Lime in mortars
Lime has been used as a binder in mortars for over 2000 years. Today, lime is still used as the primary binder in many mixes, usually in the form of lime putty or Hydraulic lime. Hydrated lime is used in modern cement based mortars mainly for its properties as a plasticiser.
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Properties of mortar
Pure lime mortars behave as if they are flexible and lime cement mortars are slower hardening and remain more flexible than cement sand mortars. Lime, therefore, enhances the ability of the brickwork to accommodate stresses caused by building movement and cyclical changes without excessive cracking.
Lime improves the plasticity and workability of mortar, while providing a high degree of cohesiveness it also spreads easily under the trowel.
Lime mortars have high water retention, creating an improved bond as there is more contact between unit and mortar. Retention of water in mortar improves carbonation in pure lime mortars and results in best conditions for early hydration of cement lime mortars – thus reducing cracking and water penetration into hardened mortar joints.
The use of lime in mortar reduces the compressive and flexural strength of the hardened mortar. In situations where structural movement takes place, lime mortars can better accommodate this movement.
Not only does lime mortar reduce the risk of water ingress, vapour permeability allows any moisture to evaporate, thus reducing the risk of freeze thaw deterioration.
Vapour permeability of mortar improves with increasing lime content. A high lime mortar can act as a ‘wick’, to allow water vapour to pass out from the building enabling the structure to effectively ‘breathe’.
Autogenous (Self) Healing
Should hairline cracks develop in the mortar, the combination of lime, moisture and carbon dioxide from the air can help to seal the crack by the formation of calcium carbonate (limestone). The crystals that are subsequently formed by this process help to plug the cracks.
Types and benefits of using lime in mortars
Air Lime gains strength slowly, by combining with atmospheric carbon dioxide to form calcium carbonate (as per the lime cycle). Air Lime or high calcium lime does not have any hydraulic component. It can be quicklime for slaking or hydrated lime. Several grades of Air Lime are identified in EN459-1 the European standard for Building Lime.
Hydrated lime is NOT hydraulic lime and will not set in contact with water; hydrated lime is added to cement mixes to give the benefits listed under ‘Benefits of using lime mortars’.
CL90 Q & CL90 S
Grades of air lime for building as described in EN459 the European standard for Building Lime. CL90 Q is the purest grade of building quicklime and CL90 S is the purest grade of hydrated lime for building. Several grades of air lime are identified in EN459 the European standard for Building Lime.
Lime with Hydraulic Properties
Lime with hydraulic or cementitious properties which will set when exposed to moisture. Several grades of Lime with Hydraulic Properties are identified in EN459-1 the European standard for Building Lime.
Natural Hydraulic Lime (NHL)
Natural Hydraulic lime which does not contain any performance enhancing additives. Its properties are as a result of the mineralogy of the calcium carbonate stone which is quarried for burning.
Lime with Hydraulic Properties based on NHL or Air Lime, which is a designer blend of constituents from a designated list. Formulated Lime may contain cement or clinker, pozzolana, ground granulated blast furnace slag or other performance enhancing additives. It is blended to give the required performance characteristics. Any additions are identified by the manufacturer.
Hydraulic binder which may contain many performance enhancing additives, including cement and clinker. There is no requirement for the manufacturer to notify the customer of its composition.
Materials for mortar
Mortars predominately consist of 3 – 4 components.
- Aggregate – this provides the bulk of the mortar.
- Binder – binds the aggregate together and the mortar to the masonry units (lime or cement).
- Water – bonds all the mortar elements together as a cohesive mass.
- Filler – materials which are added to a mortar to bulk up the mix and fill voids.
In most cases ‘soft’ building sands conforming to either Type S (BS 1199) or Type G (BS 1200).
Lime mortars – Pure lime mortars usually require a well graded sharp sand to perform adequately, a typical specification would be to BS882:1992.T.4 Grades ‘C’ & ‘M’.
Preparation of the mix
Mortar mixes are in general, proportioned by volume. Lime can be measured as putty or as dry hydrated/hydraulic lime, as the weights of calcium hydroxide per unit are in roughly equal. Mixing should either be done by hand on a clean surface or in a mechanical mixer.
A lime:sand mortar mix may be produced by either:
- mixing the specified proportions of lime putty with sand, or
- mixing the specified proportions of hydrated/hydraulic lime, sand and then water
The workability of lime:sand mortar is improved by allowing it to stand overnight before use, (alternatively it can also be purchased ready prepared). A major benefit of factory-mixed mortars is the high level of consistency that is maintained.
Building lime standards
- Limes for building purposes are specified in CEN Standard EN 459-1 – ‘Building Lime’.
Description Form Classificaton Calcium lime 90 Q,H,P CL 90 Calcium lime 80 Q,H,P CL 80 Calcium lime 70 Q,H,P CL 70 Dolomitic lime 85 H DL 85 Dolomitic lime 80 H DL 80 Hydraulic lime 2 H HL 2 Hydraulic lime 3.5 H HL 3.5 Hydraulic lime 5 H HL 5
Key: Q = quicklimes, H = hydrated limes, P = putties/milks of lime
- Designations of mortar mixes are specified in BS 5628 – ‘Code of practice for use of masonry’.
Mix designation, compressive strength and cement gauging
|Traditional mortar designation||BS EN 998-2 mortar class||Mortar by volume Cement:Lime:Sand||Factory produced by volume lime:sand||Site mixing Cement:factory produced|
|By Volume||Lime: sand by weight kg:tonne|
|Air entrained||Non Air entrained|
- More information can be found on lime in mortars from the Mortar Industry Association website: www.mortar.org.uk/whichmortar
- Download ‘The use of lime in mortar’ datasheet from the Mortar Industry Association website: www.mortar.org.uk/documents/miadata18.pdf
- Download ‘Factory produced lime:sand mortar for masonry’ datasheet from the Mortar Industry Association website: www.mortar.org.uk/documents/miadata01.pdf
- Download ‘Factory produced lime:sand mortar for paving’ datasheet from the Mortar Industry Association website:www.mortar.org.uk/documents/miadata06.pdf
- Download ‘The benefits of factory produced lime:sand mortar for masonry’ datasheet from the Mortar Industry Association website: www.mortar.org.uk/documents/miadata11.pdf