Guide to mixing natural hydraulic lime mortar
Hydraulic limes differ from non-hydraulic limes in that they have a chemical set as well as the process of carbonation. The limestones from which hydraulic limes are formed naturally contain a varied range of minerals of which silica and alumina are the main ones for creating hydraulic lime. When these limestones are heated in the kiln at temperatures of around 1200oC, the resulting lime has different properties. From the pure limestone, the silica and alumina combine with the lime to form active compounds. These compounds combine in water to create a chemical set. The percentage of silica and alumina contained in the limestone will determine the main characteristics of the lime and of course the resulting mortar or plaster.
The main characteristics:
- Setting time
- Frost resistance
A conventional cement mixer can be used although for larger projects a paddle mixer is preferable. The mix is typically 1 part lime : 2.5 parts sand. Measuring the material must always be with a gauging box or bucket. A shovel is not acceptable since quantities are too inconsistent.
Lime mortars mixed in drum mixers can be prone to balling. Use of particular mixing techniques can reduce this. We recommend the following procedure:
- Start with an empty mixer
- Add 1 part sand
- Mix in 1 part lime
- Followed by 1.5 parts sand
- Mix dry for at least 5 minutes
- After 5 minutes slowly add water until the desired consistency is reached, it is very important not to drown the mix by adding too much water
- Once the desired consistency is reached, mix for a further 20 minutes
The mix, to begin with, should appear rather dry but as mixing time increases the render will become much ‘fattier’. If too much water is added the risk of shrinkage will increase and the final strength reduced. Do not use any Plasticisers/water proofers.
If the walls are dry, damp down to reduce the effects of suction.
Sand and larger sized aggregates make up the larger proportion of most mortars. Colour, texture and overall strength are all strongly affected by the choice of aggregate.
The aggregates most commonly used with hydraulic lime are sand and grit, although for the purpose of matching historic mortars various impurities may have to be added. A good sand should be a washed sharp sand with angular grains to ensure good bonding qualities. Soft building sands should be avoided as their rounded grain shape can result in excessive shrinkage.
Sands used should be well graded with a range of grain sizes, which for most plaster, render and mortar work will range from 5mm down to 75 micron. Larger sized aggregates may be used in some mortar or pointing work. As a rule of thumb for pointing, the maximum size of aggregate should be no bigger than one third of the joint width. Sands, which contain a clay or silt content of more than 4% should be avoided, as these will inhibit the contact between lime binder and aggregate.
Sands which have a high fines content should also be avoided as the larger surface area of these will require more water in the mixing. This higher water content will induce shrinkage and can affect flexural and compressive strengths. Monogranular sands should be avoided as they will possess poor workability qualities and will inhibit good vapour exchange i.e. the ability to breathe.
Use clean water. The addition of water should be considered carefully, as it will directly affect the ultimate strength and durability of a mortar. The more water introduced into the mortar mix, the weaker will be the final result. However too little water will prevent the chemical processes taking place and weaken the material. Generally, water should be added sparingly, until a useable consistency is achieved. Adjust quantities to give a workability suitable for the application. It is important to use the minimum amount of water necessary so as to reduce shrinkage.
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