How Lime for Stonemasonry Building Work Is Made

Here at StonemasonsGlasgow.Com, we always use lime mortar on all the stonework we work with.

Despite their being lots of modern alternatives such as cement, the fact is there is no better mortar for stone than lime. Unlike cement, lime mortar let’s buildings and their stonework breathe, is kinder on the surrounding stonework and will ultimately last longer than any modern alternative. But where does it come from and how is it made? We take a look below.

What is lime?

Lime is made from calcium carbonate. It can be found in a variety of forms including limestone, chalk or seashells. In the British Isles, lime is almost always sourced from limestone or chalk.

How is lime mortar made?

Once quarried, the raw material needs to be converted into lime. Heating to extreme temperatures is required, between 800 C to 1000 C. When the raw material is heated at these temperatures, the limestone begins to breakdown and giving off carbon dioxide. This leaves calcium oxide which is what is commonly known as quicklime. As a substance though quicklime is unstable and can react very quickly when it comes into contact with water, forming calcium hydroxide.

This chemical process produces heat and is known as hydration.

The resulting calcium hydroxide can react again when exposed to the atmosphere by absorbing carbon dioxide and once again becoming calcium carbonate. This cyclical process is known as the Lime Cycle.

As stonemasons though, it is the calcium hydroxide stage of the Lime Cycle that we are most interested in. If the calcium hydroxide is hydrated with an excess of water, lime putty is produced which is then mixed with sand to produce various lime mortars. There are essentially two forms of lime mortar, hydraulic lime mortar and non-hydraulic lime mortar.

Hydraulic lime mortar

Hydraulic limes are set by a reaction with water called hydrolysis. The lime used is obtained from limestone that is less than 95% calcium hydroxide, containing silica and or alumina too. This type of lime mortar causes a faster and harder set which makes it particularly useful for exterior work especially where there is exposure to wet or damp conditions.

Non-hydraulic lime mortar

Non-hydraulic limes are putties that have been set by a process known as carbonation from limestone that is more than 95% calcium hydroxide. This, compared to hydraulic lime mortar is much slower set and the lime remains more breathable and softer.

These are particularly useful if to be used in the conservation of old buildings where maximum permeability is required to ensure the stonework is not damaged and is conserved in the very best condition.

• Lime mortar has been made using these methods for centuries. Although today the process have been refined and modern technology has made the process quicker and more efficient, the fact is that the underlying process remains the same.

• Despite the proliferation of more modern mortars, for the conservation and repair of older stonework and buildings, there simply is nothing better to use than lime mortar.