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Lime mortar: an introduction

Various types of lime are used in building as mortars, renders, plasters, slurries and washes, and all are made from limestone. Most limestone was laid down in the Cretaceous period (60-150 million years ago). When limestones containing silica and clay are burned, the clay decomposes at between 400ºC to 600ºC and combines at 950ºC to 1250ºC with some of the lime, forming silicates and aluminates. The lime produced consists of a mixture of quicklime, cementitious material and inert material. Such limes need to be slaked with enough water to convert the quicklime to calcium hydroxide, but not so much that a chemical set begins.

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: limestone is burnt and CO₂ is driven off 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 putty lime is that it is often highly compatible with weakened, weathered stones and bricks, tends to act sacrificially within the masonry face and is ideal in consistency for pointing, face repair and plastering. In other words, it is often the perfect material for the conservator.

Lime putty should be matured in excess of one month; it should be wet stored with aggregates or blended in a mortar mill with aggregates. In the absence of a mill, hand ramming and beating is the ancient practice to produce the best results. Slow curing and humidity control are significant to the final performance.

Hydraulic lime

Hydraulic lime is produced from limestone containing clay, and has the added benefit of being able to set underwater. Its hydraulic characteristic is produced by 'impurities' of silica and clay in the limestone from which it is burnt. From the Romans to the mid-18th century, cement was composed of lime plus volcanic ash or other additives. Modern Portland cements date from around the early 20th century.

Hydraulic limes are usually imported from France, Switzerland and Italy; they can be used in a similar way to cement, but not in all seasons. In common with other limes, in spite of the fact that these limes will set in water, a slow curing period of up to one week is recommended. Work 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 for plastering. Hydraulic limes retain good water vapour permeability and the ability to accommodate movement, coupled with salt and frost resistance.

Environmental benefits

One of its main benefits is that lime mortar is carbon neutral. Lime, like cement, gives off CO₂ during its manufacture, however, it re-absorbs CO₂ 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;
CO₂ 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, i.e. any house made from timber, straw bales, or earth.

Other benefits

Lime naturally breathes; any water that enters a structure through a crack can escape by evaporation/transpiration. This isn’t the case with cement; when using cement mortars, the only way moisture can escape is through the brick, which can begin to erode away.

Lime is soft and flexible, and has been called self-healing, so if a building moves slightly it won’t crack like cement and let water in.

Information Sources

More information on green roofs is available on The Construction Information Service. Subscribers to The Construction Information Service can link directly to the documents from the links below.

The Construction Information Service – Mortar

Architects' Journal

Architects' Journal Technical Prophet without honour. AJ 07.1.99
Looks at the widespread use of natural lime in France, compared to the rather precious approach to the use of this material in England. Describes the production of the different grades of lime and their use in building for renders and mortars

British Cement Association

BCA guide to materials for masonry mortar
Addresses definition of masonry mortar, the properties expected from it, constituent materials and standard documents, types of mortar, mortar specification, and recommended mortars for various applications.

BRE

Good Building Guide 66 Building masonry with lime-based bedding mortars
Gives guidance on the properties of lime mortars, mortar mix design and working with lime mortars.

BSI

BS EN 459-1:2001 Building lime. Definitions, specifications and conformity criteria

Cadw: Welsh Historic Monuments

Technical Conservation Note 2 Use of lime mortar in historic structures
Discusses the use of lime mortar in historic structures, covering types of lime, matching the mortar, aggregates, mixes, pozzolanic additives, mixing the mortar and filling joints.

English Heritage

Hybrid mortar mixes containing a blend of both non-hydraulic lime and hydraulic lime binders
Covers both non-hydraulic lime and hydraulic lime binders.

Practical building conservation. Volume 3: Mortars, plasters and renders
Explores the use of lime, cements, mortars, rendering and plaster. It provides a detailed description of the use of hydraulic and non-hydraulic limes. There is also an analysis of traditional and modern mortar additives, a guide to external renderings, to gypsum plaster and to lime washes and lime paints. Techniques are illustrated with two case studies.

Society for the Protection of Ancient Buildings

Technical Pamphlet 5 Pointing stone and brick walling (3rd ed.)
This pamphlet describes and illustrates the repointing of old stone and brick walling.

Quarry Products Association

Lime stabilisation manual. 2nd edition
This document is divided into seven main parts; introduction; application of stabilisation; mechanism of lime stabilisation; lime for soil stabilisation; construction method; laboratory testing; and site control testing.

Other sources of information

Ashurst, John. Mortars, Plasters and Renders in Conservation. Second 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.

Pasley Sr, CW. Observations on Limes, Calcareous Cements, Mortars, Puzzolanas, Natural and Artificial, together with Rules deduced from Numerous Experiments for Making an Artificial Water Cement Equal in Efficiency to the best Natural Cements of England. 2nd Edition. John Weale Architectural Library, 1847.

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.