Green shorts - Lime mortar

 

 

What is lime mortar?
When limestones containing silica and clay are burned, the clay decomposes at between 400ºC - 600ºC and combines at 950ºC - 1250ºC with some of the lime, forming silicates and aluminates. The lime thus produced consists of a mixture of quicklime, cementitious material and inert material. When slaked with enough water to convert the quicklime to calcium hydroxide, it becomes building lime. There are two main types of lime mortar, Lime putty and Hydraulic lime; this latter is further split into moderately and eminently hydraulic limes.

 

Non-hydraulic lime
Non-hydraulic lime, lime putty or quicklime, is the purest form, made from rocks containing at least 95% calcium carbonate. The limestone is processed by burning, which drives off the carbon dioxide to produce calcium oxide (CaO). Non-hydraulic lime is available from many suppliers in the form of putty and from most builders’ merchants in the form of bagged hydrate.
The great merit of mature lime putty is that it is often highly compatible with weakened, weathered stones and bricks, and tends to act sacrificially within the masonry face.
Lime putty should be matured, usually in excess of one month and wet stored with aggregates, or blended in a mortar mill with aggregates.

Hydraulic lime
Hydraulic lime is produced from limestone which contains clay. It has the special added benefit of being able to set underwater. The hydraulic characteristic of the finished lime is produced by 'impurities' of silica and clay in the limestone from which it was burnt.
Modern hydraulic limes are usually imported to the UK, and in common with other limes, a slow curing period, of up to one week, is recommended, in spite of the fact that they will set in water. Working with hydraulic limes must not take place when the temperature is below 5ºC.
Hydraulic limes may be used on copings, chimneys, weatherings and pavings, as well as for bedding ashlars, rubble and plastering. Hydraulic limes retain good water vapour permeability and the ability to accommodate movement, coupled with salt and frost resistance, when cured.

Environmental benefits
One of the main benefits is that lime mortar is carbon neutral. Lime, like cement, gives off carbon dioxide during its manufacture, however, it re-absorbs this when it sets, whereas cement does not. Other benefits include;

• Lime is recyclable and biodegradable.
• Limestone is burnt at around 900°C compared to around 1300°C for cement. This saves on fuel consumption and emissions of pollution and greenhouse gases.
• carbon emissions in the manufacture of lime are 20% less than for cement.
• Lime mortars allow bricks to be recycled; the mortar can be removed from bricks and blocks, unlike cement.
• Lime is an important part of any natural house; any house involving timber, straw bales, lime and earth.

 

Background documents
Ashurst, John. Mortars, plasters and renders in conservation. 2nd edition. Easa Publication, 1997.

BS 890:1995 Specification for building limes. British Standards Institution, 1995.

English Heritage. The Lime directory. Donhead Publishing, 1997.

Holmes, Stafford and Wingate, Michael. Building with lime. Intermediate Technology Publications, 1997.

Natural plaster book: earth, lime and gypsum plasters for natural homes. New Society Publishers, 2003.

Searle, Alfred B. Limestone and its products, their nature, production and uses. Ernest and Benn Ltd, 1935.

Vicat, Louis J. A Practical and scientific treatise on calcareous mortars and cements. (Translated by J T Smith). John Weale, 1837.