Constitution

Nontraditional gypsum plasters are made with products premixed in factory, made mostly with fine gypsum, eventually slaked lime in powder, mineral aggregates, admixtures and sometimes additions (such as fibres and lightweight fillers). They come to the working site in powder bags to which only the mixing water recommended by the manufacturer must be added. Some of the products can be applied by spraying and others are applied manually (Figs. 3.17 and 3.18). According to BS 5492: 1990 (internal plastering), the application by spraying has advantages to coat big areas as long as planning ensures work continuity, since it increases the rate of application and reduces the labour needed.

As for synthetic plasters, also called synthetic binder thin coatings due to their thickness (1-3 mm), they are made with products marketed as ready-to-apply pastes, made essentially of synthetic resins in water dispersions (binder), which are mixed with fillers, normally of silica or calcite, various admixtures and, when necessary or intended, with pigments and extra water.

Premixed gypsum plasters are usually applied in two coats—smoothing coat (10-20 mm), applied generally by mechanical projection, and finishing coat (1-3 mm) applied manually. In general terms, they are made with similar composition products, the main difference being that the first coat is made with coarser materials and is thicker whilst the finishing coat is made of finer materials and is applied over

Table 3.5 Most adequate pigments to be used in gypsum plasters’ composition [28]

White pigments

Rose pigments

– Lithopone, zinc white, titanium dioxide

– Black with weak colouring capacity

Black pigments

Yellow pigments

– Black of natural or artificial irons, with high Fe2O3 content, black of magnesium

– Mixture of yellow ochre and sienna earth, chromium, zinc, cadmium, antimony yellow – of-Paris (based on lead oxide)

Grey pigments

Cream pigments

– Black with weak colouring capacity

– Very small amount of yellow ochre

Red pigments

Orange pigments

– Iron oxides with high Fe2O3 content (red ochre), artificial iron oxides, calcined sienna iron

– Red iron oxides, natural or artificial, with yellow iron oxides and yellow ochre

the previous one in a very thin layer, almost a pellicle. However, in some cases, these coats can be made with exactly the same product [27].

The products used to make the smoothing and finishing coats are marketed in powder, in Kraft paper bags, containing a maximum of 40 kg of product and comprising a valve to allow filling them by injection and automatic zipping with specific equipment. They are generally white or grey (the natural colours of gypsum used as raw material). For non-painted coatings, the product for the finishing coat may incorporate pigments of various light colours (Table 3.5). The bags must contain the following information: commercial designation and reference of the product, name of the manufacturer, colour reference (for pigmented products), amount of product, validity, CE marking (compulsory requirement), reference of the manufacturing lot and, if it is homologated, identification of the corresponding homologation/application document [27].

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