What Is Fat Lime?

Mature Lime putty (fat lime).

Lime is a highly versatile building material that has been used for thousands of years. Over the years, the terminology used to describe the different limes has expanded. This can make it confusing to understand, especially if you are new to lime building materials. So, what is fat lime?

Fat lime is a synonym for lime putty, slaked lime and hydrated lime. This lime is a thick wet white material that looks like clotted cream. Fat lime can be used to make mortars, plasters and renders for all building projects. The building materials made using fat lime (lime putty) are highly breathable and flexible, perfect for restoration work and eco-builds.

How Is Fat Lime Made?

Fat lime, lime putty, slaked lime and hydrated lime are all different names for the chemical compound calcium hydroxide, which is made from quicklime. Quicklime is the resulting powder from kiln-burning calcium carbonate-containing rocks such as limestone and chalk.

A chemical reaction occurs when water is added to quicklime in a process called slaking. While slaking, the chemical reaction causes the lime and water mixture to heat up tremendously. The lime will boil and spit hot lime; in some cases, it can even produce a small explosion. Slaking lime is not for the faint-hearted, and protective gear must always be worn.

After slaking, the lime is known as calcium hydroxide, otherwise known as lime putty or fat lime. Fat limes are usually matured. This greatly improves workability and plasticity characteristics. The lime putty used to make mortars should be matured for around one month and plasters for around three months.

What Are Fat Lime Mortars, Plasters And Renders?

When mixing mortar, plaster or render, a binder is needed. The binder is the part of a mix that is added to help the material set, harden and adhere to other surfaces. In today’s construction industry, the most commonly used binder is cement. However, various limes have been used as binders for thousands of years! Fat lime is a type of binder.

Fat lime is a thick white material that has already been slaked at an earlier time. It usually will have matured for at least a month. To make a fat lime mortar or plaster, various amounts of fat lime are mixed with aggregates like a well-graded sharp washed sand.

Fat lime and fat lime mortar, plaster and render differ from other building materials. They will not begin to set if they are kept in airtight containers. Because of this, fat lime putty and pre-mixed fat lime mortars and plasters can be stored and matured almost indefinitely.

Pre-mixed fat lime mortars, usually just called pre-mixed lime mortar, are usually mixed and stored in plastic 25kg bags or tubs. They are extremely convenient as they are ready mixed with all the water, lime and aggregates needed. They must only be ‘knocked up’ (have a quick mix) before use.

The Difference Between Fat Lime And Hydraulic Lime?

Fat lime and hydraulic limes are the two main types of lime used for construction. There are numerous differences between fat lime and hydraulic lime, the main one being how they cure. Let’s dive deeper into these differences.

Fat Lime

Fat lime is made from a relatively pure form of quicklime, with little to no clay impurities within. As a result, fat lime mortar, plaster or render set by absorbing carbon dioxide (CO2) out of the air in a process called carbonation. Carbonation takes a long time; for example, lime plasters take around a month to cure for every inch of thickness!

Fat lime mortar, plaster and render are the softest, most flexible, most breathable and most environmentally friendly mortars. Because of this, they are perfect for making repairs to buildings with old and soft masonry. In addition, the flexibility and breathability makes fat lime mortars ideal for eco-homes and timber-framed buildings, which require flexibility that cement mortars cannot provide.

Related article: Does Lime Mortar Absorb Water?

Hydraulic Lime

Hydraulic limes, on the other hand, are made from quicklimes which have higher amounts of clay. Weaker hydraulic limes (NHL 2) have less clay, whilst stronger ones (NHL 5) have more. This is because, unlike fat limes, they react with water and set in a process called hydrolysis. Hydraulic limes are sold as a powder in a bag.

hydraulic lime in paper bags
Natural hydraulic lime 3.5 bags.

Hydrolysis is the same way cement cures and produces a harder set than the carbonation of fat limes. Hydraulic limes are used in exposed areas where fat limes aren’t strong enough or where it’s too wet for fat limes to cure properly.

Generally, the more hydraulic the lime, the harder the set. While many think the harder the mortar, the better, this isn’t the case. Mortar and plaster should always be weaker than masonry. In addition to the set being harder, more hydraulic limes have less flexibility and less breathability. These are important factors to consider when repairing historic or older buildings.

Related article: What Are The 3 Main Types of Lime Used in Construction?

The Confusing Difference Between Fat Lime And Hydrated Lime (in some cases)

Most builders merchants in the UK sell hydrated lime powder (sometimes called ‘builders lime’) in bags. In this sense, hydrated lime is a quicklime with a controlled amount of water added so that it remains a powder and doesn’t turn into fat lime or putty.

Hydrated lime (not to be confused with hydraulic lime) sold as a powder is an inferior type of lime and should never be used to make mortar, plaster or render by itself. These hydrated limes should only be used as an additive to cement mortars to help improve workability and plasticity.

Whilst you can mix hydrated lime with water to make fat lime, it will produce a far inferior putty. Usually, mortars made using only hydrated lime will fail or gain little to no strength. Hydrated lime should never be added to cement and then used as a ‘lime mortar’; this could cause significant damage to a building, especially if it’s older or historical. In these cases, a true fat lime mortar should be used.