Lime has been used as a binder for stones and brick, and as a plaster or render, for thousands of years. The knowledge of its properties and how to use it has only been lost to current practice in the UK in the last 100 years; other European countries still use lime extensively within construction. Almost all buildings constructed before 1900 will have been built using lime, which is still the vast majority of our housing stock, and yet there is now a huge ignorance about lime and its properties.

University degree courses do not teach the use of lime, and new graduates are unaware of the properties, uses and benefits of the material. This leads to major problems in construction, as architects and managers specify the use of cement, a modern material whose properties and failings over the long term are only just being recognised. Problems of damp and durability associated with the use of cement may not become apparent for 50 years or more from the time of build. English Heritage and Historic Scotland have banned the use of cement on all historic buildings because it encourages damp and can actually destroy buildings that have stood for hundreds of years.

Cement companies are claiming that cement has been around for hundreds of years, implying that Portland cement has been used throughout this time - this is simply not true. Cement is a word we use to mean a binder for aggregate, and in this sense can be clay, lime, or Portland cement. Clay has been used for thousands of years, lime has been used for nearly as long, and hydraulic lime, which is often called Roman cement, has also been around since at least the time of the Romans. The properties of lime are significantly different from those of modern cement, which we know as Portland cement. All buildings constructed before the 20th century will almost certainly have been built using lime, because cement was only invented in 1824, by Joseph Aspdin, and did not begin to be used extensively for another 100 years.

Lime mortars and plasters are:

1. Permeable. This means that vapour can pass through them at an almost imperceptible level, which is a healthier option for inhabited buildings as it regulates humidity.
   
2. Flexible. Stone or brick laid with lime can move as the earth moves through changing seasons, without cracking the structure or causing instability. There is no need for expansion joints.
   
3. Soft. Plasters and mortars should not be harder/stronger than the backing surface to which they are applied.
   
4. Weatherproof. Not waterproof, thus protecting the building without sealing it.
   
5. Not susceptible to frost. They do not freeze as they are not ‘wet’, therefore do not require foundations to be below the frost line.
   
6. Do not attract moisture. They are not a ‘wet’ material, and so don’t need to be covered with a waterproof barrier in order to protect other materials around them.
   
7. Deal with moisture effectively. They can hold excess moisture from the atmosphere in humid conditions e.g. in a shower, without becoming ‘wet’ and then release it slowly back as humidity drops.
   
8. Proven over centuries. The Romans used lime very effectively for many applications including major engineering projects such as bridges, domes, suspended floors and heated floor slabs. The earliest known use of lime is 4000 years ago.
   
9. Reduces green house gas effect. Over its lifetime, due to the cycle of lime changing from limestone to quicklime and back to limestone again, most of the CO2 released during the manufacturing process is re-absorbed during the lifetime of the plaster, thus being close to carbon neutral.

On the other hand, cement is

1. Not permeable. Creates a sealed surface that does not allow vapour passage.
   
2. Rigid. Requires expansion joints to allow for natural earth movement without cracking.
   
3. Hard. A great property in the right place but often cement is too strong for the materials it is used with.
   
4. Waterproof. Completely seals mortar joints or walls.
   
5. Susceptible to frost. Will crack in very cold conditions and therefore if used in foundations, needs to be in deep trenches that make contact with the warmth of the earth to avoid problems associated with frost heave.
   
6. Attracts moisture. Other materials around cement need to be protected from it as it holds water and can cause rot to develop.
   
7. Does not deal with moisture. Can cause condensation problems in bathrooms, kitchens, bedrooms etc. as it does not regulate moisture.
   
8. Does not have a long history. It was invented in the mid 19th century and began to be used extensively from 1930 onwards. We are seeing some major damp and durability problems now, caused by the inappropriate use of cement from 50 or so years ago e.g. collapse of cob walls re-plastered with cement, the need to re-inforce some motorway bridges, excessive cracking in town houses leading to difficulty in re-selling.
   
9. Causes green house gas effect. The manufacture of cement is one of the major causes of the green house gas effect globally as it releases tonnes of carbon dioxide into the atmosphere, none of which is re-absorbed by cement plaster.

From the above list of properties it becomes clear why lime would be used in buildings in preference to cement. In terms of sustainability, lime allows us to use low-impact foundations, that do not intrude into the earth too much because we do not need to dig 450mm or deeper trenches simply to protect cement from frost. The vast majority of older properties in the UK have very little foundation, often being simply larger stones at the base of the wall, 100 – 150mm below ground level, laid with a hydraulic lime mortar. These buildings have stood for hundreds of years, and have moved with the movement of the earth over time. Often older buildings have changed shape, but without becoming dangerous, and we usually think these features are a bonus. Rigid buildings, on the other hand, will crack if subjected to earth movement, and this can be dangerous; cracking in buildings did not become a problem until about the 1930s.

Building breathable buildings is a healthier option than building sealed buildings, and causes fewer damp problems. Often the waterproofness of cement, coupled with its rigidity and hardness, can cause damp and erosion problems. In a traditional stone or brick wall laid with lime mortar, the wall works as a weatherproof surface because the stone keeps the rain out, and the lime absorbs water whilst it is raining, and then releases it when it stops. Any moisture that enters the wall either through the mortar or from inside moving outwards (eg when you have a shower) will leave the wall once the cause of the moisture stops. If you replace the lime with cement, you are relying on the cement to make a permanent tight join with the stones or brick to keep water out. In practice this doesn’t happen because of the rigidity and hardness of the cement, which causes tiny cracks to develop as the wall moves, that allow moisture into the wall. This trickles down inside the wall and then cannot escape because the cement is not permeable, thus creating a damp problem inside the base of the wall. Also, water collects at the join between the stone/brick and cement and begins to erode the stone/brick, because the stone/brick is softer than the cement. Many old houses that have been re-pointed with cement show signs of erosion of the stone many years later and suffer from damp problems.

Hydraulic Lime can be made in two ways:

1. Fat lime or putty lime, made from quicklime, with a pozzolan added to it, to make the lime hydraulic by artificial means. The Romans used pumice or other volcanic rock but ground up brick dust can also be used.
   
2. Naturally occurring hydraulic lime, that is produced from limestone that is high in silica (clay) and aluminium, is slaked and used straight away.

Portland cement is produced from a slurried mixture of limestone and clay, burned at a high temperature in a kiln and the resulting clinker is then ground up, with ground calcium sulphate added to regulate its set. It is highly processed partly to ensure no variation in product. It is nothing like a hydraulic lime. It is called Portland cement because it was thought to set as hard as Portland stone, and to look like it.

The essential feature of a lime mortar or plaster is that it needs to carbonate in order to cure. A transformation takes place within it as calcium hydroxide changes to calcium carbonate, a process that requires moisture, as carbon dioxide in the atmosphere has to be carried into the lime plaster in solution. A hydraulic lime has the extra feature of also having a chemical set, so that it can feel hard to the touch within a few hours to a couple of days, depending on the strength of the hydraulic lime. It can remain workable for several hours or days, but it still needs to carbonate after it has set. Lime strengths vary greatly depending on the type of lime used, but it takes about 28 days to reach full strength.

Cement does not carbonate. It hardens because of a chemical reaction with water, usually setting within half an hour to an hour, after which time it cannot be re-worked. It takes about 28 days to reach its full strength, and is generally very much stronger than lime.

Essential Reading:

Building with Lime: A Practical Introduction by Stafford Holmes and Michael Wingate.
ITDG Publishing ISBN 1 85339 547 1

Lime In Building: A Practical Guide by Jane Schofield Black Dog Press jane@blackdog.ukf.net
ISBN 0 9524341 2 1