Hydrated Lime vs Hydraulic Lime: What Are The Differences?

limestone and quicklime

When it comes to construction, lime is a versatile material that has been used for centuries. It is a key ingredient in mortar, plaster, and concrete. However, different types of lime are available, and it is important to understand the differences between them to ensure that the right type is used for a specific application.

Two types of lime that are commonly used in construction are hydrated lime and hydraulic lime. While they may sound similar, they have different properties and uses. Hydrated lime is produced by adding water to quicklime in a process called slaking. On the other hand, hydraulic lime is produced by burning limestone that contains clay, making it able to set underwater. Generally, hydraulic lime produces a harder set than hydrated lime.

Hydrated lime and hydraulic lime have different strengths and properties, and they are used for different purposes. Understanding their differences is important for selecting the right type of lime for a specific application. In this article, we will explore the differences between hydrated lime and hydraulic lime to help you make an informed decision.

What is Hydrated Lime?

Hydrated lime is a type of lime that is produced by adding water to quicklime. Quicklime is made by heating limestone in a kiln. When water is added to quicklime, it undergoes a chemical reaction that produces hydrated lime.

Quicklime powder.

Hydrated lime is also known as calcium hydroxide. Hydrated lime can either be a white powder or white putty-like material. It can be used in various applications, including construction, agriculture, and chemical manufacturing. It is commonly used as a building material, particularly as a mortar for bricklaying and plastering.

Hydrated lime has several advantages over other types of lime. It is easy to handle and store as a wet putty or dry powder. It is also less caustic than quicklime, making it safer to handle. Additionally, it has a longer shelf life than quicklime, as it does not react with air as quickly.

Related article: Quicklime vs Fat Lime: The Differences and Their Uses

Despite its advantages, hydrated lime does have some limitations. It is unsuitable for underwater applications, as it does not set well in damp conditions. It also has a lower compressive strength than hydraulic lime, making it less suitable for use in high-rise structures.

Builders Lime

Not all hydrated lime is created equal. Powdered hydrated lime is often considered inferior to fat lime or lime putty. Powdered hydrated lime is manufactured by adding a controlled amount of water to quicklime so that it fully reacts but remains a powder.

This powdered form of lime (sometimes called ‘builders lime’) is far more likely to cause failures if used to create a lime mortar or plaster on its own. Therefore, powdered hydrated lime should only ever be added to help increase the plasticity of cement mortars and plasters.

If you need to make a proper lime mortar, use quicklime (which requires slaking), lime putty (also known as fat lime) or hydraulic lime.

What is Hydraulic Lime?

Hydraulic lime is a type of lime that is produced by burning limestone that contains clay, silica, or other minerals. The resulting lime contains a small amount of hydraulic materials that allow it to set and harden when mixed with water. This process is called hydraulic setting, meaning the lime sets and hardens through a chemical reaction with water rather than just absorbing carbon dioxide (CO2) like non-hydraulic lime.

Hydraulic lime is typically classified into three categories: NHL 2, NHL 3.5, and NHL 5. These classifications refer to the strength of the lime, with NHL 2 being the weakest and NHL 5 being the strongest. NHL 3.5 is the most commonly used type of hydraulic lime and is suitable for most construction applications.

Hydraulic lime is often used to restore historic buildings and monuments due to its compatibility with traditional building materials and techniques. It can also be used as a binder for mortars and plasters in new construction projects.

What is Non-Hydraulic Lime?

Non-hydraulic lime is a type of lime that sets only by absorbing CO2 from the air. It is also called pure lime, lime putty, or fat lime. Non-hydraulic lime is sold as hydrated lime powder, quicklime or lime putty. The thick white liquid can be mixed with various sands and additives to form non-hydraulic mortars and plasters. These types of building materials harden by absorbing carbon dioxide from the air.

Related article: Fat Lime in Construction: Uses, Advantages, and Limitations

Mature Lime putty (fat lime).

Non-hydraulic lime is the softest and most breathable lime available. This means that it has a very slow set, and CO2 is only absorbed when certain conditions are met. Because of this, lime mortars and plasters made with non-hydraulic lime are the most suitable for conservation projects.

Non-hydraulic lime is typically used for conservation work, such as plastering, rendering, and pointing, due to its soft and breathable nature. Non-hydraulic-based mortars and plasters are the most compatible with traditional building materials and techniques. Non-hydraulic lime is usually sold as a wet putty. This is because slaking quick lime can be dangerous if you aren’t experienced.

When using non-hydraulic lime, it is important to ensure the conditions are right for it to set properly. The lime should be protected from rain and frost during the curing process, and the temperature should be above freezing. Additionally, this type of lime should be protected from hot conditions by using sheeting such as hessian. 

Related article: Using Lime Mortar in Winter: What You Need to Know

Key Differences between Hydrated Lime and Hydraulic Lime


Hydrated lime is produced by adding water to quicklime, which is made by heating limestone. The water causes a chemical reaction, resulting in either a dry powder or a lime putty that is more stable and easier to handle than quicklime.

In contrast, hydraulic lime is produced by heating limestone that contains impurities, such as clay or silica. This results in a lime that sets and hardens when exposed to water, making it ideal for areas in harsh conditions where non-hydraulic limes would not set.

Setting Time

Hydrated lime typically sets more slowly than hydraulic lime, which can set underwater. The setting time of hydraulic lime can vary depending on the type of lime and the environmental conditions, but it generally sets more quickly than hydrated lime.


Hydraulic lime is generally stronger than hydrated lime, making it more suitable for use in applications that require greater durability and resistance to water. This is because hydraulic lime sets through a chemical reaction with water, resulting in a stronger bond between the lime and other materials.

Hydrated lime, on the other hand, is generally softer and weaker than hydraulic lime. But this makes it more breathable and flexible, making it perfect for eco-builds and conservation projects.


Hydrated lime is commonly used in a variety of applications, such as soil stabilization, water treatment, and the production of plaster and mortar. Generally, hydrated limes are used on conversation projects, eco-builds or just old houses.

Hydraulic limes are used for new builds, some conservation projects and in areas exposed to harsh weather conditions. Strong hydraulic lime, NHL 5 is the hardest and fastest setting type of lime.


It is important to choose the right type of lime for the job. If you are working on a project that requires a material that can set underwater or in extreme conditions, hydraulic lime is the way to go. However, hydrated lime may be a better option if you are working on a project that does not require these properties.

Overall, it is important to understand the differences between hydrated lime and hydraulic lime and choose the right one for your specific project. By doing so, you can ensure that your project is successful and stands the test of time.