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Issue 2 May 2012 Yq4 The use of lime in mortar

GeneralInformationThe benefit of using lime in mortar hasbeen recognised by buildersthroughout the ages. The ancientGreeks and Romans produced mortarby burning limestone and slaking(mixing with water) the resultantproduct, then mixing this lime withsand. Many ancient buildings standtoday as monuments to the efficiencyand durability of these traditional limemortars, and much of the lime usedtherein was hydraulic, that is, it gainedstrength by hydrating and courtesy of the British Lime AssociationThe use of lime inmortarmaterial is calcined at a temperature in therange 850 C - 1100 C. The calcium carbonateis changed into calcium oxide (quicklime)and carbon dioxide.

There is a real danger of contact dermatitis or serious burns if skin comes into contact with wet mortar. Wear suitable protective clothing and eye protection.

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Transcription of Issue 2 May 2012 Yq4 The use of lime in mortar

1 GeneralInformationThe benefit of using lime in mortar hasbeen recognised by buildersthroughout the ages. The ancientGreeks and Romans produced mortarby burning limestone and slaking(mixing with water) the resultantproduct, then mixing this lime withsand. Many ancient buildings standtoday as monuments to the efficiencyand durability of these traditional limemortars, and much of the lime usedtherein was hydraulic, that is, it gainedstrength by hydrating and courtesy of the British Lime AssociationThe use of lime inmortarmaterial is calcined at a temperature in therange 850 C - 1100 C. The calcium carbonateis changed into calcium oxide (quicklime)and carbon dioxide.

2 The calcium oxide isthen reacted with water (slaked) in a closelymonitored, computer controlled hydratingplant to produce a fine powder of the terms hydration and slakingare synonymous, however hydration isgenerally taken to mean a process, whichyields a dry hydrate powder, whereas slakinginvolves more water, producing wet , an excess of water was added toquicklime and the material allowed to soakfor several days or weeks in pits (the processcalled slaking) to produce a material calledlime putty. A more dilute mixture ofquicklime with water is milk of lime wherethe solids content ranges from 1 to 20%.

3 QualityLime is classified according to its chemicalcomposition. All the hydrated lime used inthe construction industry in the UnitedKingdom conforms to BS EN 459-1 and isdesignated CL90. This designation indicatesthat the calcium oxide content plus anymagnesium oxide present is in excess of 90%.MaterialsLime mortars can be supplied to the buildingsite from modern, computer controlledbatching plants, guaranteeing the correctmix proportions for each , lime sand mortar can beobtained from MIA member companiesready blended with the correct quantity oflime, sand and an air entraining admixture togive the optimum workability and freezethaw the rapid increase in building output,natural hydraulic lime mortars sufferedbecause of their variable performance andcement based mortars were preferred fortheir more rapid strength the early part of the last century.

4 Theprocess of adding hydrated lime to a cementmix was developed to produce compositemortars, which retain many of the beneficialproperties of traditional lime mortars. Morerecently the practice has developed of usingadmixtures to improve the properties data sheet concentrates on the use ofhydrated lime in mortar . Fresh and hardenedproperties of mortar containing hydratedlime are excellent and, when manufacturedunder factory controlled conditions,consistent quality is achieved to meet theexacting standards required should be noted that modern naturalhydraulic limes are now available that areproduced under rigorous quality controlprocedures and natural hydraulic limemortars are becoming more widely available,especially for restoration and production of lime commences with thequarrying of limestone or chalk (chemicallythese materials are classified as calciumcarbonate).

5 The extracted rock is thencrushed and fed to a kiln. The calcinationprocess was traditionally undertaken in smallmixed feed kilns and examples of the remainsof these can still be seen around the countrytoday. Modern methods of lime productioninvolve feeding the limestone or chalk toeither shaft or rotary kilns where the rawmia mortar industry associationMineral Products AssociationYq4 Issue 2 May 2012(SUPERSEDESI ssue 1 May 2004)CI/SfBData sheet18unsightly staining. Lime based mortarsminimise this effect by reducing theamounts of water that can penetrate themasonry units. Furthermore, they can alsoprecipitate the soluble salts in the insolublecalcium form before they reach the practicesLime has a higher sand carrying capacitythan any other bonding agents.

6 The excellentworking properties of the mortars producedfrom it lead to higher productivity from thebricklayers and the quality of the finishedwork is better than with other to the weather tightness and durabilityof the finished work subsequentmaintenance is material merely requires accurateaddition of the correct quantities of cementand water on site to produce use of lime in mortars is recommendedby the Building Research Establishment intheir publication BRE Digest 362 BuildingMortar and by the Brick DevelopmentAssociation. The use of the material isapproved in the Building Regulations and bythe National House Builders mortarpropertiesWorkabilityLime is well known for its ability to improvethe plasticity and workability of mortar .

7 Whileproviding a high degree of cohesiveness itspreads easily under the trowel. Cohesivenessreduces the wastage produced by materialsliding off the trowel. The improved settingproperties of mortar containing lime allowadequate time for tooling up the joints aswork retentivityThis characteristic becomes important wheremortar is to be used with bricks, or masonryunits that have a moderate or high suctionrate. The inclusion of lime in mortar generallyleads to improved water retentivity. Thisprovides an improved bond, as there is moreintimate contact between the unit and themortar. The retention of water in the mortarresults in the best conditions for the earlyhydration of the cement, thus reducingcracking and water penetration into thehardened mortar contentEntrained air provides improved workabilityof fresh mortar and improved freeze thawresistance of hardened mortar .

8 The inclusionof lime into mortar can assist in stabilising theentrained air strengthIt has long been recognised that excessivelystrong mortars can lead to reduced bondand cracking, which may result in crackingbeing induced in the bricks and blocksinstead of the mortar joints. The use of limein a cement based mortar however, tends toreduce the compressive and flexural strengthof the hardened mortar . In a situation wherestructural movement takes place, limemortars can better accommodate thismovement, whereas excessively strongmortars will tend to resist movement perhapsuntil some cracking occurs.

9 For example, limebased mortars have always been used for tallfactory chimneys as they can accommodatea considerable movement in high healingWhen lime based mortars crack they tend todo so in the form of a much reduced numberof micro cracks. Subsequent movement ofrainwater through the surface of the mortarjoints dissolves the free lime, which isdeposited in the micro cracks as the waterevaporates. The lime subsequently reactswith the carbon dioxide in the air and isconverted to calcium carbonate, acementitious reaction. In a short period oftime the cracks are healed, a process knownas autogenous tightness The inclusion of lime in a mortar promotesmore intimate contact between the mortarand the masonry units.

10 For example, theincrease in plasticity and cohesion results in amore effective filling of the vertical joints andresults in a bond which subsequently resistspenetration by wind driven rain better thansome non-lime mortars. Furthermore,reduced moisture contents in walls resultingfrom their greater impermeability increasesthe thermal insulation of the structure as wellas reducing internal damp reduced water penetration achievedwith lime-based mortars can minimise therisk of freeze thaw damage. The inclusion oflime will also help to resist some forms ofsulfate attack. Together, combinedsulfate/freeze thaw attack can be verydamaging to masonry building materials contain soluble salts,which can be transported to the externalsurface by the migration of water throughthe structure.


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