What's the difference between cement mortar and lime mortar?

It is common for contractors to misunderstand the distinction between cementitious mortars (containing cement) and lime mortars. This can often lead to mis-pricing at tender stage and unexpected construction cost increases.

Note: This is aimed at mortars used for new build single leaf (a.k.a. single wythe masonry in North America). For historic or heritage use of lime mortars, different materials and methods often apply – please get in contact.

The use of cementitious mortars can be traced back to Roman times, but despite some use in the late 1900s, only became widespread during the 1950s. Before then lime mortars were much more generally used, but what is the difference?

Cementitious mortars are mixed from a combination of ordinary Portland cement (OPC), hydrated lime and sharp sand, typically at a 1:1:6 proportion. The cement provides the binder and the hydrated lime provides workability. The hydrated lime does not bind the materials, and should not be confused with hydraulic lime.

Lime mortars are a traditional product and are substantially different from cementitious mortars, mainly in that they do not contain cement. A lime mortar is typically a mixture of hydraulic lime and sand. Hydraulic lime is a binder and sets hard and binds the sand in much the same way as cement, and is completely different to the hydrated lime used in cementitious mortars which simply provides workability. They are often confused due to their similarity in name,, but their function is distance and a cementitious mortar containing hydrated lime cannot be considered to be a lime mortar.

Despite lime mortars being a traditional material, these days they are in fairly widespread use in new build masonry construction and are available from multiple suppliers. They are often selected because they offer significant advantages over cementitious mortars: Lime mortars potentially have a lower embodied carbon equivalent compared to cementitious mortars, and absorb C02 as they set. They are better able to accommodate movement and have greater permeability, allowing saturated brickwork to dry more quickly. They also have a much more attractive appearance which is important because mortars make a considerable difference to the overall appearance of brickwork.

It is often suggested that they allow for future reuse of bricks but in reality, modern lime mortars are equally hard to separate from bricks at the end of a building’s life. Lime mortars are significantly more expensive compared to cementitious mortars, have slower setting times and are more susceptible to freezing during cold temperature working.

?The proportion of hydraulic lime to sand in a lime mortar mix varies according to the strength of the lime itself, and varies between producers. For example, a natural hydraulic lime with 5 MPa crushing strength (defined as ‘NHL5’ in BS EN 459-1 2015) will be mixed with a far greater proportion of sand compared to say, a weaker 5 MPa lime to achieve the same overall mortar strength.

The mix proportion is often not prescribed in specifications, to allow contractors procurement flexibility, but the strength requirement at 91 days should be provided, which good lime mortar suppliers will be able to use to match to their own products.

The colour of lime mortars will be significantly influenced by the colour of the sand and requires important scrutiny, thus strict verification requirements should be provided in specifications. As with cementitious mortar, by far the most convenient way of ensuring consistency of strength and colour is to use ‘ready mix’ (factory produced) lime:sand pre mixed mortars. These are typically supplied in 25kg sacks, one tonne bulk bags or silos and are added to water on site.

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