Why has CAT avoided using cement, and what are the alternatives?

At CAT, we have avoided using cement-based concrete, mortars and renders mainly as a way to reduce 'embodied energy' (the energy used to extract, manufacture, and transport materials). Less energy use means reduced carbon dioxide emissions from the construction of a building.

In some cases we have used unfired earth to create bricks or walls. We have also used lime-based render, mortar and concrete in a number of projects.

Less energy is needed to produce lime-based building materials, which means less fuel and lower carbon emissions. In addition, lime reabsorbs carbon dioxide from the atmosphere, which cement does not.

Also, lime is more readily reusable at the end of a building's life, as bricks and stones joined with lime based mortar or render will separate and clean up. It's very difficult to separate bricks joined with cement mortar - their only practical reuse is as hardcore. Lime will gradually convert back to it's raw form - calcium carbonate. This won't happen with cement. Old lime putty plasters and renders can even be removed, soaked, remixed and put back on the wall.

Where the greater strength of a cement-based material is needed, the impact can be reduced by using an alternative such as fly ash to replace a proportion of cement used.

Because lime is breathable, building materials including straw, hemp, earth, and clay can be easily used without fear of damage from trapped moisture. Using lime instead of cement in the mortar for a brick construction allows water entering the masonry to evaporate out.

Lime bonds well to earth because it is also a soft and porous material. Lime and earth also expand and contract at the same rate, so the lime finish will remain stable and well adhered. Cement is hard and brittle and doesn't bond well.

Lime can absorb and hold moisture in periods of high humidity, and then release tis moisture when the humidity levels drop. This helps to prevent condensation in interior spaces (the main cause of moulds and fungi).

One issue with using lime is that it takes a relatively long time to harden. This is because pure lime relies on a slow process of carbonation, which reabsorbs the CO2 which has been driven off in the firing. It is possible to improve this by using a hydraulic lime made from limestone with certain impurities which gives a quick chemical set similar to cement, followed by slow carbonation. This process will reabsorb about 60 per cent of the CO2 produced chemically during manufacture.

An advantage of lime setting more slowly then cement or gypsum, is that it can usually be re-worked up to 24 hours after application. it can also self-repair any small cracks that appear immediately after applying.

The properties of lime must be well understood - it cannot be used in exactly the same way as cement. Accurate batching is critical, as is minimal and carefully administered water content. The mixing time is longer than for cement (10-15 minutes), but overmixing can turn the whole thing to jelly! Finally, 'limecrete' must be protected from drying out too quickly - damp hessian is the favoured covering material, and it is essential to protect from frost for the first week or so.

Companies that renovate old buildings (e.e. stone & timber) should be used to working with lime, and you may be able to find a local company to give advice on a particular project. Otherwise you can find local eco-builders that may know about using lime by looking through the AECB listings.

The production of cement and lime

The main ingredients of cement - limestone, silica & alumina - are crushed and then burned at high temperatures (1500°C). Carbon dioxide (CO2) is emitted from fossil fuels used to fire the kilns and is also released by the chemical reaction that takes place inside the kiln. In the UK, the cement industry is the biggest CO2 producer after the electricity generating industry, and in some developing countries cement production can account for up to two-thirds of total energy use. Globally, cement production is responsible for 8-10% of all CO2 emissions. In addition, heavy metals are emitted during the firing and other associated atmospheric pollutants are sulphur dioxide (SO2), nitrogen dioxide (NO2) and dust. Pollution to water courses can occur, with the highly alkaline washout being toxic to fish.

The production of Lime-based products still has an environmental impact, but it is less. The manufacture of lime involves heating crushed limestone or chalk to high temperatures (800-900°C) to produce quicklime or calcium oxide. The quicklime, which is caustic and unstable, is then slaked or hydrated by adding water, to form calcium hydroxide or lime putty. In the production process, large amounts of CO2 are given off from a chemical reaction in the kiln, but unlike cement, pure lime mortars will absorb all of that CO2 as they dry out and harden over a number of years, converting back into calcium carbonate or limestone.

Contacts

AECB: the Sustainable Building Association - http://www.aecb.net - 0845 456 9773
  Network of builders, architects, manufacturers, and organisations. Aims to develop, share and promote best practice in environmentally sustainable building. There is a list of members on the website.

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