Lime Mortar

Lime Mortars

Lime mortar is made up of lime & aggregate and is used in the restoration conservation of old buildings. Lime mortars date back thousands of years, there’s evidence that lime was used to build the pyramids at Giza. Portland cement was introduced in the 19 century and is commonly used in new construction, it is not recommended in masonry restoration works.

There are different types of lime mortar but when we talk traditional lime mortars we are talking about lime being the binder and the only other component to be added is the aggregate and sometimes a reactive silica, when cement or other additives are used it is no longer classed as a traditional lime mortar and then becomes a modern cement mortar.

The different types of limes and mortars.

  • Lime putty - also known as an air lime or fat lime and is produced by adding water to quicklime (slaking).
  • NHL - natural hydraulic lime
  • Quicklime - produced by the burning of limestone or chalk
  • Hydrated - produced when minimal water is added to quicklime
  • Hydraulic lime - sets by hydration and ranges from feebly hydraulic to eminently hydraulic, can have natural or added impurities.
  • None hydraulic lime - known as an air lime or fat lime that sets through carbonation, in which there’s a reaction between the calcium hydroxide and carbon dioxide which then forms to calcium carbonate
  • Hot lime - this would have been used in traditional construction back in the day as the binder, it was easier to transport and tests show it also performed better. It is made up of quicklime aggregate and clean water

Older buildings in Melbourne were built using traditional lime mortars and when it comes to restoration work the first rule is ‘like for like’ this means whatever is taken out needs to be put back, traditional mortars need to flex move breathe, and that’s the beauty of the lime.

  • The Lime Cycle

    Lime has been used for thousands of years as the binder in mortars in all kinds of construction, and when it comes to lime mortar it’s all about the lime cycle.

    Lime starts off as limestone (calcium carbonate – CaCO3) then heat is added to around 900c to convert limestone into quicklime (calcium oxide – CaO) at the early stage the water is vaporised into a gas and as it heats up the C02 is driven out to become quicklime.

    Next water is introduced (slaking) and this is when it creates its own heat (exothermic reaction) and turns to lime putty. In time the lime putty (calcium hydroxide – Ca(OH)2) turns into a lovely creamy texture, with age it matures and the better it becomes.

    Now we introduce the aggregate by knocking up to a workable consistency, once worked it’s all about the curing (carbonation) this is another very important part of the procedure and also often overlooked.

    Once cured is when it has converted back to limestone from which it originated, The Lime Cycle

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