Non-Hydraulic Lime

Non-Hydraulic Lime

Also referred to as 'Fat' lime, is derived from limestone that has been heated to a high temperature in a kiln –a process known as calcination– and then slaked (hydrated) with water. This produces a smooth, creamy substance known as lime putty. Unlike hydraulic limes, which contain clay and set even underwater, fat lime putty requires exposure to air to set.

Calcination Temperature

To produce non-hydraulic lime, the limestone needs to be heated to a temperature between 900°C and 1,000°C* (about 1,650°F to 1,830°F) in the kiln. This process is called calcination, where the limestone (calcium carbonate, CaCO₃) breaks down into lime (calcium oxide, CaO) and carbon dioxide (CO₂):

CaCO3 (limestone) → CaO (lime) + CO2 (gas)

 * The optimal temperature of the kiln is key, harms the quality of the product.

- Too low temperature and the limestone may not fully convert into lime, resulting in a product that is under-calcinated, meaning the lime may not react effectively when slaked with water and produces poor-quality, reduced durability lime putty.

- Too high temperature, around 1,200°C causes over-calcination. Meaning the lime becomes too “reactive”. Overheated lime tends to be fine and “powdery”, leading to a weaker, less durable product. It’s also trickier to handle during the slaking process.

The best quality lime comes from carefully controlled temperature and the use of high-purity limestone.

It works by absorbing carbon dioxide (CO₂) from the atmosphere during the curing process, a phenomenon known as carbonation. As the lime reacts with CO₂, it hardens into a solid material. This slow setting process means that ‘fat’ lime putty does not set under water, making it unsuitable for underwater applications or places where moisture is constant. However, in dry or well-ventilated conditions, it can be very durable and is highly valued for its flexibility, breathability, and ability to self-heal small cracks over time.

This property of setting through exposure to air is what makes ‘fat’ lime ideal for use in building materials like plaster, mortar, and limewash, where both longevity and breathability are important. It allows buildings to "breathe," preventing moisture from becoming trapped inside walls, which can lead to decay and mould growth.

Non-hydraulic lime can exist in both powder and putty forms:

1. Quicklime (Calcium Oxide):

When non-hydraulic lime is first produced, it comes as quicklime (CaO), which is a dry, reactive powder.

2. Slaking to Make Lime Putty:

When quicklime is slaked (mixed with water), it reacts to form lime putty (calcium hydroxide, Ca(OH)₂). This putty is soft, malleable, and ideal for creating smooth plasters or mortars for non-exposed applications. Lime putty is often preferred for interior use due to its workability and smooth texture.

3. Dry Powdered Lime:

In some cases, non-hydraulic lime is used in its dry powder form—often mixed with sand or animal hair and water to make a mortar or plaster. The powder can still be used for construction, but it requires exposure to air to harden, unlike hydraulic lime, which sets more quickly in moisture.