Understanding the complexities of building materials is fundamental to successful construction and restoration projects. One pair of materials that are often misunderstood or used interchangeably are hydraulic and non-hydraulic lime. Despite their shared origin from limestone, these two types of lime possess distinct properties and uses that make them suitable for different construction applications. In this blog post, we will delve deep into the fascinating world of limes; the binding materials used in construction and restoration for centuries!

We’ll explore the differences between hydraulic and non-hydraulic lime, how they’re produced, their distinct setting processes, their strengths and flexibility, and the ideal situations to use each.

What is hydraulic lime?

Hydraulic lime is a type of binder used in building construction. It is produced from limestone that contains impurities such as silica, alumina, and iron oxides. The proportion of these impurities determines the hydraulicity, or setting characteristics, of the lime. Hydraulic lime sets by a chemical reaction with water, a process known as hydration. This allows it to harden even in damp or underwater conditions.

The hydraulicity of the lime is typically categorised as feebly hydraulic, moderately hydraulic, or eminently hydraulic. The differences in these categories correspond to the proportion of impurities in the lime. Feebly hydraulic lime has the lowest amount and eminently hydraulic lime has the highest.

Hydraulic lime offers several benefits in construction, particularly its faster initial set and higher compressive strength compared to non-hydraulic lime. It also has a lower porosity due to the water involved in its reaction, which can be beneficial in certain construction applications. However, its greater strength also comes with less flexibility and can be less breathable than non-hydraulic lime.

What is non-hydraulic lime?

Non-hydraulic lime, often referred to as air lime or fat lime, is derived almost purely from limestone, with very few impurities. Unlike hydraulic lime, non-hydraulic lime sets by a process of carbonation, reacting with carbon dioxide in the air to harden. This process can take several weeks and requires the lime to be exposed to the air during this time.

One of the key characteristics of non-hydraulic lime is its flexibility. It is softer and more flexible than hydraulic lime, which allows it to accommodate movement within a building over time. This makes it particularly suitable for use in older or historical buildings, which may be subject to more movement and shifting than newer, more rigid structures.

Non-hydraulic lime is also more breathable than hydraulic lime. This means it can allow moisture to evaporate more easily, which can help to prevent damage from trapped moisture in a building. However, it also has lower compressive strength and can take longer to set, which can be drawbacks in some construction applications.

Key differences

Setting Process

Hydraulic lime undergoes a chemical process known as a hydraulic set. It reacts with water to create strong bonds that harden the material. This is different from the process undergone by non-hydraulic lime, which requires carbonation to set. Carbonation is a reaction with carbon dioxide in the air, which binds the lime together and hardens it. The fundamental distinction between these setting processes lies in their differing relationships with water. Whereas water initiates the hardening process for hydraulic lime, it must be substantially evaporated from non-hydraulic lime to allow carbon dioxide from the air to react and cause setting.

Speed of Set

The setting process is much faster in hydraulic lime due to the chemical reaction with water. This also enables it to be set underwater or in very damp conditions. Conversely, non-hydraulic lime has a slower setting process because it requires enough time for carbon dioxide to penetrate the material and react with it. This process can take weeks and requires exposure to the air. The drying out and then slow carbonation process means that non-hydraulic lime typically cures slower than its hydraulic counterpart.

Strength and Flexibility

Hydraulic lime, once set, tends to be stronger and less flexible. This is a result of the strong chemical bonds formed during the reaction with water. On the other hand, non-hydraulic lime, while less strong, offers greater flexibility. It can better accommodate movement and shifting in structures over time due to the less rigid bonds formed during carbonation. This property makes non-hydraulic lime particularly suitable for use in older, historic buildings. They often require materials that can handle movement without cracking or damage.

Porosity

Non-hydraulic lime is more porous, allowing for higher breathability within the masonry. This can be a significant advantage in managing moisture within structures, as it allows for the evaporation of water and helps prevent issues like dampness and mould growth. Hydraulic lime, in contrast, is less porous due to the water involved in its reaction, offering more resistance to water and frost. However, this lower porosity can also mean that it doesn’t manage moisture as effectively as non-hydraulic lime, potentially leading to trapped dampness in certain conditions.

Uses of hydraulic and non-hydraulic lime

Hydraulic lime is often used in conditions where water and frost resistance are crucial, and in instances where a faster set and higher compressive strength are required.

1. External plastering and rendering: Given its water resistance and strength, hydraulic lime is often used for external plastering and rendering of buildings, especially in regions with damp climates or harsh weather conditions.
2. Underground and marine structures: Because hydraulic lime can be set underwater, it’s particularly suited for constructing structures that are below the water level, such as basements, foundations, or marine structures.
3. Structural repairs: The higher compressive strength of hydraulic lime makes it a suitable choice for structural repairs in older buildings.
4. Historic restoration: Although non-hydraulic lime is typically preferred, hydraulic lime can be used in certain situations where additional strength and water resistance are required, for instance, on external surfaces or damp locations.

Non-hydraulic lime, on the other hand, is used in situations where breathability and flexibility are required.

1. Historic building restoration: Non-hydraulic lime is often used in the restoration of historic buildings. It’s flexible and breathable, which means it can accommodate building movement and allows moisture to escape. This reduces the risk of trapped dampness and resultant damage.
2. Internal plastering: The breathability of non-hydraulic lime makes it ideal for internal plastering. It aids in maintaining a healthy indoor climate by regulating humidity levels.
3. Lime wash: Non-hydraulic lime is often used to create lime wash, a traditional type of paint. It gives a breathable coating and is often applied to walls in historic buildings.
4. Mortars for soft masonry: Non-hydraulic lime is used in mortars for soft masonry work where the mortar needs to be weaker than the brick or stone. This way, any movement in the building will be taken up by the mortar rather than damaging the masonry units.