Quicklime vs Fat Lime: The Differences and Their Uses


Lime has been used in construction for thousands of years worldwide. It has only been replaced by cement in the last 100 or so years. This makes lime and information on the restoration of lime incredibly important. A common question is; What is the difference between quicklime and fat lime?

Quicklime is the powdery material used to make various lime mortars, plasters or renders. When quicklime is mixed with water, it undergoes a reaction called slaking. Slaked lime is a thick white material which resembles very thick cream. This creamy material is known by a few different names, lime putty, fat lime, milk of lime and hydrated lime. Most commonly, it’s referred to as lime putty or fat lime.

What is Quicklime?

Calcium Oxide (CaO), also known as quicklime, burnt lime, lump lime, unslaked lime and pebble lime, is a powdery white material. It’s obtained by crushing and burning calcium carbonate-containing rocks, such as limestone and chalk.

This process is much more sustainable when compared to cement production. Cement uses some of the same ingredients, like lime, with other additives and is heated to a much higher temperature (1400°C compared to around 700°C). In addition, lime absorbs carbon dioxide as it cures, helping to reduce the overall carbon footprint.

Quicklime is very reactive with water. Once water is added to quicklime, it will begin bubbling and spitting hot lime. In some cases, especially for very fine quicklimes, explosions can occur.

This actually happened to a colleague of mine. While mixing a large volume of superfine Buxton quicklime, water was not added quickly enough as we had drained the water tank. The lime, probably around 200kg, exploded, causing the roughly 5-tonne mixer to leap around a foot off the floor.

Luckily nobody was harmed, apart from the mixer, of course. I’m not mentioning this to scare you off but to ensure slaking quicklime is respected. It can be extremely dangerous, especially if you belittle it.

Quicklime will produce a large amount of heat while slaking with water. Quicklime is not flammable. However, the heat produced by the reaction with water has been known to start fires. Quicklime should never be stored near flammable materials for this reason.

What is Fat Lime?

Calcium Hydroxide ( Ca(OH)2 ) is quicklime, which has been mixed (slaked) with water. Usually, calcium hydroxide is known as fat lime, lime putty, or slaked lime. Fat lime is a thick white creamy substance which can be mixed with sands and aggregates to make lime mortars, plaster or renders.

Generally, it’s advised that fat lime is matured before use. This reduces the amount of “drowned particles” within the lime. Drowned particles are lime particles which haven’t properly reacted with water. If used in a mortar or plaster, they can blow out, causing small craters, usually with a small white speck in the middle. This is the drowned particle that caused the blowout.

Mature Lime putty (fat lime).

For making mortars, it’s recommended that fat lime is matured for at least one month before application. For plasters, it’s recommended that the lime is matured for at least three months. Sometimes, it’s possible to purchase lime which has matured for well over a year. Generally, the longer it’s matured, the better the quality.

Additionally, it’s important to note that fat lime is technically a type of hydrated lime, but it differs from the powdered hydrated or “builders lime” lime sold in many builders’ merchants. Powdered hydrated lime (not to be confused with hydraulic lime) should not be used to make lime mortars as it is an inferior lime. Hydrated lime should only be added to cement mortars to help improve workability and plasticity.

Related article: What is Fat Lime?

Comparison of Quicklime and Fat Lime

For most people, it’s recommended that fat lime is used. It is a much safer material to work with when compared to quicklime, as it has already been slaked. You can also purchase very mature fat limes, which will increase workability and plasticity.

In contrast, quicklime degrades with age. This is because it can air slake, whereby the lime reacts with moisture in the air. It is a best practice to only purchase quicklime if you will be using it quickly, within a month or so. This ensures it’s still highly reactive and will make good fat lime.

In some cases, it’s beneficial to use hot lime mortars. This type of mortar is made by mixing in additives and sand while the fat lime is still hot, just after slaking. This results in a much stickier and more workable mix, which can help to reduce labour time.

Hot lime mortars can be mixed and used hot on-site or mixed hot and left to mature and cool down. Leaving the hot limes to mature will reduce the possibility of drowned particles, but you don’t get the benefits of using it while it’s still hot, which can make moisture management easier, and the material is more workable.

Related article: The Effects of Freezing on Lime Mortar and Plaster

mixing putty
Mixing fat lime before use.

Advantages and Disadvantages of Quicklime and Fat Lime

Quicklime is usually cheaper than fat lime. If you are comfortable with slaking, then you will be able to save some money. On the other hand, you have to ensure the quicklime has fully slaked and has no drowned particles. In addition, slaking can be a highly dangerous process, especially if you have little experience with lime.

Fat lime is usually more expensive, and purchasing more mature lime putties will likely set you back even more. However, you have the benefit of ensuring there are no drowned particles. Also, purchasing fat lime is far more convenient than buying quicklime and having to slake it.

Additionally, pre-mixed fat lime mortars and plasters can be purchased for even more convenience. These are ready mixed with the lime and various sands; you just need to give them a quick mix before use.

Finally, in some cases, there is an advantage to making and using hot lime mortars. Especially if they are used while they are still hot; however, they will have to be mixed on-site in small batches, which isn’t always the most cost-effective way of purchasing lime products.

Applications of Quicklime and Fat Lime

As discussed, both quicklime and fat lime have many applications within the building industry. With quicklime being used to make fat lime and hot lime mortar, plaster and render. But limes are used in many other industries as well.

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

Lime is extensively used in agriculture. Due to lime’s high alkalinity can be spread on fields to help reduce acidity and balance the pH to ensure the best growing conditions possible. Agricultural lime (also known as garden lime) does not require burning in a kiln; it is just crushed and milled calcium carbonate-containing rocks, such as limestone and chalk. It’s important to note quicklime and powdered hydrated lime can also be used to balance pH levels.

Finally, limes are used extensively in water treatment. Lime is used in purification plants to soften, reduce cloudiness, remove impurities, disinfect and neutralise drinking water. Surprisingly, most of the lime produced today is used to make water suitable for drinking!