The exact definition of lime plaster has been clouded with misconceptions for some time now. People tend to misunderstand lime plaster for other types of plaster. So, before we move further let’s get introduced to the real lime plaster!
Introduction to Lime Plaster
It’s often thought that any mortar with lime as the main ingredient is lime plaster. However, that’s incorrect. Such a mixture is usually referred to as ‘stucco,’ ‘lime based stucco’ among other things. And a mixture of calcium hydroxide (lime) and sand or any other inert filler as aggregate is known as lime plaster. When the lime in the lime plaster reacts with carbon dioxide present in the atmosphere, the resulting calcium carbonate, also known as lime stone, is formed leading to solidification of the wet mix. Furthermore, if aggregates such as sand are added to ‘hydrated’ lime or ‘air lime,’ a perfect lime plaster is formed!
However, if cement is added to lime, the change is not limited to just the physical appearance. It alters the chemical as well as mechanical properties such as durability, elasticity, porosity, and colour. Many-a-time, contractors add lime to cement in an attempt to lessen the effect of these properties.
Lime plaster has many uses. The most common is for frescoes that are created using colour pigments diluted with water and applied on a wet plastered surface.Portland cement is not lime plaster, as is commonly misunderstood.
Advantages of Lime Plaster
Lime mortar has several important advantages over other types of mortars. In the current context, where in we are living in a world that’s facing significant threat due to rapid industrialization, use of lime plaster can really prove to be beneficial. Some of the main advantages of lime mortar are elaborated below:
- Carbon Dioxide Emissions: By using lime plaster, the world can save at least 80% of carbon dioxide emission when compared to regular stucco finishes. For ex. If an average sized house used lime plaster as wall and ceiling material, it will help save approximately 2,500 to 5,000 kgs of carbon dioxide emission. Switching to hydraulic lime will help reduce many hundreds of thousands of pounds of carbon dioxide from being released in our atmosphere, leading to a cleaner and healthy environment.
- Better Flexibility: Lime mortar has a good level of elasticity. Therefore, it is able to adjust and settle on walls and other surface rather easily as it doesn’t set quickly. On the other hands, cement based mortar is more rigid in nature and is unable to adjust quickly to changes around it.
The lime plaster used in historic buildings (and many modern structures) had compressive strength of approximately 400psi. However, natural hydraulic lime with a compressive strength of between 800 to 1600 psi was lower than Portland cement’s 3600 psi. Higher compressive strength leads to lesser flexibility. Since lime plaster has lower psi, it offers enhanced flexibility that is necessary a long life. Perhaps it is this quality that has ensured that many lime mortar structures have lasted for almost 10-20 decades!
- Increased Porosity: Since lime plaster doesn’t set completely in a short time period, it can adjust to any early movements in structure. As the plaster carbonates, any small cracks that emerge can be quickly resealed automatically by drawing water from the atmosphere. This ensures that you benefit from smooth surfaces for a long time to come.
- Autogenous Healing: Since NHL (natural hydraulic lime) is much more breathable than cement, it tends to shed water easily too. While cement mortar tends to crack under undue stress or movement after it sets allowing for water to pass through, lime mortar adjusts quickly to stress and other changes at the onset due to its slow carbonation setting process. Simply said, lime plaster can reabsorb carbon dioxide and reseal any cracks that may appear leading to self-healing. This characteristic of lime is known as ‘autogenous healing.’
However, because of the slow setting characteristic of lime plaster, it tends to remain soft for a long period of time (read months). Besides this lime is a very caustic material when mixed with water and calls for use of safety measures such as protective gear at all times.
Uses of Lime Plaster
Since lime putty is highly plastic, other mitigating materials have to be added to it to improve its plasticity and workability. Today gypsum plaster has replaced lime plaster as the preferred choice for interior base coat. Lime mortar is almost always used to give a finishing coat to interior surfaces.