The facts about lime mortar.

As a Conservation Architect working on many types and ages of building, a common subject which I am questioned about is Lime Mortar. The most common questions are:

·         What is lime mortar? The facts about lime mortar.

·         What’s the difference between cement and lime mortar?

·         Why use lime mortar?

·         When should I use lime mortar?

·         How to tell the difference between lime mortar and cement based mortar?

·         Can’t I just use cement mortar? Does it really matter if I use Lime?

So, what is lime mortar? What are the main facts about it?

The story of lime could be described as a result of people requiring a binder for a mixture which could be used in building applications, in some regions and periods of time this was mud, but for many this was a lime binder. It would have (as it still does), involve the burning of calcium carbonate in the form of limestone, shells, chalk or coral rocks, any of these can be fired in a basic kiln at around 900 degrees C. This makes the calcium carbonate undergo a chemical change as the heat drives off carbon dioxide and moisture, turning the lime stone into calcium oxide, commonly called quicklime or lump-lime. From this point you have a versatile binder which can be used in the different forms.

Mortar is commonly made by slaking the quicklime, this can be done by sand slaking it, or by slaking the lime on its own to create a lime putty which can be stored and used for plastering, lime wash or mortar.  Lime does not like being rushed, in 19^th Century books the recommendation was for the mortar mixers to go on site and mix the mortar two weeks prior to work starting to allow the lime to mature and be ready for the masons when they arrived on site.

 

A photo of a piece of late 19^th century brickwork in Kent, see the white lumps, this can often be a sign of the mortar being sand slaked and not sieved very well, leaving under-slaked or over-slaked lime left as lumps.

 

See photo above of mortar sample taken from the core of a 13^th Century wall in Kent, see the same white lumps of lime.

It is difficult to generalise the subject of lime as there are many variants which dictate different results of the lime which is created. However in the interests of keeping to the point for those of you not familiar with traditional limes and do not know or understand them, here are 5 basic facts which should be remembered:

1.       The more pure/refined the lime stone(calcium carbonate) which is burnt the weaker the quicklime is created, this is referred to as non hydraulic.

2.       The more contaminated (with naturally occurring clays etc) lime stone(calcium carbonate)  the stronger, more hydraulic, the lime is created.

3.       The more contaminated the sands and aggregate with clays and other impurities the stronger (more hydraulic) the mortar become. It is possible to introduce contaminants into a weak lime mortar to make is stronger, these are called pozzalans, although these increase strength they often reduce the breathability and permeability of the mortar. In the late part of the 19^th century all the way through to the present day cement was added to lime mortars, the early cements were weaker than the cement of today. Today adding cement to a lime mortar can have hugely detrimental consequences, which can and does make the mortar fail if not done correctly, see the Smeaton Project which English Heritage undertook over a period ten years looking into this issue please see  https://www.uvm.edu/histpres/203/eh/RESEARCH1.HTML and http://www.buildingconservation.com/articles/cement/cement.htm . Many 19th century books talk about gauging a non hydraulic lime mortar with a hydrated hydraulic lime to provide the desired strength, this is was more successful.

4.       Most people are familiar with the NHL bags of lime which a more readily available from builders merchants. This is a hydrated lime in powdered form, the principles of its source are the same as mentioned at the start of this article with the limestone fired in a kiln. However the resulting quicklime is then pulverised and sent on a conveyor belt which is sprayed with water, this slakes the lime and allows any excess moisture to evaporate off in the heat generated by the slaking, this produces a dry powder at the end which can be bagged and used quickly and easily on site by adding water and sand etc.

Some people who do not understand lime can get confused with the NHL, on these bags, this does not stand for Non-Hydraulic Lime, it stands for Natural Hydraulic Lime, as these are derived from limestone with varying degrees of natural contaminants in them (the more natural contaminants the stronger, more hydraulic, the lime).

Current thinking is now suggesting that these processed hydrated limes are less breathable due to the lack of ‘free limes’ and the actual strength of the lime you have in each bag, NHL 2, 3.5, 5, will vary depending on the manufacture and the bed of rock from which it came in the quarry, the BS and EU standards are not sufficient to provide precise information on the grading of each bag.

This may sound a little negative, but it must be remembered that they are still vastly better and more sympathetic than a modern cement mortar. To put it in perspective, a strong hydrated lime mortar such as an NHL5 has a strength of 5, modern cement mortar would be in the 40-50 range of strength, maybe even stronger with some different types.

 

Photo of a wall re-pointed with an NHL lime mortar, the lumps of lime are not there and it has a more even appearance.

 

5.       The strength grading of a bag of lime is based on the BS and EU standards, this relates to the strength which will be achieved in 28 days. For cement this is fine as cement will reach its maximum set within this period of time. For lime it can take 12+ months to achieve it final set, therefore the strength stated does not relate to its final strength which will not be achieved in 28 days.

There are many sources for information on lime mortar, the best and most reliable of all these is the Building Limes Forum (https://www.buildinglimesforum.org.uk). I would urge you to join this forum, as they will teach you more in their journals about lime than you will learn anywhere else.

Thanks for reading

James