Fat lime is a brilliant and versatile building material. Not only is it easy to use, but it creates extremely plastic and workable mortars, plasters and renders. So what are the common uses of fat lime?
Fat lime, also known as lime putty, slaked lime and calcium lime, is a thick, creamy-looking material. Fat lime is the binder. When mixed with various aggregates such as sands, ash and stone dusts, fat limes can create various types of lime mortar, render and plaster. Fat lime creates the softest, most breathable and most flexible lime building materials. It’s especially suited to restoration projects and new eco-builds.
Now we have a brief overview of what fat lime is; let’s take a closer look at how its made and the various uses of fat lime.
What is Fat Lime and How is it Made?
Fat limes are a thick white creamy material and are made from quicklime. Quicklime is a powder produced from burning calcium carbonate-containing rocks in a kiln. Once burnt, the resulting quicklime is highly reactive with water.
To make fat lime, quicklime is mixed with water and allowed to react in a process called slaking. Slaking creates a great amount of heat. This reaction can be very dangerous, so it’s usually advised beginners purchase the ready-slaked fat lime.
After it’s slaked, the fat lime is usually matured. It should be matured for at least one month for making lime mortars. And for plaster or render, it should be matured for at least three months. It’s common to find fat lime that has matured for over a year. Generally speaking, fat lime will improve with age.
Related article: Quicklime vs Fat Lime: The Differences and Their Uses
Fat limes are matured to reduce the possibility of drowned particles within the mixture. Drowned particles are particles of lime which did not properly react with water in the slaking process. Several factors, such as poor mixing, insufficient quantity of water or poor-quality lime, can cause this. Maturing the fat lime will, over time, reduce the number of these particles as they get saturated with water.
Mortars, plasters and renders made from fat lime set via carbonation. This is a slow process where the materials absorb carbon dioxide (CO2) from the air. This way of curing is different and much slower than hydrolysis, which is how hydraulic limes and cement cure.
Historical Use of Fat Lime
Fat lime mortar, plaster and render are among the oldest and longest-used building materials. To begin with, mud and clay-based mortars, plasters and renders were used, but after lime was discovered, they were largely replaced.
Lime mortars were used extensively by the Indus valley civilisation in around 2600 BCE. The ancient Egyptians also made good use of lime to help construct and plaster pyramids, temples and other important buildings.
Later, the Romans used lime mortars mixed with volcanic ash to help create early hydraulic lime mortars. These lime mortars have sometimes been referred to as limecrete, as they were much harder than normal lime. They set via a reaction with water in addition to the usual carbonation.
Many of these extremely old lime mortars, plasters and renders are still in good condition today, a testament to their long-term durability and performance.
The Uses of Fat Lime in Construction Today
Modern portland cement alternatives have largely replaced fat lime. Cement and gypsum plasters have mostly replaced limes for their ease of use, convenience and fast set times. Overall, this makes cement cheaper to use when compared to lime.
Related article: Can Lime be Used Instead of Cement?
However, fat limes still have their place. Today, they are mainly used for restoration projects. Old buildings originally constructed with lime materials should always be repaired using similar lime-based materials.
After WW2, many historic buildings began being repaired with cement. Years later, people noticed the detrimental effects the cement was having. Because cement is so hard, brittle and waterproof, it contributed to the quick decline of the masonry and structure of these old buildings.
After noticing the damage cement was causing to these older buildings, lime was used again to repair and restore them properly. This is the most common use of fat lime mortar, plaster and render. These materials allow the building to breathe. In turn, this leads to far less trapped water, which can cause serious damage to buildings.
In addition to being used for restoration. Using lime-based building materials, whether fat lime or hydraulic lime, is becoming increasingly popular for eco-builds. This is because lime is flexible and breathable – qualities which are very important when working with natural materials that eco-homes are built with, like timber, wood, straw and mud.
Advantages of Using Fat Lime in Construction
There are many advantages to using fat lime for building projects. Some of which we have already briefly looked at. Fat limes’ main advantages are their breathability, flexibility, durability, versatility and sustainability. Let’s take a quick look at each of these advantages.
Fat limes, made via slaking quicklime, will produce the most breathable mortar, render and plaster of all the lime types. Breathability allows fat limes to control the moisture content within the fabric of a building.
Stable humidity greatly contributes to overall comfort. This not only protects the structure but also helps to maintain stable humidity inside the building as well. It also helps to avoid issues with condensation, mould and dampness.
Because non-hydraulic fat lime mortars make the softest mortar and plaster, it allows them to flex and subtly move with the building. Hard mortars often become more brittle the harder they are. This means small building movements (especially common in old and historic buildings) can cause cracking. Over time, the mortar or plaster will crumble.
Softer, fat-lime building materials can move with the building. Instead of large cracks forming, thousands of ‘micro cracks’ form. Amazingly, free lime within the mortar or plaster will fill these cracks over time. This self-healing is one of the reasons lime building materials have such fantastic longevity.
Durability and Longevity
As we have already touched on in this article. Lime has been used as a building binder for thousands of years. Limes have various benefits (as discussed), which naturally lead to higher durability, and their mild self-healing means they can last for incredibly long periods.
There are still many examples of Egyptian, Roman, Greek and Indian lime plasters and mortars that are still in great condition thousands of years after they were used for construction. This is a testament to the durability of lime building materials.
Lime is a very versatile building material. There is essentially an endless amount of mortars, plasters and renders that can be created—all with different colours, textures, hardness, breathability and flexibility characteristics.
This means you can tailor the lime mortar, plaster, or render to suit your specific project exactly. Also, old mortars and plasters can be matched, so you cannot tell the difference between them.
Whatever your project is, there will be an additive, aggregate, stone dust and lime to suit. This is why lime is probably our planet’s most versatile building material.
Unlike cement, lime is a great eco-friendly building material. Non-hydraulic fat lime sets via carbonation, a reaction where carbon dioxide is absorbed for the material to set. This can largely offset the emissions caused by manufacturing the material. Because of this, limes are often referred to as “carbon neutral”.
In addition to absorbing CO2 from the air, fat limes can be easily recycled. It isn’t uncommon for old lime to be scraped off masonry and mixed with fresh lime and aggregates to create a new mortar. Doing this is almost impossible for cement as it’s far too hard, meaning you are usually unable to remove the mortar without damaging the masonry.
So this means that not only can the old mortar be recycled, but also the original masonry. This reduces the need for new masonry and also maintains the originality of the building itself.
Limitations of Fat Lime in Construction
Although fat limes have many fantastic properties, they also have some drawbacks. The main one is their slow set times. They set via carbonation (the absorption of carbon dioxide) which is a much slower process than cement or hydraulic lime (which react with water).
Instead of taking a couple of days to cure, like cement, fat limes can take weeks and even longer in poor conditions. This slow set time generally means a slower pace of construction. This isn’t feasible today as it would lead to a much higher labour cost. The potentially high labour cost is the main barrier stopping people from using lime products.
Additionally, lime requires more curing time, so it requires more highly skilled tradespeople to use it properly. Again, this can lead to increased labour costs. As well as the increased labour cost, usually, lime materials are more expensive than cement alternatives because they are more specialised.
Lime is a brilliant building material. Not only does it create a beautiful finish, but it also protects the building and the environment itself. Limes are a truly sustainable building solution. It would be great to see lime building making a comeback, with more and more eco-buildings being constructed as people become more conscious of sustainability.
If you can look past the added labour cost, you will be rewarded with a far superior (in my opinion) finish over what can be achieved with conventional building materials.