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Technical

Lime in mortars and renders

Benefits of using lime in mortars and renders

Incorporating lime into mortars and renders benefits the fresh and hardened properties of the materials and associated construction.

Fresh properties:
  • Improves workability and water retention
  • Promotes economy in use
  • Enables good workmanship
  • Improves quality of bond to substrates
Hardened properties:
  • Reduces potential for water penetration
  • Improves ability to withstand normal movement (e.g. thermal expansion and contraction, creep) without excessive cracking or debonding
  • Improves frost resistance
  • Enhances general durability
  • Reduces potential for efflorescence (staining caused by salt deposits)

Introduction to lime in mortars and renders

Lime has been used in mortars and renders for thousands of years, both as the only binder material, and also in conjunction with other binders that were usually used with the aim of obtaining faster set and strength gain.

Mortars and renders made with cement as the only binder ingredient tend to be hard, impermeable and brittle, which may lead to issues with cracking, water penetration and poor durability. Scientific and industry institutions support the incorporation of lime into mortar and render mixes. See the 'downloads' section at the bottom of this page for more information.

Construction material design standards and codes continue to emphasise compressive strength characteristics, and therefore do not reflect the many benefits of incorporating lime, particularly in accommodating movement, and on performance and durability.

How does lime give benefits?

Workability and water retention

Incorporating an appropriate proportion of hydrated lime into a cement-based mixture improves plasticity and workability, making the product easier to handle on the trowel. Hydrated lime also increases water retention which helps to improve the contact and bond with the substrate.

Cracking and movement

Incorporating hydrated lime helps the construction withstand the minor movements that occur, for example, as a result of thermal expansion and contraction. These movements can cause a hard and brittle product to develop large cracks or to 'debond' from the substrate, often damaging the substrate as well. Incorporating hydrated lime into the mixture encourages crack formation to be in the form of gradual 'micro-cracking' within the material. These micro-cracks repair themselves naturally by a process of hydrated lime diffusing into the tiny fissures and then hardening by reacting with atmospheric carbon dioxide to form calcium carbonate (limestone). This process is known as 'autogenous healing'.

Moisture movement, frost resistance and durability

The improved quality of the bond, and absence of large cracks, which comes from incorporating hydrated lime helps to reduce the risk of water ingress. Mixtures containing appropriate proportions of hydrated lime also have a greater ability to transmit water vapour (vapour permeability) than cement-only mixtures. This helps moisture to dissipate, allowing the structure to 'breathe' and reducing the risk of frost damage due to saturation. All these factors contribute to improved durability.

Aesthetics

Largely due to the increased permeability of mortars and renders using hydrated lime, there is a reduced the risk of unsightly efflorescence, a powdery deposit of water-soluble salts that can appear on the surface of the building.

Lime products

There are three basic types of lime used in mortars and renders:

Hydrated lime

Hydrated lime is the type of lime most widely used as a component in mortars and renders.

Hydrated lime, and quicklime (see below) are commonly referred to as 'air lime' products. This is because they do not react with the water in the mix to form a 'set'. Instead, they react with carbon dioxide from the air in order to harden (carbonation). This is a gradual process and mixtures made with air lime products as the only binder may take days or even weeks to harden.

Hydraulic lime

Not to be confused with hydrated lime, hydraulic lime products react with water in the mix to form a 'set'. Hydraulic lime products can be either manufactured from naturally occurring rock (Natural Hydraulic Lime or NHL), or from a formulated mixture of hydrated lime and reactive binder components (Formulated Lime or FL).

Quicklime

Quicklime is used to produce 'hot lime mixes' and 'lime putty' which are extensively used in the renovation and conservation of historic buildings, and to a limited extent in new build construction. Please refer to the safe handling information below. The reaction of quicklime with water is exothermic (generates heat).

Mortars and mixtures

Factory produced mortars

Silo mortars are supplied to BS EN 998-2. They are classified by nominal compressive strength tested in accordance with BS EN 1015 Part 11:

  • M2 - that is a compressive strength of 2 N/mm2 - generally suitable for internal work or sheltered locations, lightweight blocks
  • M4 - that is a compressive strength of 4 N/mm2 - suitable for most general brick and masonry above ground
  • M6 – that is a compressive strength of 6 N/mm2 - often used for masonry below ground or in severely exposed locations

1. Silo mortar

Silo mortars are a complete system consisting of a portable silo containing the dry ingredients (sand, cement, hydrated lime, additives) with a built-in mixing system to blend the dry ingredients and combine with water from site supply. The mix proportions are pre-set by the supplier according to the grade requirements given by the customer.

2. Ready to use wet mortar

Ready to use wet mortars are factory produced fully mixed (cement-hydrated lime-sand-additives-water) mortars for immediate use, delivered to site in tubs. Additives are incorporated in the mix to retard the set and extend the workable period.

3. Lime-sand mortar

Lime-sand mortars are a factory produced mixture of damp sand and hydrated lime delivered to site in specified proportions, to which cement, additives and water are added in a mixer prior to use. The proportioning and mixing guide for site-made mortar (below) also applies to lime-sand mortar.

Site-made mortar

For site-made mortars, mix proportioning is normally by volume. For accuracy, use buckets or gauge boxes rather than shovels. Add some water to the mixer, then sand and hydrated lime. Mix for at least 5 minutes then add cement and finally water to adjust the workability.

A guide to volumetric proportions for mortars containing hydrated lime to correspond with the strength classifications of BS EN 998-2 is given in a National Annexe to the standard, as below:

Masonry type Cement : lime : sand mix proportions by volume Nominal strength class (N/mm2)

Internal masonry

1 : 2 : 8 to 9

2

External/general masonry

1 : 1 : 5 to 6

4

Below ground/high exposure

1 : 0.5 : 4 to 4.5

6


Historic buildings

Nearly all masonry up to the early 20th century was constructed using mortars where lime was the only binder in the mortar. Renders and plasters were also made in this way. Most masonry construction was of solid walls and the use of lime binders allowed the moisture to quite freely move within the structure. It is important that mortars used in repair and renovation of these buildings are sympathetic and compatible with the other materials in the structure, are able to accommodate minor movements and to allow water vapour to escape and not trap moisture within the structure.

In the recent past, cement-based mortars have often been used for repair and renovation work and the hard, brittle and impermeable nature of these materials has resulted in damage to historic buildings and structures. Damage is caused by issues including trapped moisture and additional stresses through the incompatibility of the unmatched repair materials.

It is now more widely appreciated that compatible mortars should be used in the repair and renovation of buildings. A thorough investigation of the building or structure is needed before repair and renovation materials are specified, and appropriate advice should be sought.

Safe handling

The Safe Handling of Lime document (available here) gives some general guidelines on the handling requirements for lime products.

However, please refer to the supplier's Safety Data Sheets for the complete safety information referring to an individual product being considered.

More information

Further advice on factory produced mortars and renders is available from the Mortar Industry Association.

Information on the use of lime-based products for historic buildings is available from a wide variety of sources including:

Downloads

The European Lime Association (EuLA) has produced a range of publications on the benefits of lime in mortars:

 
 
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