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CEMENT Use Ohorongo CEM II B-LL 32.5N cement or Ohorongo CEM II A-LL 42.5N cement, depending on the purpose and function of the plaster.
AGGREGATES: SAND As sand is the major constituent of a plaster mixture, its quality has a significant influence on the performance and material cost of the plaster mix.
WATER Use clean, drinkable water
Depending on the exposure to weather conditions of the plastered surface, different types of plaster mixes are recommended. Refer to section overleaf for the recommended mix proportions.
Machine mixing is preferable to hand mixing. However, if the plaster is mixed by hand, special care should be taken to follow these instructions:
Warning: If the plaster mix has hardened and is not workable anymore, that batch of plaster mix has to be discarded. Do not attempt to mix additional water into the mix, as this reduces the strength and adhesive properties of the mixture. At any one time, only prepare the amount of plaster that can be used up within 90 minutes or before the mixture dries out.
Note: If a very smooth texture is required, a steel trowel may be used on the surface. However, such a surface is generally not recommended, as it tends to craze and show up imperfections. Various decorative finishes are possible. Techniques include brushing, flicking plaster onto the surface and lightly floating, etc.
Warning: Plaster should be protected from the sun and drying winds. Any batch of prepared plaster should be used up within 90 minutes of being mixed and never be re-tempered by mixing with additional water.
SMOOTH CONCRETE Spatterdash is a mixture of one part of cement to one and a half parts of coarse sand, with enough water for the concrete to have a sluggishly pourable consistency. Throw the mixture forcibly on the wall, using a scoop or a brush with long, stiff bristles. The spatterdash should cover the substrate surface completely and form a rough texture with nodules about 5 mm high. Spatterdash should be kept moist for at least three days. The surface should be tested for adhesion and strength by probing with a screwdriver or knife before plaster is applied to it.
CONCRETE MASONRY The texture of the masonry units should be sufficiently rough without need of further treatment. If not, apply a spatterdash coat. It should not be necessary to control suction of the surface by pre-moistening, unless the units are very absorbent.
BURNT CLAY STOCK BRICKWORK The texture of the bricks should be sufficiently rough without need of further treatment. If not, apply a spatterdash coat. Burnt clay stock bricks normally have a high level of water absorption, which results in rapid drying out of the plaster. If so, pre-moisten the wall and allow it to become surface-dry before applying the plaster.
Name | Description | Cause | Solution |
Grinning | Positions of the motar joints are clearly visible through the plaster | Different rates of suction between the mortar and the bricks | Apply plaster undercoat or spatterdash coat before plastering. Moisten Bricks |
Crazing | Network of closely spaced, fine cracks | Over-trowelling a rich mixture or use of sand that contains too much fine particles | Use better quality plaster sand. Do not over trowel |
Cracking | Larger cracks randomly spaced | Movement of the wall or shrinkage of the plaster caused by excessive loss of water from the plaste. Using badly graded sand that lacks fine material. Excessive suction by the bricks or blocks. Exposure to direct sun or wind | Do not use very rich mixes (too much cement). Use good quality sands. Limit plaster thickness to a maximum of 15mm per coat. |
Lack of hardiness | Plastere is easily chipped away or is easily scraped off another hardening | Plastering in full sun and wind. Not wetting absorbent bricks. Addition of extra water after first mixing. Using a very lean mixture (too little cement) | Avoid the causes listed |
Debonding | Plaster not adhearing to the wall after hardening | Dust on the wall when plastering. Over-rich mixtures. Very thick layers of plaster (>15mm) | Prepare surface properly before plastering. Limit plaster thickness to a maximum of 15mm. Do not use very rich mixes |
CEM II A-LL 42.5N / B-LL 32.5N | Bags | Sand | Stone | Volume m³ |
EXTERIOR / DRY MORTAR AND PLASTER
Mortar and plaster exposed to dampness - mix
designs are based on the use of good quality sand
|
1 | 2 | - | 0.1 |
INTERIOR / DRY MORTAR AND PLASTER Mortar and plaster not exposed to dampness - mix designs are based on the use of good quality sand |
1 | 3 | - | 0.14m³ |
The volume of the wheelbarrow is 60 litres.
Click on the link below to download the full plastering made easy with Ohorongo Cement document.
Ohorongo (Pty) Ltd. is one of the first of Namibia's cement manufacturers and owns one of the most modern cement plants in Africa. It was constructed over the course of two years by leading international engine...
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