Why the best mortars use lime!

Background

The use of lime in construction is as old as civilization itself, and there are many examples of buildings that have stood for millennia in testament to the qualities of lime. Lime putty and Natural Hydraulic Limes are often recommended for the restoration of older buildings, although modern construction techniques prefer the strength properties conferred by Portland cement.

Lime in modern construction

Lime is no longer a local material burnt in a rudimentary kiln and slaked in a tin bath. Modern lime is a high purity industrial chemical produced in sophisticated, fuel efficient kilns to demanding standards, such as EN459 for use in the construction industry. As an inclusion in a mortar mix, together with cement and aggregates, lime confers some excellent benefits.

Benefits of Lime

The many benefits of using lime in mortars are discussed at length in numerous documents, including MIA data sheets, BLA technical information and publications by the BRE. They are summarised as follows;

• Workability
• Water retention
• Reduced mortar strength
• Flexibility
• Vapour permeability
• Freeze-thaw resistance
• Autogenous healing
• Reduce efflorescence

So why don’t all mortars use lime?

Mortar producers can add other materials to achieve some of these benefits, although no additive will confer all of the benefits that can be achieved by the simple addition of this time-honoured material. These additives can have unwelcome side effects, but under pressure to reduce costs, lime will sometimes be removed from the mix unless it is specified. We recommend that you specify lime to EN459 in your mortar to ensure that you get the best available mortar for modern construction.