Lime Mortar VS Cement Mortar

Lime Mortar VS Cement Mortar

In my previous two blogs we have looked at Lime and the different types of lime mortar and how these can be made and what strength they can have. So if we take this subject to the next stage we have a number of questions which are often asked:

·         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? Does it really matter if I use Lime mortar?

All of the answers to the above answers are interlinked, which is why I have bundled them up into one blog.  So, cement, what is it and why is it so bad for traditional buildings which have been built with a lime mortar?

Cement has its origins similar to lime, there are historically many types of cement, it could be considered that Ordinary Portland Cement (OPC) is the best known. The difference between cement and lime is that the calcined product taken from the cement kiln has virtually no free lime, this is due to the high temperatures which the raw limestone, with over 25% of contaminates of clay or other additives, is fired to over 1400 degrees C, and this high temperature creates a complex combination calcium silicates and aluminates.

The problem with modern cement, is it is very good at what it does, it sets fast, is very strong and is impervious to moisture once set and it is very easy for unskilled labour to mix up in most temperatures and weather conditions.  These are all the reasons why it superseded the use of lime mortar, for the builder it saved time and ultimately saved money, there did not seem to be any drawbacks. As was mentioned in the last blog, for the typical building site the mixing gang would slake and mix the mortar up two weeks prior to the masonry starting, this would allow the lime for mature and good to work with, once laid it would need care and attention while it set and then would not reach its full potential until 12+ months. Some refer to cement mortar, or any mortar as ‘muck’ this symbolises the simplicity of mixing cement mortar, the mortar mixer of the two millennia previous to the 20^th Century would have been skilled at their craft and would have a huge understanding of the issues which would need to be overcome to produce the right mortar for the task for which is was intended.

Just after the mid 20^th Century there was a realisation that cement mortar may not be the wonder product that everyone thought it was, in the 19^th and very early 20^th Century cement mortar varied and could be rather feeble, comparable now to what today would be classed as a NHL5. As manufacturing process were refined and ‘improved’ during the 20^th Century the cement produced was stronger and stronger and more impervious to moisture. As can seen in the graph below, the research carried has shown this development.

 

Chart showing the development of the strength of cement of the centuries. Source Building Limes Forum volume 22 (Their source: The evolution of Portland cement Paul Livesey).

The best way to explain the impact of a cement mortar on a historic masonry structure is through the sketch below.

Sketch showing the effect rain has on masonry with lime and cement mortar.

It doesn’t take long to see the effects of cement as one walks around.

Brickwork showing signs of damage due to the use of cement mortar

With the development of hard cements has come the development of harder bricks and therefore these bricks are able to cope with the strength and lack of porosity of the cement. The real issues come when the cement is used with a brick or stone which is soft and will not be able to cope with the change in moisture and salt levels which the introduction of cement will create in the structure. Therefore the importance of understanding the type and strength of the mortar and the masonry before planning any repair or maintenance work cannot be over emphasised.

The main rule to follow is that the mortar should always be softer than the masonry which it is being used with, as it is the role of the mortar to be sacrificial to protect the masonry.