Sheets for cladding

External and internal cladding as well as subfloors can be done with mineral sheeting. There are three main types of mineral sheeting for these applications: cement-based, calcium silicate-based and gypsum-based. In addition to the binder, they often contain fibrous reinforcement. The mixtures can also include polymer emulsions to suppress efflorescence and pigments to give colour. When the sheeting is used as internal cladding the joints must be filled. The filling material is, almost without exception, based on plastic binders, mainly polyvinyl-acetate or acrylate glues. Joints can also be covered by timber strips or the products can be tongued and grooved.

The energy needs and polluting emissions associated with production are large for the cement-based sheets, moderate for the gypsum products and low for those of calcium silicate. This is largely proportional to the calcium content.

In the finished buildings there are few problems with these materials, although asbestos may be found in older products. The use of plastic-based mastics and jointing between sheets for internal use could result in emissions of monomers and other organic substances that can reduce the quality of the indoor climate.

Robust sheets can often be re-used, especially if fillers are not used in the joints. However, by taping the joints with an easy removable, broad tape, before filling the sheets can be kept well preserved even then. Pure plasterboard will often be too weak to be re-used, but the material can be recycled as a 5-15% constituent in new material. The gypsum industry is, however, very centralized, which makes it economically unviable to recycle the products. Calcium silicate products can be crushed and recycled as aggregate in concrete. If finely ground, they can be used in mortars and plasters.

The waste is inert and can be used as fill, as can most cement products. If there are high levels of organic substances in the products, when they become waste they may increase the amount of nutrients seeping into the ground water. Sulphur pollution can develop from waste gypsum through decomposition by microbes; this can be reduced by adding lime.

Cement-based sheets for cladding are very similar to roofing sheets. They are strong and can be used externally without rendering as they withstand severe climate conditions. As reinforcement cellulose fibres and polyvinyl alcohol (PVA) fibres are much used, but also chippings from spruce or birch give good results, especially for interior use and in subflooring. The amount of wood chippings is then usually about 25% by weight. The cell content of the wood should be washed out by boiling or leaving the chippings in fresh air for a year. As a final treatment the chippings can have substances added which accelerate the setting of the concrete. Waterglass, calcium chloride or magnesium chloride can be used. The wood is left to lie in a 3 to 5% solution for a while and is then mixed with Portland cement and water, after which the sheets are formed in a hydraulic press for seven to eight hours, then set in a special curing chamber.

Calcium silicate sheets are used as both external and internal cladding. The sheets have very good fireproofing properties and offer slightly better thermal insulation than other mineral sheets. They also have good humidity characteristics. Calcium silicate sheets are produced by reacting up to 92% by weight of quartz with lime and with a little cellulose fibre as reinforcement. Vermiculite can be used as aggregate.

Plasterboard was first produced about 100 years ago. The sheeting is the most commonly used of all internal wall claddings, either painted or covered by wallpaper or thin fibreglass woven sheeting for painting. Gypsum products also have an important role as climatic products, for example as acoustic ceilings and wind barriers. Some products for wind-proofing have shown a tendency towards mould growth (Lis0 et al., 2007). For the same reason, it may be suspect to use plasterboard in bathrooms.

The standard products are manufactured from 95% gypsum with fibreglass reinforcement (about 0.1% by weight). Industrial gypsum, a by-product from coal-fired power stations, is commonly used in portions of up to 80% of the total. The following substances are also added to a total of about 1% by weight: calcium lignosulphate, ammonium sulphate and an organic retardant, usually trinatriumcitrate. The sheets are covered with thin cardboard glued with potato flour paste or polyvinyl-acetate adhesive. Pure gypsum sheeting is not particularly strong, but with more reinforcement added (e.g. wood shavings and recycled cellulose materials), the strength will increase. There are also products with granulated cork added, and even recycled tyre fibres (Eires etal., 2007).

15.2.3 Plaster

Many cementitious plasters are available, depending upon the surface to be rendered and the actual climate conditions. The alternative binders are lime, waterglass, cement, gypsum and sulphur, as well as mixtures of these substances. Additives can make the plaster bind better or improve elasticity or thermal insulation; they include steel fibres, mineral fibres, perlite, hacked straw, or even hair from cows, pigs and horses. In polymer modified plasters polyvinyl-acetate is added, up to 5% by weight. Colour pigments can be added; these should be fine grained and resistant to alkalis; usually metallic oxides are used. For external rendering or rendering in rooms such as bathrooms, water-repelling, hydrophobic substances are most often added, such as silicone products. Sand is also added, its grain size depending upon the surface quality required and how many layers of plaster are to be used. The final ingredient is water.

Rendering is labour-intensive work, but the result has a long lifespan. Well-applied lime plaster can last from 40 to 60 years, if it is not exposed to aggressive air pollution. Many organic substances added to increase waterproofing and make application easier have a detrimental effect on the durability of the rendering.

Products based on a large proportion of Portland cement will imply considerable emissions of the greenhouse gas carbon dioxide during production of the cement. This is reduced in the case of lime-based renders and products with a larger pozzolana content.

Pure plasters produce no problems within a building. Lime and gypsum-based products have good moisture regulating properties. Pure lime plaster can be recycled, in theory, by being re-fired, but this is mostly impracticable in reality. Bricks and concrete blocks rendered with cement plasters are difficult to cleanse for re-use. Lime and cement-based plasters can be classified as inert, so their waste products can be used as fill. Pure lime plaster can be ground up and used to improve the soil. Dumping sulphur and gypsum waste can lead to sulphur pollution, which can be reduced by adding lime.

Limeplasterconsists of slaked lime, sand and water. The proportion of lime to sand is 1:3 by volume. The plaster is put on in several layers until it is about 1.5 cm thick. It is most suitable for internal use, such as in bathrooms, but can also be used externally. For exterior use it should be protected against driving rain and continuous damp, otherwise it may be destroyed by frost because of its high porosity.

Was this article helpful?

0 0

Post a comment