mud coat is quite thirsty, although I did mist it thoroughly several times before putting on the final coat of lime render.

    Reply
  • Deri Brown

    I have been rendering a stone property with nhl3.5. I applied a harl coat first mixed 1:1 and am now applying the scratch coat at a mixed ratio of 2.5:1 with fibres added.I am thoroughly wetting down the walls before and during application however I find that the scratch coat cracks as it is drying. Some say I should not be concerned about this coat cracking, however the last wall I rendered in line back in the autumn hardly cracked at all?

    Reply
    • Adam Brown

      Hi Deri

      It is difficult to say without knowing the brand of lime, the type of sand and the exact substrate you are working with. A common cause of cracking is excess or rapid moisture loss or the render drying too quickly. I know that you have said you are dampening the background and the render but you may find with the heatwave we have been experiencing of late that both the background and the render require more dampening than usual. Also if you are working in direct sunlight this will speed up the drying time significantly. I would recommend dampening the background more before applying the render, ensuring that the render remains damp once applied and protecting it from sun by using hessian. As i am unable to see the cracking i cannot comment as to whether the render is compromised, if you only have isolated or minor shrinkage cracking (less than 1 mm) you may be ok, if the cracking is deeper/larger and/or extensive than this, the render may be compromised.

      Kind Regards

      Adam

      Reply

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