Into the limelight
With demands for more flexible buildings and energy efficient materials, it's no wonder lime is experiencing a resurgence
Lime is the principal binder of most traditional mortars, plasters and renders. The Romans introduced the 'technology' to Britain and it was used extensively in mortars and surface finishes from then until the 19th century, when patent cements, such as Portland cement, were introduced.
Although lime was mainly used in pre-1900 construction, and it is still seen primarily as a method of maintaining and repairing traditional buildings, an increasing number of professionals are discovering that modern buildings can benefit greatly from its use.
The use of lime decreased throughout most of the 20th century, but in the past 20 years it has been recognised that hard, cement-rich mortars are unsuitable for use on old buildings, and lime is enjoying a strong revival in the building conservation industry.
Much of the skill and knowledge needed to use lime successfully had dwindled, so when lime began to be specified more widely for conservation work there was a lack of suitably experienced and skilled craftsmen. But the practical techniques required for the use of lime can be mastered by anyone with good building skills or a craft aptitude, and the adoption of certain basic principles.
Lime has many different applications. Lime mixed with sand to form lime mortar can be used for bedding masonry and for pointing, rendering and plastering (often with animal hair added). For the very fine joints in ashlar masonry or gauged brickwork, pure lime putty can be used for bedding, pointing and repairs. Lime can also be mixed with carefully chosen aggregates (sands and stone dusts) to make repair mortars for damaged stonework. It has valuable applications for specialist stone cleaning and conservation techniques. Furthermore, lime putty can be diluted with water to make a lime wash for painting both internal and external walls, with pigments being added to produce a coloured lime wash.
Traditional building construction is based on the use of relatively soft and porous materials such as stone, brick, timber and earth, together with a lime-based mortar for bedding and plastering. Usually the buildings have solid walls, with no cavity, and are often built on insubstantial foundations.
They are therefore liable to settlement and movement associated with seasonal changes in ground conditions.
Lime mortar is softer and weaker than the stone or brick to which it bonds and so is able to accommodate slight movements caused by settlement or temperature changes without significant cracking. It is permeable and allows evaporation of rising and penetrating damp from within the wall. It is this permeability, or 'breathability', that helps to keep many old buildings dry inside without a dampproof course or chemical treatments.
Cement has very different properties to lime. Cement pointing is very hard and brittle, much less porous and sometimes completely waterproof. Its use on traditional masonry - especially if used with scant regard for the movement, moisture and material conditions pertaining - will have a detrimental effect in one of several ways. Cement pointing is harder than soft brick or stone and is too rigid to accommodate settlement or movement in the wall. When movement occurs, the edges of the stone or brick are forced against the hard mortar, spalling and cracking the masonry.
Further damage is caused by rainwater and dampness within the masonry. Because the mortar is not permeable, this moisture cannot evaporate from the mortar joint once rain stops; instead, it is forced to evaporate through the face of the brick or stone. Soluble salts present in the water crystallise in the surface layers of the masonry, leading to crumbling and decay. This effect is sometimes so severe that the entire face of the stone is lost and the hard cement pointing is left standing proud. To compound the damage, rainwater becomes trapped, the decay continues and the concentration of trapped water in the masonry increases its susceptibility to frost damage in winter.
However, because it is more porous than the masonry, soft lime mortar allows moisture movement, encouraging evaporation and salt deposition in the mortar joints. It is the mortar that decays, not the stone or brick. It is much easier and cheaper to repoint a wall than to repair or replace damaged brick or stone, with reduced risk to important historic fabric.
Using cement render causes other problems. Hairline shrinkage cracks inevitably form in the surface of the render as it sets, or afterwards by slight movement in the wall. Rainwater is drawn by capillary action into these cracks, which diffuses into the wall.
Once inside the wall, this moisture, together with any rising damp and everyday water vapour, is trapped because it cannot evaporate through the hard, impermeable render. The moisture levels start to build up in the wall and show on its inner surface, resulting in internal dampness and damage to plaster and decorations.
Strange as it may seem, applying a waterproof render can actually increase levels of damp inside a house.
Conversely, a porous lime render encourages evaporation of moisture from its surface, helping to minimise the effects of moisture within the wall and keeping surfaces dry.
About 150 years ago, Portland cement was considered a specialist material; no one thought it would catch on in mainstream building because it required grinding during its manufacturing process. But advances in industrial processes and the demand for faster building, particularly after the Second World War, changed the market conditions and cement took off. Portland cement is a fantastic material for mass concrete and engineering structures, but the past 50 years have shown it is not always the best option for mortars, plasters and renders. This has been particularly noticeable where it has been used on historic buildings.
More than 3,000 million bricks are fired annually in the UK, consuming large quantities of fuel and adding significantly to CO2 emissions. Buildings constructed with lime mortars can be altered easily and bricks/stones reused. Indeed, the masonry can be reclaimed entirely if a building has completed its useful life. This is why architectural salvage yards have second-hand bricks to sell. Bricks bound together with cement mortars, however, can generally never be recycled, except as hardcore. This is especially pertinent to modern commercial buildings, which may be demolished after only a few years. If we carry on using cement mortars we will have to continue making billions of highenergy bricks each year.
During the manufacturing process, the respective firing temperatures of cement and lime are significantly different. Lime is produced at a temperature of between 900-1,100degreesC.
Cement is produced at 1,200-1,500degreesC.
More energy is required to produce a tonne of cement than a tonne of hydraulic lime, thereby increasing CO2 emissions. Additionally, the bulk density of lime is half that of cement, leading to overall energy savings when using lime of approximately 30-50 per cent. Lime mortars will also re-absorb some CO2 during their setting process.
Since global cement production is responsible for 1,500 million tonnes of CO2 production), the environmental argument for lime in new building is a compelling one.
We are now at a crossroads because we need to build millions of homes over the coming decades but environmental issues are becoming more important. Some people think lime will never take off again in mainstream building, but small, subtle changes in the market conditions could change the future. The combination of lime with modern technology and higher demand could bring down costs. The introduction of a carbon tax or legislation setting targets for recycling of buildings could make cement impractical and lime the natural choice.
TYPES OF LIME
Lime is made by quarrying limestone and heating it in a kiln. The purity of the limestone and the burning temperature will affect the qualities of the finished product. Impure limestone (containing silica and alumina) produces hydraulic limes. These set harder and faster than pure limes. Limes can also set in wet (even underwater) conditions. These types of limes are suitable for modern construction because they set relatively quickly and are strong enough to cope with the stresses of cavity construction.
Ian Pritchett is managing director of Lime Technology, tel 0118 9696949, email ian@ijp. co. uk