A condition for the use of lime plaster on earth walls is that the walls must be well dried, and that the surface is even and without cracks. A thin clay gruel is first applied to the wall to give it a rough surface as a key for the lime rendering. The gruel consists of one part clay gruel and two parts sand with a grain size of around 4 mm. Pieces of hacked strawor hay 3 cm long are added and the mixture is then applied in two layers, in quick succession. The first layer is about 2 mm thick and the next is 5 to 8 mm thick. This is then left for two to three days.
The lime plaster is applied in two layers by a trowel, without dampening the surface before application.The first layer consists of one part slaked lime, one part sand with a grain size of 4 mm and three parts hempf ibre or the equivalent.The next layer consists of one part finely sieved lime dough and three parts marble powder. In Japan, where this plaster originates, a small percentage of gelatine from seaweed is added. This makes the surface water repelling, although it is not vapour proof.
For coloured plaster, pigment is added in the second layer (seeTablel 8.5).The surface is matt from the beginning, but a smooth shiny surface can be achieved by adding a third layer that is only 1 mm thick, consisting of one part finely sieved slaked lime, one part white marble dust and one part pigment.The thin layerof plaster is put on with a trowel and smoothed out until it gels to a lustre finish.Then the surface is polished intensively, preferably with the palm of the hand.The result is of very high quality, but the process is of course very labour intensive.
Hydraulic lime plaster gives a more weather-resistant result than pure lime plaster. It still needs to be applied in several layers to achieve a high durability. The first layer consists of one part hydraulic lime and two parts sand with a grain size of up to 7 mm. The second and the third layers both consist of one part of hydraulic lime and three parts sand with a grain size of up to 5 mm.
Lime cement plaster is used a great deal externally. It is somewhat stronger than lime render and more elastic than pure cement render. From 30 to 50% of the binder is usually cement.
Cement plaster is very widely used as external rendering on retaining walls, tanks, pools, etc. as well as on solid concrete walls, concrete blocks, brickwork, lightweight concrete blocks, etc. First cracks or damage to the surface should be smoothed out with a cement mortar of proportions 1:3. Then the surface should be brushed with a cement gruel of the same mix proportions and, finally, rendered with a cement mortar of 1:1 on concrete walls or 1:3 on concrete block or lightweight block walls. The last treatment can be repeated, giving a surface that is as watertight as possible.
Silicate plaster is based on waterglass as a binder and is an outdoor rendering with good strength, whilst being open to moisture. Small amounts of synthetic polymers are sometimes added.
Gypsum plaster can be used externally, except in areas with harsh climate conditions, but is mainly for internal use, especially as a moisture buffering layer. A mix of one part gypsum to two parts sand is common. This sets in 10 to 30 minutes. For stucco work, a mix of three parts liquid lime and one part gypsum powder is used. More gypsum is needed for relief work in proportions of one part lime and two parts gypsum. A final coating can be one part lime and one part marble dust.
Sulphur plaster can be produced by melting sulphur at temperatures ranging from 120 to 150 °C. Sand, wood flour or the equivalent can be added. It is waterproof but cannot be used on materials with a high lime content.
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