You are here: Lime Mortar, Plaster and Render

Building a Lime Kiln is not quite as difficult as one might believe. A university student contacted me for help (when his dissertation was due) and apart from the use of lime, he also enquired about lime kilns. Rather than describe the process, I showed him how to build a simple kiln. The local stone in Hampshire is chalk and some was burnt in the kiln, which was fired on the same day that the kiln was built (not good working practice). Once burnt, the lime was recovered and slaked the following morning. Limestone is calcium carbonate, and during the firing process any water in the limestone is driven off together with the carbon dioxide. Once fired the product converts into calcium oxide, (sometimes referred to as ‘quicklime’, burnt lime’ or ‘lump lime’). When slaked in cold in water (never hot water as the exothermic reaction can be very violent), the lime putty is stored away from the air, either in a sealed container or under water. Once the lime putty is mixed with an appropriate aggregate, the resulting mortar is suitable for bedding masonry, plastering and rendering. It should be noted that once exposed to the atmosphere it cures by absorbing carbon dioxide.

THE KILN built with lime putty mortar 19 YEARS ON

The selection of a suitable aggregate was emphasised and a simple illustration given to show how dependant the resulting mortar can be on the choice of the material. A simple guideline is to determine the proposed mortar joint size (on modern brickwork the joint is often 10mm, rather than the 5mm. we find on historic buildings). Divide the proposed joint size by three to determine the maximum particle size, and the choice of aggregate, which is sharp in particle profile, and well graded through the range to fines. It is also most important to ensure that the water added to the mix is clean. Traces of clay or other contaminant can reduce the performance of the mortar. Whether it is a lime mortar or cement mortar, both products contain water. When water freezes it expands in volume by about 9% and the latent force is sufficient to disrupt brick and stone work to the point that a rebuild is necessary.

On site, it is recommended that a thermometer and humidity metre, are placed close to where the work is being carried out. Immediately after signing the day book each morning, the site manager should record the readings. A second reading should be made at midday and a final reading before the manager leaves site. A glance at the readings will indicate whether there is a need to protect the work. One important point, check that the thermometer has been placed properly as in one case, I found that the work was on the east wall but the thermometer had been placed on the west wall!
Any mortar contains a proportion of water and if this freezes it will disrupt the work. The simple rule is:- Make sure that you have 5 degrees and rising, with no risk of frost for 28 days. If for some obscure reason you are forced to work when the weather is not on your side, write to the client disclaiming any responsibility for spoilt mortar.
The suggestion that a hot air blower or other form of forced drying will ‘dry the mortar’ faster is quite correct, it will dry the mortar, but it is likely to inhibit the cure as the natural process of absorbing carbon dioxide will be affected.
A sheet of polythene may not suffice as protection, if it is too close to the surface of the mortar, as it will inhibit the transfer of vapour. Hessian is also vulnerable to damp from the mortar which will inhibit the ability of the mortar to dry naturally. The underlying message is that work should not be carried out when temperature and humidity are not suitable. Forced drying of the mortar will increase expenditure and will never guarantee success or a satisfied client.

Straw bale ‘Llama house’

Test results carried out in accordance with BS459 and the European Norms show that lime mortars more than equal Portland Cement in several ways. 
Always keep in mind that solid masonry walls need to breathe. Serious damp problems can follow if permeability is inhibited.
Limewash is permeable and has a dual refractive index.  Using modern impermeable paint on historic buildings is never a good idea. This straw bale ‘Llama house’, (built 18 years ago on one day), is lime rendered and lime washed. On a cold, misty winters day the limewash almost glows.

This paper was written and presented at a CPD Conferences Course by R. H. Bennett M.B.E.
Bob was a much respected long time presenter at CPD Conferences courses, and is sorely missed since passing away in early 2017.