Nonmetallic Mineral Surface Materials

Basic info is found in Chapter 6.

Cementitious substances can be used to produce materials for all surfaces, either cast in situ as plasters or as prefabricated components, including units for cladding, underlay for floors and other basic elements.

The first concrete roof tile was made in Bavaria in 1844. Since the 1920s, concrete roof tiles have been in strong competition with clay tiles. Whether they can be as beautiful as clay tiles has always been a matter of debate. As early as the beginning of the twentieth century the Norwegian engineer Bugge advised: 'Don't spend much time putting concrete tiles on dwellings because their form is usually unattractive, and their colours, in particular, are most ungraceful' (Bugge, 1918). Colouring has improved somewhat since then, to the extent that it can be difficult to tell the difference between concrete and clay tiles. The concrete tile has taken on both the colour and form of the clay tile, but the difference is more apparent when ageing; the clay tile is usually still considered as having a more dignified ageing process.

In situ cast floors have a long history. They have been found in 7000-year-old ruins in the Middle East. The mixes were of pure lime; today they are cement-based or made of concrete slabs.

Mineral sheets consist of lime, cement or gypsum-based substances with other constituents added, such as reinforcing fibres. They are used widely as roofing sheets, exterior and interior claddings, as well as for wind bracing and subfloors. These products also have good acoustic and fire-resistant properties. Many of them are useful as interior claddings due to their humidity regulating properties. Fairly thick sheets will contribute usefully to the thermal mass and stabilize interior temperatures.

Plasters provide finished surfaces that often do not need further treatment. This is especially the case with lime plasters, which can be given a matt or polished finish. The treatment of walls with plasters also dates back thousands of years. As well as its function as a surface treatment, plaster can also be considered a highly valuable climatic material, as it can provide wind-proofing and moisture control.

The most relevant non-metal minerals have rich reserves but the extraction often entails heavy defacing of the environment, which can lead to changed water table levels and damage to biotopes. Products based on Portland cement and lime also entail large emissions of the greenhouse gas carbon dioxide during the calcination process. Part of this is later reabsorbed into the material through the building's lifetime, especially in the case of pure lime products. Pollution can be caused by a variety of additives to these products, both during production and if they are dumped after demolition.

Surface products based on non-mineral materials usually present no problems in the indoor climate. Some of the additives can entail a risk of unhealthy dust and fumes. If steel reinforcement is used, the electromagnetic fields in a building can increase. Many products can be reused if they are easy to dismantle. They are usually inert and can, if nothing else, be used as fill material. An exception is gypsum products that can degrade to polluting sulphur compounds.

15.2.1 Roofing

There are two main types of cementitious roofing: tiles and corrugated sheeting. Certain amounts of fibre must be added to both to give them the required tensile strength. Polymer emulsions or a coating of acrylic paint are often added to suppress efflorescence, provide colour and help seal sanded and textured surface finishes. More than any other concrete product, roofing needs particular care given to the proportions of the ingredients and the design of the sheeting or tile. One important aspect is that the finished product must have very low moisture absorption.

Concrete tiles (see later Figure 15.3) are usually made with about 25% Portland cement content. Other hydraulic cements can also be used. Sheets consist of up to 45% cement, and 5 to 10% silicate powder is often added. The rest is mostly sand, fibre and water. The fibre mixture can be natural fibres from hemp, sisal, steel, jute, reed, goat hair and cellulose, synthetic organic fibres from polyvinyl alcohol, polyacrylonitrile, polypropylene and polyester, or mineral fibres based on silica dust, steel, carbon, asbestos or fibreglass. Organic fibres are more easily decomposed. Research has, however, shown that even if this happens most of the strength is conserved (Parry, 1984). The reason for this is partly that the fibres play their most important role during the setting process; it is during this period that the dangers of damage through shrinkage are greatest. Roof sheeting was originally produced mainly with asbestos fibre, but this is now generally banned and has been replaced by other fibres mostly from cellulose and polyvinyl alcohol for health reasons, in a proportion of 2% by weight.

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