Lime has been used for many years both as a single binder material and as a material used in combination with other binders to achieve a stronger result and quicker set time. Cement only based renders and mortars create a finish that is impermeable, brittle and hard which can result in problems with cracking, poor durability and water penetration when used with brickwork and stonework. Lime has been advocated as an effective material that can be incorporated into mortar and render products because of its properties and benefits.
There are multiple benefits associated with the use of lime including an overall improvement in workability and flexibility enabling it to be applied using a trowel with ease. When you use hydraulic lime, this also improves contact and bond with the substrate due to its enhanced ability to retain and release water.
Another significant benefit associated with lime is that it can better withstand structural movement over time as the ground moves or in response to thermal contraction and expansion from season to season. When traditional mortars are used, this can cause large cracks as the mortar begins to separate and the bond with the substrate breaks down. This not only causes damage to the substrate but can result in crumbling and large cracks to form, reducing the watertight bond and increasing the risk of water ingress into the building. When hydraulic or putty based lime is used as an alternative, cracks still form but they are very faint and happen over time within the material. In addition, these cracks can naturally repair themselves. The hydraulic and putty limes diffuse and harden as they react with carbon dioxide from the atmosphere to form limestone. This is a process known as autogenous healing. As a result, you don’t get the large cracks in the brickwork and other masonry, ultimately retaining the integrity of the building for a longer period of time.
Frost and moisture
Brickwork mortars that are used without the addition of lime often present problems with water ingress in buildings. When you used a lime based building mortar mix water is better transmitted which allows moisture to dissipate. As a result, the structure of the building is more breathable which reduces the likelihood of frost damage and increase the longevity of the building.
If you have ever noticed a white powder forming on the exterior of a building, this is known as efflorescence which is a powdery combination of water soluble salts which will sit on the surface of the brick. When you use lime based mortars, this is significantly reduced, increasing the overall aesthetics of a building or wall.
Now that you have a good understanding of some of the benefits of lime, let’s take a look at some of the different types of lime that can be used in construction products.
Lime Benefits and Products
In the construction industry there are three types of lime which can be used in render and mortar products:
This is the most popular and widely used varieties of lime used as a key part of mortar and render. Hydraulic lime sets after water has been added. This makes it more suitable for exposed areas. Hydraulic lime comes in 3 strengths, NHL 2, NHL 3.5 and NHL 5. The higher numbers represent a stronger hydraulic lime more suited to exposed and wetter conditions.
Quicklime and Lime Putty
The second type of lime is used to make lime putty by a process called ‘slaking’, this is where water is added to quicklime resulting in a reaction, once mature this lime putty can be used to make mortars and renders. Putty or hot lime mixes are used frequently in conservation projects or renovation work on historical buildings, they can also be used on new buildings.
This is the third type of lime and is sometimes confused with hydraulic lime. Hydrated lime is most commonly added to cements to plasticize the mix, improving the workability.
A Silo Mortar
This type of mortar will include a portable silo featuring a selection of dry components such as additives, cement, sand and hydrated lime, along with a mixing system to combine the dry ingredients and water on site. The proportions of the mix are pre-determined by the supplier depending on the grade requirements specified by the project team applying the product.
A wet mortar can be ready to use, and they are manufactured in a factory. You don’t need to do any mixing all that has been taken care of for you and the mix will usually include lime putty (produced from quicklime), stonedusts and a well graded sand. These are delivered to your building site ready to be “knocked up” before use, they usually come in 25kg bags, tubs, half tonne and tonne dumpy bags.
Lime putty and Sand Mortar Matching
Lime putty and sand mortar matching is also undertaken in the factory. Usually this is used in restoration works and a sample or picture of the original mortar must be sent. Once a mortar has been matched correctly it can be produced in batches to the customers specifications. This is delivered to the site to be used on the project. For the mortar to be used on site it will need to be “knocked up” or lightly mixed before use.
Mortar created on site (hydraulic lime based)
There are also some lime based mortars that can be made on site and the proportions used will depend on the scale of the project. Buckets or measuring boxes should be used to measure out accurate quantities as opposed to shovels. Place the water into the mixer first followed by sand and then lastly the hydraulic lime. The mortar should be mixed for five minutes at least before you can add more water if needed to get the mix to the correct consistency.
There are some benefits of using lime based mortars:
- Improved application, workability and retention of water
- Enhances workmanship
- Strengthens the bond to the substrate
In addition, when the mortar becomes hardened it offers the following properties:
- Reduced likelihood of water to penetrate into the building
- Able to withstand building movement
- Enhanced resistance to frost and cold temperatures
- Increased durability for the long term
- Decreased likelihood of efflorescence
Lime benefits are excellent when you compare them with more modern cement based mortars and they can maintain the condition and durability of buildings over the long term.