Guide to mixing and storage of non-hydraulic lime mortar

Non-hydraulic lime mortars were generally speaking the most common type of mortar used in the construction of historic buildings, these mortars were often a mixture of lime, earth, shells, crushed stone, pit or beach sand. The aggregate used basically depended on what was available locally and was often the direct result of years of local knowledge being passed down from generation to generation, as opposed to formal teaching techniques. These mortars performed and do perform in a very different way from modern cement or gypsum based mortars.

Non-hydraulic limes, sometimes called “fat limes” are limes which are very pure with regard to their calcium carbonate (CaCo3) content, the purest forms of this material can have upwards of 95% CaCo3, these “fat limes” have no or little chemical set in the way that modern materials harden, they harden by a process called carbonation i.e. reaction with the air.

The carbonation process is a slow ongoing reaction which can take weeks, months and to a degree years to complete. The mortars and renders therefore need good protection from the elements whilst carbonation is taking place.

Mixing non-hydraulic lime mortar.

Today, non-hydraulic lime mortars are mixed from high calcium non-hydraulic lime putty and aggregates. Lime putties for these mortars should be at least 3 months old and wherever possible older, in fact the older the better, as the lime putty improves with age. The aggregates required for the lime mortar will of course vary from job to job, particularly where mortars are being matched, in the past generally speaking the nearest local aggregate to the job would be used, whether this was sand, crushed stone or even plain earth, clay earth being the oldest known mortar used in Ireland. Today good quality sands and aggregates are easily available, therefore when selecting your sand you should wherever possible use washed sharp gritty sand, well graded with the minimum amount of silt in the sample. Of course certain tasks will call for certain sands, a fine internal lime plastering finish would call for fine silica sand, whereas pointing on ancient rubble stone masonry would in all likelihood call for a mixture of aggregate from fine silica sand up to small pieces of crushed stone.

Wherever possible, non-hydraulic lime mortars or plasters should be mixed in a mortar mill. The action of the mortar mill, using a combination of revolving cast iron wheels and scrapers cuts and squeezes the lime putty and aggregates together, and by doing so there is no need for extra water to be added, other than the moisture within the putty itself. The great benefits of this are that mortars made this way are far less prone to shrinkage as the correct amount of water is within the mix.

Another method of mixing lime mortars is to use a pan mixer. It is in many ways very similar to a baker’s dough mixer, using a mixture of paddles and scrapers to combine the components together, again the moisture within the lime putty should be adequate for making the mortar, but if more water is needed, it should be kept to the minimum.

If the use of this equipment is not available then the possibility of purchasing readymade mortars should be considered, but should this option not be available, then mixing by drum mixer and further mixing by hand will be the only option.

When using a drum mixer, add the components in small amounts and keep the drum at a more horizontal angle, almost certainly more water will need to be added to stop balling and the only way to counter this is to stop the mixer at different times and ram the mortar with a timber rammer. The mix will need further hand mixing on a banker board and should be chopped and rammed as well as turned by shovel until the putty and aggregates are well blended and mixed together. Should hair be required in a mix, this should be added no more than two weeks before use, as the lime mortar in its wet state may rot the hair.

Storage

In the past lime mortar was heaped up and allowed to stand with little more protection than a sheet thrown over it, and in many cases no protection at all, the outer skin hardened and the mortar underneath remained moist with just occasional wetting down. The reason for allowing the mortar to stand or mature is to allow a better bond to take place between lime and sand. When using large amounts of mortar the best way of storage is to construct large timber mortar bins, three sided, which will allow the mortar to drain off any excess water and with little more than a polythene sheet keep the mortar air tight, wetting the top in warm weather. When using small amounts, sealed plastic tubs offer the best way of storage.

Hot Lime Mix

This is the most traditional method of preparing mortar in Ireland. This process combines slaking and the mixing of the aggregate in one operation. This method was most commonly used in the preparation of mortar for the construction of rubble stone walling. The mixing took place on or very close to the construction site.

The aggregate was normally spread in a circle and the lump lime was added to the centre of the heap, water was added and the mixture was turned a number of times. The end product is a very workable and “plastic” mortar that is ideal for rubble wall construction. You can use this fresh or it can be stored and reworked at a later date. Traditionally the mixing of mortars was carried out in the winter and they were left to sour out in pits until the building season started in the spring.

When producing hot lime mixes judging the correct ratio of lime-aggregate is rather difficult. Lump lime increases in volume when it slakes. In general a useful rule of thumb is to use four parts aggregate to one part lump lime.

This type of mortar is unsuitable for plastering as it may contain un-slaked particles. These un-slaked particles tend to blow and give the plastered surface a pock marked appearance.

There are many health and safety issues to be addressed when working with hot lime mixes and great care should be taken. Skilled operatives who understand the process and the dangers are essential when working with hot lime mixes.

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